Cantilever insulation/sealing?

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pex006

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Hi. I have a cantilever that extends over my back wall about 12 inches. The rim joist is the outer part of this cantilever. The rimjoist has foamboard on the outside then vinyl siding over that foamboard. The soffit under the cantilever is 2 by wood with seams sealed with caulk and vinyl soffit over it. The inside joist bays of the cantilever have no insulation right now. Do I have to do anything to insulate or seal these joist bays or is the foamboard insulation on the outside enough?
 

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Hi. I have a cantilever that extends over my back wall about 12 inches. The rim joist is the outer part of this cantilever. The rimjoist has foamboard on the outside then vinyl siding over that foamboard. The soffit under the cantilever is 2 by wood with seams sealed with caulk and vinyl soffit over it. The inside joist bays of the cantilever have no insulation right now. Do I have to do anything to insulate or seal these joist bays or is the foamboard insulation on the outside enough?

Anyone?
 

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Hi. I have a cantilever that extends over my back wall about 12 inches. The rim joist is the outer part of this cantilever. The rimjoist has foamboard on the outside then vinyl siding over that foamboard. The soffit under the cantilever is 2 by wood with seams sealed with caulk and vinyl soffit over it. The inside joist bays of the cantilever have no insulation right now. Do I have to do anything to insulate or seal these joist bays or is the foamboard insulation on the outside enough?

Are you saying there is currently NO insulation between the floor joists? That would be both a pretty huge heat leak and a potential air leak. It's even worse if there is no air-barrier in the joist bays to keep air leakage into that joist bay from passing into/above the basement ceiling (or under the floor if it's a second floor.)

If it's a basement, is the foundation insulated?

If it's a second floor, an air-barrier where the joist bay passes over the supporting wall is necessary, after which you can safely dense-pack the mini-cavity with cellulose or high-density batts:

TE322_cantilever8_CV_PNNL_04-20-12.jpg
 

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Thanks for the reply Dana. This is a walkout basement. The 12 inch cantilever is over the walkout. It used to just have fiberglass insulation just balled up and shoved in the spaces between the floor joists. I removed all the fiberglass and was wondering if I just need to seal any gaps in the cantilever spaces with caulk since there is foamboard insulation on the outside under the vinyl siding. The walls of the basement are insulated with 2 inch XPS foamboard. There is no insulation up in the floor joists between the basement and the first floor above.
 

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Thanks for the reply Dana. This is a walkout basement. The 12 inch cantilever is over the walkout. It used to just have fiberglass insulation just balled up and shoved in the spaces between the floor joists. I removed all the fiberglass and was wondering if I just need to seal any gaps in the cantilever spaces with caulk since there is foamboard insulation on the outside under the vinyl siding. The walls of the basement are insulated with 2 inch XPS foamboard. There is no insulation up in the floor joists between the basement and the first floor above.

You need to seal any gaps in the subfloor, and seal whatever air-dam there is between the mini-cavity and the joist bay as it transitions into the basement to both the joists and the subfloor, making it air-tight between that mini-cavity and the rest of the house.

Then insulate that foot-wide section of floor with the insulation in full contact with the subfloor. Dense-packed cellulose (3lbs/cubic foot min) or dense-packed fiberglass (1.8lbs min) would be ideal, but short of that, rock wool batts or "cathedral ceiling" batts carefully trimmed & fitted will work, as long as you caulk or can-foam every corner & seam where wood meets wood (including plywood/OSB.)

In your climate (US zone 5) you don't need or want a vapor barrier of any type in the cantilever, but if you DO have a vapor barrier it needs to be at the subfloor or at the air-barrier in the joist bay where it crosses the supporting wall.

80476d1390982868-another-cantilever-insulation-question-slide2.jpg


This ^diagram^ shows polystyrene (XPS or EPS) being used as both the air dam, and an interior-side vapor retarder under the floor in a cantilevered overhang. Air sealing that air dam and air sealing the joists to the subflooring is critical to getting the performance out of the insulation if you are using fiber insulation. Also critical is filling the bay completely with a perfectly snug fit at the subfloor-joist and subfloor-air dam transistion. To achieve that with batts requires careful trimming, then tucking/compressing the edges in to fill any void/channels in those corner transitions, then tugging out lightly so that it's still a compression fit for the board you put on the bottom.

If the joists are 2x10s compressing a low-density unfaced R38s (probably the cheapest option) will fit snugly enough, and deliver pretty good R32-ish performance:

60610d1354245933-compressed-insulation-r-values-compressed-fiberglass.jpg


Alternatively, stacking an R23 +R15 rock wool batt (Roxul is sold by both of the big box store chains now) would deliver a true R38 center-cavity performance.

