Cantilever insulation/sealing?

[Dana]

The Mohawk smart cushion states it isn’t breathable(permeable?) but this one I linked below says it’s breathable:

Https://hw.menardc.com/main/items/media/HEALT001/Prod_Tech_Spec/HCGoldSpec.pdf

I’m just cautious because under my living room there seem to be considerable air leaks through the closed cell insulation. It’ll be very hard to close now as I don’t have the funds to take care of it for a year or so. Do you think the moisture/air flow penetration would cause an issue with this pad?

*Edit- Thinking back on your 1sq inch graphic and how much moisture comes in and out, knowing I have air gaps under my living room from a crappy open cell install, does this all really matter so much? Should I just go for the ‘breathable moisture barrier’ carpet pad and be more comfortable?

Maybe in the future I am open the garage ceiling, remove the box open cell job and spray in closed cell.

Is there any reason you can't or don't want to use a vapor open fiber-type carpet underlayment, or a rubberized mesh type?

Describe the leaks through the closed cell foam. At 4" when it's fully adhered it's usually extremely air tight, unless it has been applied a bit unevenly with folds & crevices.

 

[Bobby E]

Is there any reason you can't or don't want to use a vapor open fiber-type carpet underlayment, or a rubberized mesh type?

Describe the leaks through the closed cell foam. At 4" when it's fully adhered it's usually extremely air tight, unless it has been applied a bit unevenly with folds & crevices.

I just don’t know anything about those types of underlayments. I’d really like something antimicrobial (I’m worried about mold from subfloor and food/drink from above penetrating the padding) but everything that has it also has a moisture barrier built into it. Do you think the pad I linked would work ok even if there’s air coming in from the garage?

It’s not the closed cell cantilever area that has air leaks(well there is one spot, but I’m going to spray canned cc around where the HVAC duct enters the wall below, where the pro guys missed), it’s the open cell in the garage ceiling, which is adjacent to the dining room/cantilever and under the living room. It looks as if there are voids along the edges. I have a picture of the top of the open cell, not the sides where the air leakage is occurring.

 

[Sponsor]

 

[Dana]

If the subfloor is pretty much at the same temperature as the room it's not going to grow mold in the carpet underlayment. The entrained air in the carpet underlayment will have about the same relative humidity as the room air, and until/unless it's humid enough in the room to grow mold on the furniture, it won't be growing under the carpet. On the now reasonably warm subfloor any spills will still dry rapidly toward the room, roughly as quickly as if the spill would on a hardwood floor.

 

[Bobby E]

If the subfloor is pretty much at the same temperature as the room it's not going to grow mold in the carpet underlayment. The entrained air in the carpet underlayment will have about the same relative humidity as the room air, and until/unless it's humid enough in the room to grow mold on the furniture, it won't be growing under the carpet. On the now reasonably warm subfloor any spills will still dry rapidly toward the room, roughly as quickly as if the spill would on a hardwood floor.

Thanks again for the response Dana you’re awesome. I have to make my decision tomorrow and I think I’ll go with the memory foam with breathable moisture barrier. I was just nervous because some of the subfloor has gaps open to cantilever below (runs perpendicular to joists and no blocking under one edge) but I’ll work some flexible floor patch into them. This padding is also antimicrobial so that should prevent mold growth too. I just have to make sure to tape all the seams and staples with moisture resistant tape. I don’t have to worry about mold and pizza grease on the plywood right? With the breathable moisture barrier carpet pad? I’m so nervous about all these things as a first time owner!

 

[Dana]

Don't sweat the pepperoni & beer stains on the subfloor. Most of it will be captured by the carpet underlayment, and it'll all dry to the interior before it becomes mold food, now that you have a fully insulated floor.

 

[Djford88]

Dana,

You've been a wealth of information so I thank you for that. I am insulating my cantilever space in zone 4A this coming week. Subfloor is continuous plywood (not plank), and I have 2x10 joists and the exterior sheathing of the cantilever is painted ply as well. Previously filled with R10 fiberglass and air leaks abound. The walls are poorly insulated as well but that's a task for another day.

I plan using polyurethane caulk to air seal all seams and joints in the joist bay, and use blocks of polyiso to form a vertical air barrier with caulking around those corners as well and fill the bays up with Roxul. I believe this plan is consistent with your recommendations for zone 4.

My question is this: my local big box stores only carry 5.5 in and 3.5in thick Roxul. The joists are 2x10, so roughly 9.5inches. The Roxul adds up to 9 inches. Is a 1/2 gap okay in this scenario? Should I use 0.5inch rigid foam on the underside of the subfloor to make up the gap?

 

[Dana]

The nominal dimensions of a milled 2 x 10 is 9.25", not 9.5". A stacked pair of R23s is 11" at full loft, but it will have no problems compressing to 9.25", and the higher density will deliver about R4.3-R4.4 per inch (x 9.25" = R40-R41) rather than it's R4.2/inch (x 5.5" = R23) at full loft.

