Cantilever insulation/sealing?

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Dana

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It's fine to compress batting to ensure a tight fit. When compressed it gives a higher R/inch and is more air-retardent than at full loft:

An R13 batt designed for 3.5" deep cavites has exactly the same amount of material as R19s designed for 5.5" deep cavities.

When compressed into 5.5" the R19 actually performs at R18, which is R3.27/inch.

Compressing it into a 3.5" space it performs at R13, which is R3.71/inch.

Compressing it further into a 2.5" cavity delivers R10, which is R4/inch.

Basically compressing the fiberglass is generally GOOD for performance, voids are always BAD. Never be reluctant to pack it in there as long as it's still springy to finger-push. It takes quite a bit of compression to where it goes beyond the optimal density.

Using 2-3" of foam is NOT better than filling the joist bays with cellulose, and it's probably going to be more expensive to boot. Cellulose at open blow densities (as in attics) costs about 3 cents per square foot per R, and at mid-density (2.2-2.8lbs per cubic foot) it's about 5 cents R-foot. Dense packed to 3.5lbs it's more, but actual cost varies depending on the complexity of the project. Ceiling joist bays are dead-easy, walls with lots of complicated window & door framing blockages to work around are harder.

Closed cell foam at about a buck a board-foot is 15-17 cents per square foot per R. So for the same cost of 3" of closed cell foam ($3 per square foot, ~R19) you can get ($3.00/$0.05=) R60 of cellulose. Clearly the most you're really going to get into a 2x12 joist bay is about R40, but it's more than 2x the performance at 2/3 the cost (a 33% savings, give or take.)

And cellulose...

* is more fire retardent using less toxic fire retardent chemicals

*doesn't have any polymeric outgassing issues, should the installer get the mix or temperature slightly off

*does not use a high global warming potential blowing agent (HFC245fa at ~1000x CO2)

*is protective of structural wood by buffering/sharing the wintertime moisture burden, without damage or loss of performance

*reduces the thermal bridging of the joists by being a deeper fill at any R, lengthing the thermal path through the ~R1.2/inch wood (which means the thermal bridge goes from R3.6 in a 3" closed cell foam solution to R13.5 in 11.25" cellulose joist-fill solution.) Even at equal R-values at center cavity, the cellulose-fill outperforms the closed cell foam partial fill by a double-digit percentage due to the thermal bridging factor.

It's basically cleaner-greener-cheaper and more forgiving stuff, and when you have a whole joist bay of space to fill, the rationale for using the more expensive higher R/inch goods just isn't there. In locations where you need the higher R/inch due to lack of space orwhere you need it for vapor retardency control (such as at band joists or foundation walls) closed cell foam can be the best option, but that's not what you have here.

If you are still enamored of polyurethane foam solutions, a full 2x 12 cavity fill of open cell foam would deliver R38-R40 and cost about the same as 3" of closed cell (maybe 10-15% more). And since it's blown with water instead of HFC245fa it doesn't have the greenhouse gas issue (though it still has the other chemical issues). It would have to be blown in two 5.5" lifts to not be a fire hazard as it cures or have shrinkage/bonding issues, but done right it is even more air-tight than closed cell foam. When applied over a garage or in a cantilever overhang you have maximal drying capacity on the "cold" side of the issue, so you don't really need the lower vapor retardency of closed cell foam to protect the ceiling drywall or the bottom side sheathing of the cantilever. But open cell foam does not have the protective hygric buffering characteristics of cellulose any moisture that gets in via diffusion ends up as adsorb in the cold-side wood until it warms up enough to release it.

The microscopic hollow fiber characteristic of cellulose allows it to adsorb and redistribute quite a bit of moisture before losing function, and it is protective even at relatively modest density (even more protective if dense-packed.) For cheap stuff it's pretty good.
 

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Ok well the front of the house I did has made a difference in at least the heat registers are encapsulated and masticked so I am getting ALOT more pressure/volume coming out and the metal vent is getting quite warm compared to how cold it would stay even if the furnace ran awhile. And our master bedroom on that side (my bed side) is alot more quiet with car noise, lawnmower, wind noise ect.

My insulation company called and they are wanting to do open cell in the cavities 9.5" tall and 17" or so wide chambers in the cantiliver and under the bedroom floor. He says that if open cell is 5.5" or deeper it is the a air barrier. Said he did his new house that way and was great. He said they could do the cellulose in the garage/bedroom but would never be a air barrier no matter how they packed it in.....
 