If they are 2 x 12s it's probably worth cutting & cobbling 1- 2" EPS or polyiso in there, gluing and sealing it to the sub-floor for a more compressive-than-usual fit on the R39s. Only use XPS if you have some scrap remainders lying around. The blowing agents used for XPS aren't very environmentally friendly (for either you or the planet), and cutting up new stock to scrap the rest would be a waste.

Alternately, a double-layer of R23 rock wool , would give you R46 performance in a 2x12 cavity, but at a higher cost.
 

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Thanks again Dana. Does the foam insulation on the outside of the rim/Band board make any differance? I thought I saw on the net (Building science site or somewhere) that if you had foam on the outside you just had to caulk. Has that way of sealing been dismissed? If so, then if I do dense pack cellulose, do I still use EPS/XPS foam in those areas in the diagram or just seal gaps and use 2x10 wood blocking to contain the cellulose? Also I have a 8 inch round HVAC in one joist bay and a Dryer exhaust duct in another. Can the wood or foam be put around and enclose these ducts or will that be a fire hazard and against code? Is there people out there that specialize this type of work? I think this may getting over my head. If so what would they be called? You also said XPS is not good for health. Is that when being made or even after cured?
 

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Thanks again Dana. Does the foam insulation on the outside of the rim/Band board make any differance? I thought I saw on the net (Building science site or somewhere) that if you had foam on the outside you just had to caulk. Has that way of sealing been dismissed? If so, then if I do dense pack cellulose, do I still use EPS/XPS foam in those areas in the diagram or just seal gaps and use 2x10 wood blocking to contain the cellulose? Also I have a 8 inch round HVAC in one joist bay and a Dryer exhaust duct in another. Can the wood or foam be put around and enclose these ducts or will that be a fire hazard and against code? Is there people out there that specialize this type of work? I think this may getting over my head. If so what would they be called? You also said XPS is not good for health. Is that when being made or even after cured?

The foam between the band-joist and siding can stay- it keeps the band joist warmer, and less likely to take on moisture, but slows the rate of drying toward the exterior (into the air behind the vinyl siding.) Air-sealing is always important, with or without exterior foam, as moving air can both bypass the insulation (undercutting it's effectiveness) AND move a lot of moisture to potentially susceptible materials. In this instance the drying path for the band-joist is inward, into the cantilevered cavity, and the drying path for the cavity is downward, not outward. The presence of the 2" XPS does not create a potential moisture trap in this situation, but it could in some others, since it's vapor permeance @ 2" is well under 1 perm. (EPS is about 2-3x as vapor permeable as XPS which in some stackups is a good thing, but would not matter in this particular assembly.)

Wrap the dryer duct in R15 rock wool (tied in place with steel wire) before insulating the rest. The HVAC duct is fine to be in contact with the insulation, but air-seal every seam & joint with duct mastic. Air-handler driven air-leakage can pump a lot more moisture into that cavity than random infiltration or vapor diffusion paths. The high end of the operating temperature range for polystyrene is about 150F, which is higher the temperature you would see in HVAC ducting, but would not be suitable for dryer ducts (especially gas-dryers.) Cellulose is good to about 160F for long term contact without degradation, but rock wool is good to 2000F or more. (Rock wool is mostly spun slag from iron making.)

The blowing agents in XPS take about 30-50 years to leak out. While the HFC134a (that comprised more than half the total blowing agent XPS) isn't super-toxic to humans, it's a powerful greenhouse gas (~1400x CO2). EPS is the same plastic as XPS (polystyrene), but is blown with pentane (about 7x CO2, and flammable), but it has outgassed nearly completely by the time it hits the distributor's yard. It has a half-life measured days, not years or decades. The fire retardents used in either are a longer term issue, as is the toxicity of the smoke in the event of a fire.
 

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Geez.. Thanks for the info Dana. I have about 50 feet of wall covered by XPS foamboards. Is it recommended to remove XPS foamboard and replace with something else. How toxic is the XPS to the people living in the house?
 

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Geez.. Thanks for the info Dana. I have about 50 feet of wall covered by XPS foamboards. Is it recommended to remove XPS foamboard and replace with something else. How toxic is the XPS to the people living in the house?

XPS isn't super-toxic when it's not actively on fire. I don't personally feel very threatened by the outgassing issues if it's in a reasonably-ventilated house. YMMV. (There are usually worse things to worry about in most houses.)