But as long as the cantilever bays are truly air tight it's not a disaster if the batts aren't compressed tightly to the subfloor above, so if it's cheaper to go with R23 + R15 (-R38) that's fine too. There really isn't much to gain by filling the extra 1/4"with cut'n'cobbled foam board.

Depending on the amount of area needed to cover and the amount of coverage per bag it may be cheaper to just go with bigger bag of R23s and double them up rather than have a bunch of R15 and R23 material left over.

 

[Djford88]

The nominal dimensions of a milled 2 x 10 is 9.25", not 9.5". A stacked pair of R23s is 11" at full loft, but it will have no problems compressing to 9.25", and the higher density will deliver about R4.3-R4.4 per inch (x 9.25" = R40-R41) rather than it's R4.2/inch (x 5.5" = R23) at full loft.

But as long as the cantilever bays are truly air tight it's not a disaster if the batts aren't compressed tightly to the subfloor above, so if it's cheaper to go with R23 + R15 (-R38) that's fine too. There really isn't much to gain by filling the extra 1/4"with cut'n'cobbled foam board.

Depending on the amount of area needed to cover and the amount of coverage per bag it may be cheaper to just go with bigger bag of R23s and double them up rather than have a bunch of R15 and R23 material left over.

Thanks for the speedy reply. Last question, I have seen you mention and linked articles that mention thermal bridging of the joist members where they contact the sheathing. Would you worry about this in Zone 4? I could take the sheathing off and place some rigid board but I do not want to make a moisture trap either!

Thanks

 

[Dana]

In most of zone 4 there won't be any "payback" in either comfort or heating/cooling energy costs from thermally breaking a 2x10 cantilevered joist. In bare feet when it gets down to -15F in you would be able to feel temperature striping at the joists since the joist is only R11-R12 whereas the insulation between them is R38 or more, but it's only part of the floor, not a pier foundation freely vented under the entire floor, and I expect negative double-digits is pretty rare in your neighborhood. There are probably better places to spend your time & money than the thermal break under the cantilever.

 

[Djford88]

Thank you for the reply and common sense approach.

 

[diydude55]

Seeking the sage wisdom of Dana on this thread.
I am located in Zone 8 (Greater Seattle area) with house built in ‘55.

I am performing an earthquake reinforcement of a shear wall on the west side of the house. This wall also has a cantilever above it from the main floor of the house running its entire length.

For the EQ retrofit I will be replacing the ship lap sheathing with 1/2” plywood. The joists openings will be reinforced with 2x10 blocking.

How much foam should I put against the subfloor above and joist blocking?

As I intend to redo the siding with a rain screen/radiant barrier, should I use foil faced foam facing the ground under the subfloor and out on the blocking?

Of note there is a black paper barrier between the subfloor and the hardwood floors on the main level above. So maybe no foil faced foam?

This wall builds a lot of UV heat during the summer, as it gets sun all day long.

I intend to fill the remaining cavity with Roxul (I love the stuff), then cover with leftover rip cuts of hardi panel.

 

[Dana]

Seeking the sage wisdom of Dana on this thread.
I am located in Zone 8 (Greater Seattle area) with house built in ‘55.

Plant hardiness zones have little relevance to construction issues. From a temperature & moisture perspective the IECC climate zones (used by the DOE and cited in building codes) are more useful. Seattle is in climate zone 4C- a temperate marine climate.

I am performing an earthquake reinforcement of a shear wall on the west side of the house. This wall also has a cantilever above it from the main floor of the house running its entire length.

For the EQ retrofit I will be replacing the ship lap sheathing with 1/2” plywood. The joists openings will be reinforced with 2x10 blocking.

Has the shear wall been analyzed by an engineer? Plywood will stiffen up the wall structure making it less susceptible to racking forces peeling the wall away, but increases uplift on the foundation sill, and more likely to walk off the foundation. It may need reinforcement to keep it on the foundation too.

How much foam should I put against the subfloor above and joist blocking?

Probably zero. In general it's better to insulate the exterior walls of the basement & band joist, not the subfloor. That is doubly true if there is a furnace/boiler/water heater in the basement.

As I intend to redo the siding with a rain screen/radiant barrier, should I use foil faced foam facing the ground under the subfloor and out on the blocking?

Radiant barriers are next to useless in your climate even in attics, and even more useless when applied to wall & floor assemblies.

Vapor barriers aren't really necessary in your climate, but if there IS going to be a very low permeance layer (such as a <0.05 perm foil facer) it's best installed on the conditioned space side of the assembly (the "warm in winter" side.)

Of note there is a black paper barrier between the subfloor and the hardwood floors on the main level above. So maybe no foil faced foam?

Tar paper has variable permeance, and becomes more vapor open when wet. When applied between a subfloor & finish floor it serves as a slip surface & air barrier.

This wall builds a lot of UV heat during the summer, as it gets sun all day long.

I intend to fill the remaining cavity with Roxul (I love the stuff), then cover with leftover rip cuts of hardi panel.

