Basement Remodel Insulation

[Dana]

If you're building in a "vapor diffusion vent" with fiber insulation only at the bottom of the wall as a drying path for the concrete, it doesn't matter at all what type of facer there is on the polyiso. If you're doing it without the diffusion path, fiber faced would allow at least some drying through the foam, but the shiny stuff none at all. It's dead-easy to air-seal foil facers (with foil tape), but most paper or fiberglass facers still work OK with housewrap tape.

The diffusion vent area fiber insulation should be full depth, from the foundation wall to wallboard or kickboards, no gaps. Yes, it will wick some moisture, but unless there's active bulk water leaks it probably won't be enough to matter. You'll find out- perfection is unachievable even in new construction. But it's better to risk mold issues at the bottom of the wallboard than in the bottom plates of your structural walls. In Canada they often leave it as a complete gap at the bottom- no fiber insulation, just air, but apparently that's not allowed by your local code. Filling any gaps between the polyiso and drywall with fiber insulation will only help, and it will also improve thermal performance.

With a radiant ceiling you'll need at least the code-minimum R on the band joist, and probably more, since you'll have a "hot spot" at the end of each joist bay where the tubing runs, well above the average indoor temp.

 

[Rossn]

Many thanks, guru Dana! Very good information.

For the radiant ceiling, I’m planning filling the joists with insulation… maybe cellulose, cotton, or spider. I will use something similar to warmboard, but because it is ceiling don't need to cover the full ceiling.

I’m finally to the critical point where I have to get the rigid materials and get them installed… I’ve already started air sealing the sill plate to the concrete.

I have two primary concerns relating to moisture, as I’m sure you are aware. Mold in the wall, and rot in the sill plate. However, I’m less concerned about rot in the sill plate due to relative humidity, and more due to any moisture that could come into the below grade stem wall and get trapped where it contacts the sill plate (no gasket, and not treated).

My gut says I should be as conservative as possible on both these topics, and that I should find 2” polyiso that is fiberglass faced. I saw a 2013 post you had on GBA, referencing <.1 perm for foil facers, < .5 perms for paper for asphalt paper facers, and .5-1 perm for fiberglass facers, and I assume this still holds true. I’d prefer reclaimed foam, and the only I can find looks like the below picture, which I assume is an asphalt paper (not fiberglass) facer… let me know if that’s accurate. So then I have to look to use new, which is not great from an environmental perspective and from a shrinkage perspective, though cost only seems to be about 30% difference (on 20 sheets, that isn’t dramatic). I’m all ears if you have any additional thoughts.

My key questions that remain are:

1. Do I have to bring the polyiso up above the stem wall, to the top of the two bottom plates? I’m assuming yes to ensure I don’t get condensation due to the temp gradient between stem wall and interior air, but this would also reduce drying… another reason for the fiberglass facer. Any additional thoughts here would be good.

2. Originally we had talked about 2” of CC foam sprayed in the stud bays for air sealing and preventing condensation on the surface. Then we learned the aluminum faced backing material had some permeability. Is with or without the CC foam a safer alternative from a mold perspective? I do think it may be hard to air seal each cavity with caulk and may not bond well to both the foil and the wood. Also, I’m not sure if both options will hold up equally well to the installation pressures for spider or cellulose. I will say that board is pretty rigid. Any input here would be valuable.

3. Do you have material recommendations for 50-year type materials, preferably green, for: air sealing/caulking wood-wood interfaces, air sealing/caulking wood/foil sheathing interfaces, wrapping the edges of the 2” polyiso (to prevent absorption), sealing the seams of the polyiso-polyiso (if new, shrinkage could be an issue), gluing or attaching the polyiso to the concrete stem wall? I’ve heard that for air sealing, sometimes a thin strip of foam is sprayed, but not sure in the exterior wall cavity @ stud/sheathing if that would be an additional surface to condensate.

4. From a spider versus cellulose perspective, do you think one presents less risk of mold under this wall’s normal conditions, would either be better in a bulk flood event, and is either superior from an acoustic baffling perspective? I'm generally fine with either from an environmental and health perspective.