If the XPS is visible, exposed to the interior space it's a code-violation on fire-safety grounds. At a minimum it needs a rated thermal barrier against ignition between the basement and the foam. This can be as little as 1/2" sheet rock/OSB, but it's long-term cost-effective in your climate zone to trap the XPS to the wall with a non-structural studwall with UNFACED R13-R15 batts, and hanging the gypsum on the studwall. For thermal & moisture transfer reasons seal the seams of the foam with duct mastic, and caulk the foam to the studs & top/bottom plates in each stud bay. As an added measure, caulk the gypsum to the studs (at least on the full-sheet edges, if not every stud) and you'll have a pretty good air-barriers for keeping the outgassing local inside the wall, not in the living area.

If the slab isn't insulated, putting an inch of EPS under the bottom plate of the studwall as a thermal & capillary break is recommended to keep the moisture content of the bottom plate under control.

Insulating and sealing the foundation sill & band joist should happen at the same time. This diagram doesn't show a cantilevered section, but the insulation at the band-joist/foundation sill should come in at LEAST as far as your wall-foam, and over the top plate of the stud is even better. You can use EPS sealed with can-foam as an air-barrier to the cantilevered cavity if it doesn't already exist

A%20Complete%20Water%20Management%20Strategy%20for%20a%20Below-Grade%20Wall.jpg



If the XPS is on the interior face of an insulated studwall in a walk-out section it's OK to just long-screw some wallboard to it (after sealing the seams) as a thermal barrier. The screws would need to penetrate the studs by at least 3/4", so select screws that are really long enough.
 

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Thanks Dana. The Foam actually has 1/2" OSB laminated to the front and then !/4" paneling over that. The seams and top are taped with Siga Rissan tape. Could I put the drywall over the Rissan tape or L over the whole top with 3/4 " boards for the thermal barrier at the top?
 

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Thanks Dana. The Foam actually has 1/2" OSB laminated to the front and then !/4" paneling over that. The seams and top are taped with Siga Rissan tape. Could I put the drywall over the Rissan tape or L over the whole top with 3/4 " boards for the thermal barrier at the top?

The half-inch OSB meets code as a thermal barrier on it's own- no need to add gypsum. That was predicated on "If the XPS is visible, exposed to the interior space..." .
 

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How about the 2 inches of exposed foam at the tops. Is this a big offgassing area and fire hazard. Right now It looks like the picture below, but is totally covered with Siga Rissan Green sealing tape. . I have some 3 inch wide EDPM sill plate gaskets from Conservation Technology that I can put on the top of the wall and go over it with 3/4 boards or is that overkill since ther is already the Siga Rissan sealing tape? I just wanted to use something between the wood boards and the top of the concrete block foundation. As it is now, the wood would be sitting on the Siga Rissan tape. Do you know if the conservation technology EDPM gaskets have any offgassing issues or toxicity?
 

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Dana

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The offgassing rates are too low to really be a human health problem unless the house is ultra-tight (like PassiveHouse tight) and has no mechanical ventilation.

Small exposures of foam without a thermal barrier are only a fire hazard if there is an ignition source nearby. If you need 8-10' flames to reach the patch of exposed foam it's safe to say the place is already engulfed, eh?

I don't know of offgassing issues with EPDM, but if you have the product name I'm sure there is an MSDS sheet available online to say what (if anything) might leak out over time.
 

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One more question. I have sealed the 2 inch wide tops of the panels with Siga Rissan airsealing tape. Now if I put 3/4 wood boards over that Siga Rissan as the thermal barrier should I seal over those boards again with more Siga Rissan or would that trap moisture between the two layers of Siga Rissan.
 

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You are overthinking this. There is little point to air-sealing the thermal barrier to an already air-sealed foam layer when the thermal barrier is smack up against it, but it would be unlikely to accumulate moisture in any event, since it's all at the same temperature as the room.

I couldn't find a vapor permeance spec for Siga Rissan tape in a quick web search, if you're worried but still want to air seal it, used duct mastic.
 

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Hello I normally would not jump into a thread but this is VERY similar to something I have going on. Our house is a 1996 colonial in Michigan, and Ive been agonizing on how to insulate our cantilever chambers for 3 years now. I began by adding a new GAF HD roof with ridge vent, added up to R50-60 in our attic, blow in. I then Had 3" closed cell sprayed in my (then unfinished) basement rims and the bonus room knee walls (was ice cold and little to no insulation) all big improvements. New Payne 96% 2 stage furnace 18 seer AC. Then when finishing the walkout basement I put up 2" pink xps on the concrete walls and r13 fiberglass in the studded wall then 1/2" drywall nice its balanced all year long with supply vents near closed except mech room its still bare concrete so a bit chilly/hotter their. Then I put in new Sunrise Windows this spring another huge difference.