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It's true that cellulose will not be as air-tight as open cell foam from an air-transported moisture point of view, but from a blower-door test numbers point of view it's nearly that tight.

From a cost point of view open cell foam is often cheaper than dense-packed cellulose, rarely as cheap as standard density ~2.5lb ish cellulose (which is also pretty air-retardent.)

In tight places with obstructions it's often difficult or impossible to spray ocSPF without at least some voids (unless you use a slow-rise half-pound pour) which may be an issue in the cantilever bays with the ducts.

Sounds like that leaky duct was blowing majorly copious quantities of heating air into that cold cantilevered joist bay before you sealed it up!
 

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I bay window bump out with a 2 foot exterior cantilever subfloor extending over the basement foundation wall. The area in the basement is finished but I can access the subfloor from inside the basement and I also am able to remove the soffit from the outside. Its hard to do much from inside the basement because of the finished interior well which 43" from the outer rim joist.

The problem its like an artic wind tunnel in the winter and there is cooper pipe that has frozen and has bursted twice in the last two winters. There are manufactured I beams sitting next to each other and the cavity between them is uninsulated and contains lots of arctic air in the winter. I live in eastern pennsylvania so it gets pretty cold here.

The builder just installed batts of yellow insulation and finished the exterior with soffit material.

I'm considering the options.
1. blow in cellulose from the basement.
OR
2. remove soffit and fit foam board attached the subfloor and rim joist
OR
3. remove soffit and spray foam the subfloor and rim joist

In additional to the options above seal the joist cavity with 1-2" foam board as shown in some of your pics and install reinstall the batt insulation that I have or replace with roxul insulation.

Based on the pictures what do you folks think.

What should I do with the I beam cavities, if I remove the soffit I think I can block off the cavity void from the outside, or drill holes into the cavities and fill with great stuff foam. These cavities are pretty drafty so I think I should seal them unless sealing everything off using the options above prevent any draft from entering therefore the it wont be necessary to fill the voids with anything. They are basically uninsulated now and the best case is to stuff batt insulation inside if I remove the soffit.

The floor joists are 2.5" x 12", the cavities between I beams are 2" in width when I beams are placed next to each other.

Here are some pics to go give a visual representation of what Im talking about.
 

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PeteSf

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One more pic of finished basement wall and ceiling view.
 

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Dana

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It looks like they installed soffit vents in cantilever? I can't imagine WHY, if it's supposedly a semi-conditioned basement and not a vented crawlspace! That's completely nuts!

It's not clear in the picture whether there is a sill gasket under the foundation sill, but even if there is it probably leaks air and should be sealed up with spray foam as part of the project.

Batts are not designed for I-joists, making it impossible to get a decent fit, which won't fix the air-leakage much. Blown cellulose is very air retardent even at 2-2.5lbs density and even more so when "dense-packed" to 3lbs+ density (which will never settle.)

It's possible to install air-blocks in the I-joist bays without ripping out the soffit boards. This is done by drilling a 2.5" hole in either the ceiling or the soffit with a hole saw, inserting a woven grain/feed bag, and dense-packing the bag full of cellulose. (Any cellulose installer worth their salt is familiar with this method, but it's also DIY-able if you fab-up something to narrow down the 2.5" hose of a rental blower to 1" or so. You can then go ahead and fill the now-plugged cantilevered bay with cellulose with a second hole.

Where you have access to the interior side of those bays you can cut'n'cobble some 1.5" EPS rigid foam as the air barrier, can-foaming it in place. (Cut the rigid-board fairly loose to make it easy to get it in there, tacking it in place with a few nails so you can foam-seal it. Then cut blowing hole with a hole saw, saving the plugs if you can, and foam/caulk the plugs in place when you're done.

Hopefully all of the plumbing is on the interior side of the foundation, not out over the cantilever?
 

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Hi Terry,

I did the following
Removed soffit
Cut cobbled 1.5 foil face rigid board to the rim joists and subfloor and top of sill plate blocking access to the basement. Next I was going to install roxul to fill the foil faced cavity.

Finally I was going to add another layer of 1.5 foil faced rigid board to the bottom of the joist and then installing the soffit.

Will there be a moisture problem if I seal off the bottom of the joists with 1.5 foiled faced rigid board?