It's not the UV, but rather the full solar spectrum (primarily visible light + infra-red, with only a tiny bit of UV) that causes the cladding to warm up. With a rainscreen gap vented both top & bottom there will be some convection cooling of the Hardi, as well as ~R1 of insulating air films between the sheathing and siding. If you're concerned about the heat radiated across the gap, don't be. If you can't help but continue to be concerned, use a lighter finish color with a high solar reflective index (SRI). In the PNW light colors tend to show the algae & moss growth a bit more, and the difference in both peak and average siding temp won't be huge (but measurable). I personally wouldn't opt for a titanium white finish in any climate, but especially not in the foggy-dew western side of the Cascades.

(FWIW: When I was in high school I lived in ZIP 98109, but it's been awhile. I still have relatives all over the Puget Sound area that I haven't visited since the pandemic got rolling- I'm overdue for a trip.)

[edited to add]

Where solar heating of the wall is a concern rather than rock wool, CELLULOSE (blown or batts) is a better choice for it's favorable thermal diffusivity. At a given R-value at about 1/4 th diffusivity of rock wool cellulose will delay and lower the peak temperatures measured at the drywall layer. (Cellulose also protects the structural wood from moisture drives far better than rock wool.) Thermal diffusivity is a dynamic characteristic , a function a the material's thermal mass in combination with it's R-value as well as change of temperature over time.

The thermal diffusivity chart below comparing different insulating materials was put together by a German vendor of (even denser) rigid fiberboard insulation, but you can see that a given R value cellulose wins the low-diffusivity game handily against other fiber insulations, 4x better than rock wool, and ever 2-3x better than rigid foam board:

 

[diydude55]

Thanks Dana.

The wall treatment is following an engineered plan, and the embedded sill plate and blocking for the sheathing will get bolted to the foundation.
Once I have the 2x10 blocking in place I will only have access to the cantilever from outside, so on the cellulose recommendation I'll go with batts.

Attached is a crude computer drawing of what I'm working with and hopefully all the layers illustrated well enough.

What should I treat the rim joist with?

As a side note, this wall was originally insulated with tar paper wrapped batts of lambs wool insulation, then at some point a previous owner got a ‘port package’ and had spray foam blown in to all the bays compromising the original tar paper weather barrier over the sheathing. They never touched the cantilever, so now that I’m ripping it all open I’d like to renovate it properly.

 

[Dana]

Thanks Dana.

The wall treatment is following an engineered plan, and the embedded sill plate and blocking for the sheathing will get bolted to the foundation.
Once I have the 2x10 blocking in place I will only have access to the cantilever from outside, so on the cellulose recommendation I'll go with batts.

A reasonable plan. Be sure to make sure the batts are in full contact with the subfloor (or air leaks will rob the performance of the batts), and air-seal the blocking & rim joist to the joists and sill plate with polyurethane caulk (or a flexible low expansion can foam on gaps bigger than ~3/8" or so.)

Since the underside of the cantilever doesn't get direct sunlight the peak surface temps never exceed that of the outdoor air, and the thermal diffusivity benefits much much smaller than when applied to sun drenched walls or roofs/attics. The 2x10 joists are nominally 9.25", an R15 cellulose batt is only a slight compression fit into 3.5" (2x4 bays), and an R20 batt a slight compression fit into 5.5" (2x6 bays), which adds up to only 9.0", which may or may not be sufficient for a reliable compression fit into the 9.25" nominal 2x10 bay. In some ways it would be better to use rock wool or fiberglass designed for 2x10s, or double-up rock wool/fiberglass designed for 2x6 to guarantee the compression fit.

What should I treat the rim joist with?

The rim joist doesn't need anything special, other than to be properly air sealed to the subfloor and to whatever the bottom of the cantilever gets sheathed with. The subfloor is a fairly robust vapor retarder limiting vapor diffusion from the indoors into the cantilever cavity, and as long as the bottom sheathing of the cantilever is at least somewhat vapor-open. MDF or asphalted fiberboard are both VERY vapor open as well as moisture tolerant and would be a reasonable choice if available. CDX plywood painted with latex stain (not full bodied paint) would be next best.

As a side note, this wall was originally insulated with tar paper wrapped batts of lambs wool insulation, then at some point a previous owner got a ‘port package’ and had spray foam blown in to all the bays compromising the original tar paper weather barrier over the sheathing. They never touched the cantilever, so now that I’m ripping it all open I’d like to renovate it properly.

There aren't many rocket scientists wasting their careers on installing spray foam, eh?

If the foam job was pre 1980 it might have been a low-expansion injection foam pours- the stuff that outgasses urea formaldehyde for years and eventually shrinks leaving big cracks & gaps. Those would fall under the "industry wide ignorance" heading. It's at least possible to do polyurethane pours and polyurethane spray foam correctly, but never over batts. Injection foams have been reformulated with multiple polymers, less outgassing, and somewhat less shrinkage and can often be an OK solution in masonry cavities, but they are pretty much never a good idea in studwalls given the range of more suitable alternatives.