Basement insulation and finishing prep questions

[ajs317]

I am refreshing my basement, which was crudely finished at some point in the 60s or 70s. I have spent many hours reading the posts on this forum about basement insulation and related topics, and I’ve been wowed by and appreciative of the depth of information and the helpfulness of the contributors here. I have some specific questions around my situation that I’m hoping you all can help me with but would also like to get feedback on whether my plan is sound.

Background:
- House was built in 1953 and is located in the lower Hudson Valley of NY right on the border between zone 4A and 5A.
- The house is built on a hill with a walkout basement that is already divided by partition walls into a finished portion that is fully above grade on the three exterior walls (not counting the internal partition) and tapering to below grade in the unfinished portion.
- The finished portion contains vinyl asbestos tile on the floor that I do not want to remove. I was finished with drywall (with no insulation), but I am gutting it completely except for the floor.
- The basement is very dry—no liquid moisture issues. I did a calcium chloride test that came in well underneath the standard threshold for wood flooring. Cardboard boxes stored on either the tiled portion or the bare slab in the unfinished portion have no visible mold growth even after months. It does have a musty basement smell that I'm hoping to fix and typical basement humidity especially in summer.

Plan:
- Based on what I’ve read here, insulate the exterior CMU block walls with 2” rigid foam and then rock wool batts inside 2x4 stud cavities. Initially, I’m going to only do this on the above grade finished half but eventually will do the below grade unfinished half.
- In addition, I'd like to insulate the rim joist using cut-and-cobbled rigid foam.
- For flooring, I’m thinking of doing high quality engineered wood. I have some questions about the floor prep below.

Now for my questions:

1. Is there any danger whatsoever of the 2” of foam leading to sill plate rot (also considering the rim joist insulation)? There is no gasket or flashing between the plate and the masonry foundation. Based on what I’ve read, the above ground portion should be fine drying to the exterior, but what about below ground portions? I probably have only about 8-12” of exposed foundation on the fully below ground wall, maybe less in some spots.

2. Some of the stud walls need to be rebuilt. Would I be ok putting those on top of the asbestos tile and attaching them through to the concrete with a ramset? I would put a foam sill guard underneath. It seems to me that this would disturb the tile only minimally. Alternatively I could remove the tile but I’m not sure that would be any better given that the cutback would still remain underneath.

3. My understanding is that the stud walls should be placed up against the rigid foam. There are a couple of walls where this is not possible due to pipes running along the length of the foundation wall. Is it a huge problem if there is a gap of a couple of inches between the foam and a batt-filled stud wall?

4. I planned to paint the entire floor (on top of the vinyl) with BIN shellac-based primer, as the floor took on cat urine, and I’d like to seal in any odors. Does this product have any negative effect on the moisture properties of the floor?

5. What should I do about insulating the floor? Headroom is an issue as the basement floor to the floor joists above is right at 7 feet now, and I am already losing some space for the finished ceiling. Is there a minimum thickness of XPS that would still be beneficial? And can I then float an engineered wood floor right over that or would I need some kind of plywood on top of that? Would leaving the vinyl asbestos tile underneath cause any issues or affect the moisture/insulation profile at all?

 

[Dana]

With 2" of EPS (R8-ish) on the exterior of unfaced batts on the interior the CMU will always be above the wintertime indoor dew point air, and will not need an interior side vapor retarder to keep the batts dry enough. With no vapor retarders (other than latex paint on wallboard) any ground moisture wicking would be able to dry toward the interior if you use 2" of unfaced EPS rather than XPS (pink, blue, green board) or foil faced foam, and the risk to the foundation sill is low. Foil or plastic facers are true vapor barriers, (less than 0.1 perms), and at 2" XPS runs about 0.5-0.7 perms a powerful Class-II vapor retarder (that would meet the Canadian building code definition of "vapour barrier".) But Type-VIII (1.25lbs per cubic foot nominal density) or Type-II (1.5lbs) EPS would still be 1.5 perms or more, which is in Class-III vapor retardency range, just like latex paint.

Standard latex on wallboard is 3-5 perms, so the lower vapor permeance of the EPS relative to the paint guarantees that the fiber insulation won't become too damp from ground moisture, but still allows enough drying toward the interior to protect the foundation sill. With a foot of above-grade exposure on the foundation even the below grade section would be mostly drying would be toward the exterior, which is fine. But in the parts where there's only 8" of exposure on the exteriror you still have the additional drying path toward the interior trh the ~1.5 perm EPS. Concrete wicks moisture vertically fairly far, and even if the bottom of the CMU is pretty damp, EPS is quite moisture tolerant (they use it for dock floats and sub-slab insulation, after all). The only place you have to worry about the moisture content of the CMU is where it meets the foundation sill. I think you're fine under all circumstance as long as you use unfaced EPS.

Regarding rebuilding the non-structural studwalls (and any new studwalls), with an uninsulated slab the bottom of the stud plate can run colder than the summertime dew points and get moldy, a major driver of the "musty basement" smell. Installing an inch of EPS (faced or unfaced) under the bottom plates of the studwall is a sufficient thermal and capillary break than you wouldn't need to use pressure treated lumber there, and it won't stink even if you don't use mechanical dehumidification. A half inch of polyiso would work too, but if it ever floods the polyiso would wick up water, and it's R-value would drop by half, and take forever to dry.