Im also considering adding a layer of luan board at the bottom of the floor joists on top of the 1.5 foil face rigid board installed at the bottom of the floor joints. But it depends if I have the room.

Please let me know if there will be a moisture problem if I seal off the bottom of the floor joist with more 1.5 foil face rigid board.
 

Dana

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Unless you have a vapor-barrier on the top side of the flooring creating a moisture trap, it's fine to use foil-faced foam on the underside of the joists. Putting on the bottom side of the joists is far superior to a cut'n'cobble, since it isn't thermally bridged by wood, and it can be made far more air-tight. Use a high quality aluminum tapeto seal the seams of the foam. (eg Nashua 324a, found in most box stores, but there are others. Anything with a temperature rating is good- doesn't matter what the temp is. Unrated foil tapes usually have the world's cheapest adhesives, guaranteed to fail over time.)

In your climate true vapor barriers such as foil or polyethylene sheeting is never necessary, but if you're going to install one (such as foil facers on polyiso) it's better to be on the exterior side rather than the conditioned space side, since exterior moisture can potentially condense if it's on the interior side of an air conditioned building. You should NEVER put a true vapor barrier on BOTH sides of a wall /floor/roof, since it's a moisture trap- any moisture that finds it's way in could take years or decades to leave. But on one side it's just fine. In colder climates it can be better if it's on the interior side, but it can live on the exterior side if there is a sufficient amount of R value on the exterior of the vapor barrier relative to the R-value on the interior- it's climate specific.
 

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Thanks for your prompt reply. Unfortunately due to the exterior stucco finish I can't really alter the finish to add 1.5" of foil-faced foam to the bottom of the floor joists. I would have to add some sort of corner detail that would hide the form board from the exterior view.

I have been following the attached diagram only I'm using Polyisocyanurate foil-faced form board in place of XPS or http://www.homedepot.com/p/Thermash...Rigid-Foam-Insulation-Board-W-N5150/100317819

I have already installed the vertical piece on the rim joist, horizontal piece on the subfloor and a vertical piece on the foundation plate.

If I understand you correctly using foil-faced foam isn't appropriate on the conditioned space side since exterior moisture can condense. With that said sounds like I may have a problem in the future if I leave whats installed now.

With that in mind I see these options

Solution A
Remove horizontal piece of foil-face foam attached to subfloor and attach to the bottom side of the floor joists nested between joists due to finishing issue identified in the first paragraph. Leave the vertical foil-faced foam that's attached to rim joist and foundation plate in place. I realize this install does not maximize use of the foam to prevent thermal bridging but I have exterior finishing constraints.

Solution B
If foil-faced foam isnt appropriate for this project I can start over and remove all foil-faced foam and use unfaced rigid foam insulation http://www.lowes.com/pd_338136-210-...d=3308704&pl=1&Ntt=rigid+insulation+sheathing ?

Solution C
Tedious as this might be I could remove the foil from each piece that has been installed and reinstall as unfaced Polyisocyanurate.

All solutions include roxul in the cavity.

I live in Northeastern PA, Nazareth area. Im trying to maximize what I can do with the space that I have without getting into spray foam or blown in cellulose.

What solution do you recommend such that I don't have moisture issues in the future?
 

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Dana

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Thanks for your prompt reply. Unfortunately due to the exterior stucco finish I can't really alter the finish to add 1.5" of foil-faced foam to the bottom of the floor joists. I would have to add some sort of corner detail that would hide the form board from the exterior view.

I have been following the attached diagram only I'm using Polyisocyanurate foil-faced form board in place of XPS or http://www.homedepot.com/p/Thermash...Rigid-Foam-Insulation-Board-W-N5150/100317819

I have already installed the vertical piece on the rim joist, horizontal piece on the subfloor and a vertical piece on the foundation plate.

If I understand you correctly using foil-faced foam isn't appropriate on the conditioned space side since exterior moisture can condense. With that said sounds like I may have a problem in the future if I leave whats installed now.

With that in mind I see these options

Solution A
Remove horizontal piece of foil-face foam attached to subfloor and attach to the bottom side of the floor joists nested between joists due to finishing issue identified in the first paragraph. Leave the vertical foil-faced foam that's attached to rim joist and foundation plate in place. I realize this install does not maximize use of the foam to prevent thermal bridging but I have exterior finishing constraints.