Now the last major things are to pull drywall in garage and spray foam under the bonus room floor. And the cantilever, It is essentially made of a 1x6 rough sawn nailed to OSB on the outside and rough sawn 5/16"? (a bit thinner then 1/4 plywood on the under side) with maybe R 19 fiberglass layed in haphazardly on top. I know this because I had to repair a corner of the house where a downspout had disconnected and rotted a large area before we bought the house and bees/ladybugs were getting in our master. So I saw the area. No house wrap, no foam really nothing...... ALSO our second floor HVAC supply runs go up the 1x4 exterior wall and do a 45 degree INTO the cantilever space...... So lots of heat loss...... Only thing I can do is fill the cantilever space to separate it from the cold air. I can tell this is having a effect on the temps upstairs because normally heat rises but as I walk up the stairs when my face and neck "break the plane" of where the rim space cuts through the house the upstairs immediately becomes MUCH colder!

Now how to put 3-4" of foam in their? One idea from my spray foam company is to roll the carpet back in each room along the front and rear of the house. Trench cut the floor OSB pull it up spray down fill the space on the outside wood, replace floor and stretch carpet. Another I came up with is to pull down the 5/16" underside wood and spray upwards and seal to the inside walls/ underside of vents. Another more difficult would be trench cut the drywall on the first floor looking up and try and spray upwards dont think this is a good idea....... Anyone have a different idea? Tips?
 

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For most applications like that packing it full of cellulose is much more forgiving than spray foam (especially closed cell foam), and nearly as air-tight. You can fill the cantilever drilling from the either the bottom (exterior) or the top (the finish floor side), but usually the bottom side is easiest and does less cosmetic damage.

If there isn't already an air-barrier in the joist cavity where it crosses the supporting wall, a common solution is to drill a 2-2.5" hole from the underside near the wall, and insert a fabric bag (typical animal-feed type bag) and fill it full of cellulose to block the bay, then drill further out to fill the bulk of the cantilever.

These approaches work equally well with fiberglass blowing wool if they dense-pack it to 1.8lbs per cubic foot or higher density. Cellulose is fairly air-retardent at any density, and while dense-packing it is good, isn't as critical as with fiberglass.

Same for the bonus room floor- you can just drill & fill with cellulose- there is no need to pull the drywall. If you drill the blowing holes with a hole saw and save the plugs (most cellulose contractors would do it that way) it makes the ceiling repair a lot cleaner & easier too. Unless the band joist has been air-sealed it's a good idea to completely fill the joist bays, since an air-gap above the top is a potential thermal bypass channel for any air leaks. When blowing a cavity completely full the air leaks in the band joist get mostly-plugged with the insulation- it'll take the infiltration down by over 90% even without dense-packing, but if blown at 3lbs/cubic foot (a minimal dense-packed density for that material) it's more like 99%.

This is DIY-able with box-store's rental blower if you're handy. The typical rental setup is a 1-stage blower, and does not include dense-packing hoses. It may take a few more holes to hit 3lbs density without snaking the blower hose in there, but you can get there, even with a single-stage blower. (The pros use 2-stage blowers and have narrower hoses that attach to the main hose to be able to snake it into the end of the cavity to get higher and more consistent density.)
 

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Ok thanks before reading your response I got frisky today and the smaller front cantilever in our master (12' long or so) I pulled down the the soffit it was one layer of 5/16 ply and 1/2 of OSB and maybe R19 laid in the space. Their is no backer wood block inside the house, I can see from one end of the house to the other under the floor. So I used 2" XPS I had left over from the basement and used my table saw to cut and friction fit pieces then caulk and can foam them into place. using your pictures as a guide. A piece glued under the floor and a piece parellel with the first floor outer sheathing in a L shape. The first duct I found had alot of leaks so I used mastic to seal it made a box of 2" on all 4 sides sealing in fiberglass insulation. And filled with unfaced R30 attic blanket thats nice and tight but not compressed. Then some 1/2 OSB I had left also from the basement to cover untill I get more ply to cover that and paint. The rear of the house I am just going to hire.... its maybe 35' long they can just make a backer out of cardboard and spray 2-3" and done...... and encapsulate all HVAC supplys. I still would think 2" foam on the bonus room floor would be better no?
 
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