5. Any reason I should consider applying a moisture or radon sealant to the area of the stem wall behind the polyiso? My guess is that would defeat the purpose of using more permeable polyiso and not allow the upper section of the stem wall to vent.

 

[Sponsor]

 

[Rossn]

almost forgot the photo of the reclaimed polyiso... will pm/mail it, as I don't seem to be able to embed it without a URL and have to head out to work, so I can't fiddle with it

 

[Dana]

The vapor permeance of polyiso itself means that even with asphalted paper facers (which are variable-permeance) most 2" polyiso will be under 2 perms.

It's not critical to bring the polyiso up flush with the top side of the bottom plate boards, but it doesn't hurt. With fiber cavity fill there is a lot more drying capacity of the bottom plate into the 3.5" in contact with the fiber than there is on the 1.5" edges.

If you install ccSPF in the cavities it has to be at a sufficient ratio for dew point control at the foam/fiber interface. Whatever the permeance of the foil faced hard board is due to the patterned perforations, it will drop dramatically when those perforations are plugged with closed cell foam. Closed cell foam is a reliable long term air seal at 2" or more.

Polyurethane caulk is your best bet for long term air sealing of concrete to wood, or other dissimilar materials. It remains somewhat flexible for decades, and adheres well to just about everything.

Cellulose will keep the structural wood drier than Spider, but will be much worse in a bulk-moisture incursion/flood situation, since it will wick & retain moisture at least a few feet above the high tide mark, maybe even to the top of the cavity if the flood waters linger. With fiberglass the wicking above the high water mark would be measured in inches.

Radon mitigation is far more about air tightness than vapor tightness. If the slab is sealed to the foundation wall and any cracks in the wall are sealed with polyurethane caulk, and the foam is air sealed, you've already gone further than most radon abatement contractor would on a retrofit. (They're more likely to just fix the big air leaks and put in a bigger fan than get into the finer details of air sealing.)

 

[Rossn]

On the radon, these guys seem to claim that up to 80% of radon can diffuse through the concrete - not an air exchange issue. I'll have to poke around to see the legitimacy versus marketing on that, as that's not intuitive to me: https://www.radonseal.com/radon-indoor.htm

I assume with your comment about polyiso not needing to be flush with the top of the bottom plates, you've taken into consideration temperature and likelihood of dew point condensation, based on a need for a well detailed smart air barrier on the interior. It sounds like you're a fan of the well detailed interior air barrier more than the 2" of ccSPF within the stud bays to maintain maximum permeability, and obvious environmental considerations.

Tonight I was reviewing some different polyiso offerings, and learned that Atlas Roofing has a roof polyiso with permeability around 4.0 perms. http://roof.atlasrwi.com/products/acfoam-polyiso-roof-insulation/acfoam-iii/

Given that due to the minor efflorescence I was considering excavating 4' deep around 150' perimeter of the home, to apply a water impermeable coating or material, to reduce the risk of mold in the wall, and the high associated costs, destructive nature, and headaches...

Do you think that by using a more highly permeable rigid foam layer, it would be reasonable to:
1) run the polyiso from sill plate all the way to the ground (might not give as much flood protection) instead of having a gap and roxul or similar at the base
2) forego the excavation and stem wall waterproofing, provided I still do some exterior work on drainage, underground 'roof', etc?

To me, if the above seems reasonable, it would well justify using new foam that is engineered for the purpose. This actually has me wondering why I wouldn't consider EPS, but I'm sure there is good reason for that.

 

[Dana]

At 4 perms the higher perm roofing foam is about as vapor permeable as standard latex paint. That's probably going to be OK, but any higher than that the humidity in the wallboard could become pretty high if the concrete was truly damp.

Even a skinny inch of Type-II EPS is more vapor tight, typically specified at 3 perms MAX. At 2" it's 1.5 perms max, at 3" it's 1 perm max. It's fine to use in this application, but at 3" won't provide dramatically more drying capacity than the typical 1 perm max fiber faced polyiso.