Solution B
If foil-faced foam isnt appropriate for this project I can start over and remove all foil-faced foam and use unfaced rigid foam insulation http://www.lowes.com/pd_338136-210-L28200LOWES___?productId=3308704&pl=1&Ntt=rigid insulation sheathing ?

Solution C
Tedious as this might be I could remove the foil from each piece that has been installed and reinstall as unfaced Polyisocyanurate.

All solutions include roxul in the cavity.

I live in Northeastern PA, Nazareth area. Im trying to maximize what I can do with the space that I have without getting into spray foam or blown in cellulose.

What solution do you recommend such that I don't have moisture issues in the future?

It's confusing that the location in your member ID is Plano, TX . the edge of US climate zone 2/3 (which is what I had assumed), rather than Nazareth PA, which is US climate zone 5. It makes a difference.

It's fine to use foil-faced foam on either side of the assembly, but never both, and only if there is a sufficient ratio of foam-R to fiber-R in the insulation stackup that condensation or adsorption into susceptible materials is limited.

index.php


As drawn you have created a severe moisture trap with foil faced foam on four sides of the cantilever. In a zone 5 location the condensation risk is on the exterior layers- the sheathing side, not the flooring side. Since the sheathing to cantilever floor can always dry toward the exterior (bottom), it only needs to be air-tight, but you can't put foam insulation there unless there is sufficient R-value that condensation would not collect at the fiberglass/foam boundary over the winter. In Zone 5 that means more than 25% of the total R value in the cavity would have to be foam, all on the exterior side of the assembly.

Polyiso has performance issues at colder temps, and when the average temp through the polyiso layer is less than ~45F (say 30F outdoors, 60F on the warm side of the layer) EPS would outperform it. So in your case it would be better to remove the foam off the band joist & the bottom of the cantilever, and put it on the top of the insulation stack-up tight to the sub-floor of the conditioned space, and at the air-dam over the foundation where it would operate at it's labeled R, and let the cantilever dry toward the exterior.

But you still need vapor permeable air-barrier on the bottom side. If you cut a sheet of OSB or plywood to fit, and can-foam seal it to the floor joists and rim joist and to the foundation sill it will be fine. OSB and plywood are "smart" vapor retarders. When bone-dry they run about 1 perm, but if moisture gets into the cavity they become far more vapor-open, which allows that moisture to escape. Better still, you could also use exterior-rated gypsum board such as GP Densglass sheathing (any thickness) for the air barrier, which is more than 15 perms. Box stores don't carry it in PA, but you can get it at Opdyke Lumber in Frenchtown, NJ, Blue Ridge Lumber in Hackettstown, NJ, or Trevdan Building Supply in Sellersville PA (call ahead to verify stock before pulling the F-150 out of the driveway.)

Even though you already installed it, since you have stucco siding it will be much safer to REMOVE the pieces from the rim joist, allowing the rim joist to dry into the fiber insulation layer. Stucco is a "reservoir cladding" that takes on rain & dew moisture, then releases it when temperatures rise- it's sort of like having a sponge in close proximity to the wall sheathing & band joist. With foil faced foam on the other side blocking drying, the moisture drives from the stucco will increase the moisture content of the rim joist, potentially to rot/mold levels.

Your summertime outdoor dew points in Nazareth rarely break into the 70s, and (almost)never hit 80F, so the risk of summertime condensation collecting on the foam on the air-conditioned side of the cavity due to the foam is ZERO. (In Plano TX there would be some risk but low unless you kept the interior ridiculously cold.

On the basement walls & interior side air-dam it's fine to go with 1.5" polyiso, despite the temperature derating. With two layers you'd be at ~R18 from a labeled R point of view. The above-grade portion would only perform at about R15 during the coldest weeks. Polyiso is much greener than XPS due to both the polymer content per R, and the drastic difference in blowing agents. XPS is comes in either 1.5lbs or 2.5lbs per cubic foot density, and is blown with HFC134a (automotive AC refrigerant), which has a global warming potential ~1400x CO2. As it leaks out over the next 50 years it's performance slowly drops to about R4.2/inch, the same as 1.5lb EPS. Polyiso is blown with pentane at about 7x CO2, and is less dependent upon the blowing agent for it's performance. Most foil-faced polyiso is only 1lb density. Fully depleted of the pentane it's between R5.5 and R6 per inch, and even with temperature derating for climate in this application fully depleted polyiso would outperform new XPS. (This would not be the case if there were insulation between the foam and the conditioned interior.)
 

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Thank you so much for the wealth of information. I will remove the rim joist foil-faced rigid board and add to the foundation plate doubling up its R value at 1.5" x 2 is an R value of 20. I will remove the bottom foil-faced rigid board and add to the subfloor doubling up its R value again at 1.5" x 2 an R value of 20. For the rest of the cavity I was going to add Roxul. Based on what I can put into the 12" cavity, 3" are used by the foil-faced foam and 9" remain. With Roxul 5.5" it gives me an R value of 23 or an R value of 4 per 1". If I fill the remaining 9" with Roxul I will have roughly an R value of 36 with the Roxul and an R value of 20 with 3" the foil-faced foam, creating a total R value of 56.

Next I will cover the bottom with OSB or plywood and foam seal. This is the trouble area because I don't have a lot of room between the bottom of the floor joists and the top of the soffit. I may have 3/16 of an inch and with that it's going to be tight. Otherwise I will need to do extra finishing work to install OSB or plywood thicker than the current space that I have.

I thought Id use some luan board for a cover or the thinnest plywood I could get and cover it with the original suffit. I have to see what I can do here to maybe get 1/4 board there.

I have attached a revised diagram detailing what you described.
 

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Dana

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The detailing of the cantilever looks good, with no moisture traps:

index.php


But the foundation insulation and foundation sill insulation detail seems to have gone missing. The portion of the original drawing detailing of the foundation wall and the interior side of the foundation sill is still worth following though, making sure that you air-seal all of the seams in the foam:

index.php
 

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Right. The wall I have in the interior side of the basement is finished so I can't really add foil-faced rigid foam on the foundation wall, sorry for the confusion there, it's just not accessible.

But I can still add the piece of rigid foam to the interior side of the sill as shown in the previous diagram. Im just wondering though isn't enough to foam seal the interior side of the sill, what will adding a 4" wide piece of foil-faced butted up against the interior side of the sill add?

One more thing, since the entire front side of my house is stucco currently Im only focused on the cantilever. But there are other parts of the front of the house where I can access the rim joist and am consider addressing that area as well. Right now those other parts are not sealed with anything but a piece of batt insulation with the vapor barrier facing the inside of the basement. I assume because of the stucco rim joist moisture trap issue, that foil faced rigid foam is still not appropriate.
The best I can do is seal the surfaces of where wood meets wood with caulk or great stuff foam and replace the batt insulation is that right? The other sides of the house is siding, know that siding is used on the other sides does that change the approach of how the rim joist should be sealed?

Thank you again
 

Dana

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An 8" thick concrete wall runs about R1. The distance from the condition area on top of the foundation to the exterior side of the concrete with a 2x4 sill is less than R0.5, which is a pretty extreme thermal bridge.

Assuming you have 1x or 2x furring or a 2x4 studwall with air between the wallboard and the concrete the air-films are worth another ~R1, and the wallboard itself is worth another R0.5, so you're looking at an R value on the order of R2.5-R3 from the interior side paint to the exposed exterior concrete. Every square inch of that R0.5 thermal bridge conducts 5x as much heat as the equivalent area of finished wall.

It's likely that the top of the cavity of the finished wall is not air-tight, and there is some performance lost to convection. Installing the rigid foam so that it's sealed at the foundation sill AND seals the top of the finish wall improves the thermal performance of that wall as well, even without adding insulation into the cavity.

For the non-cantilevered section UNFACED rigid EPS foam at 1.0 or 1.5lbs per cubic foot density aka "Type I or Type II" (looks like a cheap foam cooler, with macroscopic bead structures visible) would be sufficiently vapor permeable to allow the rim joist to drying at thicknesses up to 1.5", applied directly to the rim joist, with batts installed on the interior side of the assembly. You can also apply it to the top of the foundation sill, and any concrete ledge from the sill to the interor. R-TECH's Type-I EPS sold at box stores usually has a plastic or foil facer reducing it's vapor permeance, which is NOT what you want here. Wallboard cut'n'cobbled to fit and air sealed is fine for the interior-side air barrier. (If painted, use only a standard latex.)
 

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Dana,

Are you still here? I know this is an older thread but I was hoping for your advice; I know you're the person to talk to as I have already read many of your post on here and GBA. Please respond and I'll post my questions, much appreciated.
 

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Still here...