Digging down and addressing the fundamental drainage issues in a serious way is good insurance against excessive moisture finding it's way to the foundation sill, and will also improve the radon situation. (The "... up to 80%..." diffusion claims by a company selling concrete sealer as radon abatement needs the supporting data. Sure, if the slab and foundation walls were truly air tight you could argue 100% of the radon entering the house would be via diffusion, so? ) Just as with water vapor, transport of radium & radon in the soil gases via direct air leakage is huge. With a slab depressurizaton fan in place direct air leakage also short-circuits the air path for the fan, pulling air from indoors under the slab rather than purging soil gases, somewhat defeating the purpose. If the air leakage in the foundation & slab is big enough the depressurization fan measurably depressurized the house, driving outdoor air infiltration. (The size of that leak is easily measured during blower door testing, by turning the radon fan off, then on, measuring the difference with the blower door.)

The soon to be added clean stone drainage on the exterior of the foundation wall is extremely air permeable, and will allow outdoor air to flow down the outside of the foundation to under the slab, diluting and purging the soil gases, but that works a lot better if the foundation and slab are reasonably air tight.

 

[Rossn]

I see where you are headed. There is some history to share.

For 50 years 1/3 has had drywall glued to the stem wall, and another 1/3, drywall furred our 1.5” from the drywall. In two rooms, there has been poly sheeting behind the drywall for 13 years.

In all walls, there is no evidence of any bulk mold, just the efflorescence and some water staining on the lumber.

Based on that, would you agree the more permeable the polyiso, the better, as it would represent the original wall. I’m thinking so, but maybe the smart membrane and air sealing changes things.

 

[Dana]

It's fine to go with 4 perm polyiso, just don't go with unfaced polyiso, which is sometimes quite a bit more permeable than that.

 

[Rossn]

Thanks, Dana -- you've been very patient and helpful with the endless questions. I'm just trying to make sure I don't end up with a long term issue.

At what range of perms on the polyiso, do you think I can get away without the bottom 'drying area', and just cover the whole lower wall e.g. 1-4 perms, etc?

Safe to assume that with the higher perm foam I don't have a risk of mold forming within the foam due to the temperature gradient? I'm assuming material resistance to mold prevents that as a concern.

 

[Dana]

You can probably get away with foil-faced <0.1 perm goods after you've fixed the drainage. Before that it's pretty much a crap shoot- we don't really have a clue as to how much moisture it needs to move. Best WAG, 1-2 perms (but it's just a WAG).

Do you have a 2-prong moisture content meter?

 

[Rossn]

I don’t have one... do you think I should get one, and any particular recommendations?

I’m coming from the perspective that if it has been this way 50 years without mold, then there is probably a way to make it work without changes to the exterior and full excavation, but would still do surface and near surface grading and drainage work. When I dug a sump pit last week, earth was damp, and high % sand.

All this makes me wish I had built new

 

[Rossn]

I have a framer coming next week, and am trying to ensure I can do what was discussed. I'm unsure if I can really airseal the wall to do the approach. My new interior studwall top plate will have a 0.5"-1.5" gap between the two parallel edges of it and the existing top plate. Assuming I spray closed cell on the rim and in this area, will that adequately air seal the top, such that I don't get any air exchange coming from the ceiling joists?

Regarding the new stud wall bottom plate, is it fair to use a good caulk (I was using Titebond Radon Seal, w/ 25% joint movement capability) at the foundation or is a tape a better solution? I had considered an EDPM gasket, but am having trouble locating those and based on existing lumber on foundation that has not yet rotted (and doesn't appear to be treated), I'm thinking that is not an issue/requirement. For the prior installed partition walls, the framer applied loctite adhesive liberally to the bottom plate to create some layer of separation, and maybe that's sufficient in this case (with the additional caulking).

Lastly, does Intello require furring strips be installed to sandwich it between the stud and furring strip, or are there other surface-only techniques that work?