Wow, thank You so much for responding Dana, I appreciate that very much.

Bought a 4 level foreclosure house not to long ago built in 65. I'm in the very north part of Illinois, so zone 5 still, but only 30 miles south of zone 6 in Wisconsin so it does get very cold here at times. I'm an efficiency minded kind of guy, moved from a house built in 96, and my current much older house is nowhere near where I want it to be. Leaky and uses a lot of boiler gas to heat up. Already redid the windows to some .23 units. Next is the cantilever area since I had to fix a few burst pipe leaks, and no wonder since there was barely any insulation in there. They are 2x10 joists. Most work on these cantilevers will be done from the walkout basement area since it's easier to redo drywall than sub floor but there are a few areas where I opened up the sub floor as you see in the pictures. In the bays with no heating pipes I was originally thinking of doing foam board against the bottom of the sub floor and a piece over the supporting wall blocking the exit to the basement ceiling, than Roxul for the rest of the bay. Now, since I read you say I really don't need an air barrier in the cantilever, I'm just thinking of either doing Roxul or Cellulose in the whole bay with no foam board at all. Not sure which would be better considering I eventually want to add foam board on the walls outside. Currently the 2x4 walls have basic old paper backed f.g. batts than a lawyer of 1/2" fiber like cardboard crumbly material, than some reflective material backed by thinner fiber, than Tyvek wrap and siding. Sorry, not sure exactly what these old product are but they are in the attached pictures. If I use cellulose in the whole cantilever do I have to caulk all of the inside or will cellulose take care of the air leaks. If I use Roxul would caulking make sense than? I'm hesitant to caulk because I don't think it will last long with the wood moving an all and it will also make the water go into the wall next time I have a pipe burst instead of going outside through the cantilever. I like Roxul better because of the higher R value but it doesn't seal air leaks as well as cellulose and if I don't caulk cellulose might actually perform better even though it would have almost 5 R less than Roxul. What do you think? Again, thank You kindly for your response.
 

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Dana

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You definitely need an air barrier between conditioned space and the cantilever's joist cavities, but it need not be foam board (though it can be.) If you use foam board it should be on the conditioned space side of the assembly.

When using fiber insulation it's important to have an air barrier on the exteiror side of the fluff, but in your climate you want it to be vapor permeable, or at least semi-permeable. Half-inch OSB or CDX would be semi-permeable and air tight (if caulked to the framing with polyurethane caulk), and OK on the under side of the cantilever, but you could also use housewrap under t & g / ship-lap wood.

Cellulose dense-packed to 3 lbs per cubic foot or higher is sufficiently air retardent from a thermal loss/infiltration point of view, but it's not adequate from an air-transported moisture perspective. Caulking the framing to the subfloor is preferred, but you'll still get away with it as long as you use something more vapor permeable than plywood on the bottom of the cantilever. Asphalted fiberboard works- it's 10+ perms even when dry, more than 10x as vapor permeable as dry plywood or OSB. If you can't find it at the local building supply, Menards carries it. Asphalted on both sides is best, but single sided is fine as long as the black side faces the exterior. If dense-packing you'll get less bowing if you use 3/4" goods. As long as it's VERY air tight on one side (either side, but interior side preferred) you can let the air retardency of the cellulose work for you on the rest.

If installing foam board over the sheathing on the exterior of the walls it has to have sufficient R-value relative to the cavity fill for dew point control. In zone 5 that would be an absolute minimum ~27% of the total R, eg: 2" R8 EPS, for 2x6 R20 framing: R8/R28= 29 % would be fine, but R6 foam with R20 cavity feel would be R6/R26= 23%.

In zone 6 it the prescriptive ratio is 36%.

Those ratios a minimums, but it includes the cool edge of the climate zone. But if you want any margin you're better off splitting the difference- go for a ratio of more than 30% if you can spit to zone 6 in a high wind.
 

dAgent

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You definitely need an air barrier between conditioned space and the cantilever's joist cavities, but it need not be foam board (though it can be.) If you use foam board it should be on the conditioned space side of the assembly.

Just to be clear you're saying I need an air barrier between the joist on top of the supporting wall to block air movement in the cantilever cavity? What foamboard and thickness would work best here and if not foam board what else could it be?

When using fiber insulation it's important to have an air barrier on the exteiror side of the fluff, but in your climate you want it to be vapor permeable, or at least semi-permeable. Half-inch OSB or CDX would be semi-permeable and air tight (if caulked to the framing with polyurethane caulk), and OK on the under side of the cantilever, but you could also use housewrap under t & g / ship-lap wood.

I have 3/8 plywood on the underside of the cantilever with aluminum soffit. I'm not removing these since I'll be renovating my walkout basement area and I'll be able to access the cantilever cavity than.

Cellulose dense-packed to 3 lbs per cubic foot or higher is sufficiently air retardent from a thermal loss/infiltration point of view, but it's not adequate from an air-transported moisture perspective. Caulking the framing to the subfloor is preferred, but you'll still get away with it as long as you use something more vapor permeable than plywood on the bottom of the cantilever. Asphalted fiberboard works- it's 10+ perms even when dry, more than 10x as vapor permeable as dry plywood or OSB. If you can't find it at the local building supply, Menards carries it. Asphalted on both sides is best, but single sided is fine as long as the black side faces the exterior. If dense-packing you'll get less bowing if you use 3/4" goods. As long as it's VERY air tight on one side (either side, but interior side preferred) you can let the air retardency of the cellulose work for you on the rest.

I'm sorry I'm getting confused. Are you saying that I can't just stuff more insulation in my current cantilever cavities, which I don't want to rebuild, whether it's Cellulose or Roxul, without caulking or changing the underside of the cavity or having an air barrier between the joists?

If installing foam board over the sheathing on the exterior of the walls it has to have sufficient R-value relative to the cavity fill for dew point control. In zone 5 that would be an absolute minimum ~27% of the total R, eg: 2" R8 EPS, for 2x6 R20 framing: R8/R28= 29 % would be fine, but R6 foam with R20 cavity feel would be R6/R26= 23%.

In zone 6 it the prescriptive ratio is 36%.

Those ratios a minimums, but it includes the cool edge of the climate zone. But if you want any margin you're better off splitting the difference- go for a ratio of more than 30% if you can spit to zone 6 in a high wind.

I understand the ratio concept. The problem is I can't due more than 1"-1.25" of foam board on my 2X4 walls without pulling out and reinstalling the windows. I was thinking of doing either 1" r6 foil faced polyiso or 1" of pink xps. I understand that the polyiso underperforms in the very cold and xps overperforms, considering this would 1" of polyiso still work for my wall stack up with the R13 f.g. batts, the 1/2" of fiber like cardboard crumbly material, than some reflective material backed by thinner fiber, than Tyvek wrap and siding? I know a typical 2x4 wall is around R10 when you factor in thermal bridging, with the additional materials of my wall what do you think my wall r value is?

I'm shopping for a caulk to seal my wall base plate, was thinking of using GE supreme silicone since I have heard good things on it lasting and the fact it can stretch 50%. Do you think polyurethane caulk would be better and if so do you have a specific suggestion?
 

Dana

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XPS only overperforms it's rated R in the first decade or less. While it's performance still increases as the average temp through the foam falls, as it's climate damaging HFC blowing agents slowly leak, the overall performance eventually falls to R4.2, at mean temp of 75F test condition required by the FTC for labeling purposes, R4.5 at a mean temp of 40F, R4.7 at a mean temp through the foam of 25F. At temperatures that matter from a dew-point control point of view don't assume better than R4.7 for an inch of the pink stuff, no matter what the manufacturer guarantees or claims.

Most pentane-blown polyiso (which is nearly all current product out there) will perform at less than R4.5 when the mean temp through the foam is 25F, (which only occurs when it's colder than 10F outside, which is less than 5% of the time), but rises rapidly into the R5s when the average temp through the foam is above freezing. Overall, and over the long term you're better off with an inch of foil-faced polyiso, with at least a 1/4" air gap between the exterior foil and the siding (or just use vinyl siding), which gives it another ~R0.5-R1 in average thermal performance. The mean outdoor temp for Rockford IL in January is about 20F, which means the mean temp through the foam will average about 30F during the coldest month, and much warmer the rest of the time.

Polyurethane sealants remain flexible for life (especially the "windows & doors" variaties), and have much better adhesion to wood than any silicone caulks I've used. Maybe the GE Supreme is the exception that makes the rule, but just about anybody's polyurethane caulk sealant is a sure thing. It can be a bit gluey, so removing a decorative baseboard after the fact might be a destructive process, but for sealing the joists to the subfloor it's definitely the right stuff.
 
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