 

[Dana]

My mental picture of the top plate separation issues are incomplete, but you can't leave half-inch gaps anywhere with fiber insulation. Filling it in with can-foam or fiber insulation, or just packing it full of fiber insulation and taping it well with housewrap tape can work.

Caulking the bottom plate with radon-sealer caulk will probably work better than most tapes.

Intello or MemBrain is usually stapled to the framing, then taped over the staples. It doesn't need furring. Ideally seams between sheets should occur where they are supported by framing, since tape can't be counted on to remain air tight forever if it's in a location that can flex a lot.

 

[Rossn]

Ok, great. For the visual... imagine a 9" thick wall with a gap between the top plate on the back wall and the front wall.

The moisture meter you mention, would probably be a really good thing to measure concrete, drywall, and wood moisture levels. Any reason you say two prong version, versus a pinless version like this (ideally cheaper): http://www.lignomatusa.com/meters/ligno-scanner-sdm-moisture-bamboo/

 

[Dana]

I've never used a pinless type, so I can't really comment. It's possible to buy fully functional 2-prong moisture meters from box stores for under $30 (don't know how accurate those are, or how long they would last, but it might not really matter much), but they have others too. Most box stores don't have them on the shelves, but they can always be ordered online.

So you're describing basically a double studwall with the studwalls very close to one another. Tapes appropriate for sealing sheathing (or even housewrap tape) can work. Fill the gap with shreds of batt packed finger-tight, it'll be fine.

 

[Rossn]

I had looked into the cheap meters -- one moisture meter review site showed again and again that the cheaper ones they reviewed were consistently way off the mark.

Figured they could just spray foam on top of the void when they spray the rim and that should air seal it.

Last night, after a few days of heavy rains, I had water entering over the top of the mud sill plate... that cemented the fact that in addition to the drainage and exterior work, I really need to make sure I don't go with a paper facer polyiso... just incase the drainage experiences a failure.

The only reclaimed polyiso I can find appears to be paper faced, which means the next best option is some discounted new discounted polyiso that is glass fiber faced (Firestone GL 95+ 2.6" $19/sheet), unless you have alternate recommendations. I guess reclaimed xps is an option, but I'm not finding any of that.

 

[Dana]

With visible bulk water moisture issues that high on the foundation wall you really need to do the exterior drainage work BEFORE insulating the foundation!

Is it possible that you have rain moisture getting in behind the brick veneer?

 

[Rossn]

This is the first time in two years I've seen this... two days of pretty solid rains. There was a clogged gutter, which I think contributed to this; however, this is an area where the soil level may 0-2" above the stem wall height (that the veneer brick sits on) - so no doubt it is an issue with moisture behind the veneer, except at the bottom. Additionally, the ground has flat to slight negative slope to the house for at least 15' from the house.

The good thing is that March-May is Colorado's wet season; in about 2-4 weeks things should fully dry up. In getting ready to frame the exterior walls (now delayed by the framer), I'm thinking at the moment of placing the polyiso sheets (pre-cut and pre-taped around the edges) loose in the wall, and later coming back to glue or screw them to the wall. Partially for this moisture issue, and partially to allow stem wall inspection after excavation and re-compaction of soil -- which I'm not sure if could damage the wall.

 

[Rossn]

Ok, polyiso cut. Sealed edges with foil tape and per another thread you commented on, duct Mastic. Ready for mounting.

What is your opinion on the ideal way to attach the glass fiber faced polyiso to the stem wall? In places, the walls are not always flat, though I knocked off concrete bumps/seams.

I’m assuming we don’t have to air seal the top and bottom of each piece, just those where there is a visible gap?

 

[Dana]

Walnut sized blobs of foam board construction adhesive on a 2' grid can work, if the foam is going to be trapped to the foundation with a studwall. Drilling into the foundatation and using cap-screws works too.

Air sealing the bottom & top is important from a moisture transfer point of view- those less-than obvious gaps are bigger than you think, and the convective drive can move a lot of air through that gap in winter. A continuous bead of polyurethane caulk or foam board construction adhesive to keep moist indoor air from finding your foundation sill in winter is important, even if it's just the top edge.