Concealed vertical spaces behind walls need fireblocking at least at the top of the space to meet code. In some jurisdictions it would need one at mid-level too. The space would also be a potential thermal bypass, letting air pass freely, and the batts need an air barrier on that side too. With up to 2" of space between the foam and the stud edges, take some 3" sound abatement batts and stack them on edge forming a continuous layer of fiber compression-fitted behind the studs, then go ahead and insulate the studwall as planned. Even at the cold edge of zone 5 you don't need more than R7.5 for dew point control on 5.5" of fiber insulation. You're on the warm edge of zone 5 and you'll have R8 or better, so even if it's 3" of space you'll be fine.

As for insulating the floor, R3 is enough to keep subfloor from getting moldy in summer (even with a rug or a few cardboard boxes on top), when the indoor dew points are high. So 3/4" of either XPS or EPS would work, but EPS is a much greener product, since it's blown with hydrocarbons (usually a pentane variant), most of which leaves the foam at the factory and is recaptured (for local air pollution requirements), whereas XPS is blown with a combination of HFCs, the most predominant of which is HFC134a (automotive AC refrigerant), a powerful greenhouse gas ~1400x CO2. The HFCs give it a ~15% higher R value than EPS of similar density, but as it diffuses out over a few decades it's performance drops to that of EPS. Fully supported by concrete from below an with a subfloor distributing the weight above the compression ratings of the foam really don't matter. With any density of foam, but especially with cheap box store Type-I (1lbs density) EPS it's important to stagger the seams of the subfloor with those of the foam, with at least a foot of overlap. If the seams are aligned directly atop one another there can be compression at the edges of the foam sheets. It's better to use TapCons than ramsets for securing the subfloor to the slab, since you may need to back off the pressure of the fasteners (which can apply far more localized pressure than anything resting or walking on top of the floor) to keep the edges of the subflooring at the same level.

There is foamboard that would work with engineered wood directly on the foam- it needs a subfloor. You can usually get away with 5/8" t & g sheathing or even half-inch if the slab is really flat, rather than the full 3/4" subflooring. The thicker subflooring is designed for minimum flex when only supported by joist 16" 0.c., but with continuous foam supported by continuous concrete there is NO flex to speak of even with half-inch. The thickness of the subflooring used is primarily an issue of fastener retention for the flooring. If the slab is uneven there may be some trampoline-effect issues if you go as thin as half inch though, and with half-inch it's a good idea to use foam board construction adhesive to avoid "potato-chipping" curl causing separation at the subfloor edges due to normal seasonal humidity changes.

Don't sweat the vinyl asbestos tile on the floor- leave it in place. It's fairly vapor retardent, protecting the subfloor from ground moisture.

Seal any cracks in the floor with self-leveling polyurethane caulk, and seal any cracks in the CMU as well as the seam between the slab & CMU with a (non self leveling type) polyurethane caulk. This is both for bulk-water moisture protection, but also for gas tightness against soil gases, air, radon, etc.

 

[Sponsor]

 

[ajs317]

Thanks, Dana. This is a huge help! To solve the walls where there's a gap between the foam board and the stud wall, am I understanding you correctly that you suggest putting something like Roxul Safe & Sound running perpendicular to the studs to fill the gap between the studs and the foam board and then putting insulation batts in the stud cavities? Would this create any risk of having too much batting material between the interior air space and the CMU block, thus inhibiting the ability of the CMU to dry to the interior? Maybe this is not a concern where the walls are entirely above grade, which is the case for the walls where I'd have gaps.

Any thoughts on how leaving the asbestos tiles in place would affect the floor setup? I'd be nervous about drilling for tapcons. Can the foam and subfloor just be floated? Or attached with foam adhesive at most? Also, I've read in other threads that you've recommended 6 mil poly barrier in the floor stack. Do you know if this would be necessary if using an underlayment like 15 lb felt paper as recommended by some flooring manufacturers?

 

[Dana]

You are understanding it well.

Batting has extremely high water vapor permeance, and does not impede moisture transfer. As long as there isn't more than 2" of EPS the foundation can dry toward the interior at a reasonable rate, protecting the foundation sill. The issue then becomes the average wintertime temperature where the fiber meets the foam, which is a function of the average outdoor & indoor temperatures, and the ratio of the foam-R to total-R. That temperature has to average slightly above the dew point of the basement air in winter to keep moisture from being drawn through the painted wallboard & fiber and collect on the surface of the foam, eventually wetting the fiber. But with 5.5" of rock wool you're looking at ~R23 rock wool, and ~R8.4 of foam, R31.4 at center cavity. So the foam R8.4/R31.4= ~27% of the total, which is sufficient to prevent moisture accumulation during the winter even on the climate zone 5/6 boundary.

To save a lot of engineering modeling and caluculation the IRC prescribes the amount of insulating sheathing it takes for dew point control on fully above grade walls when there is only a Class-III vapor retarder (like standard latex paint on wallboard) on the interior for different framing depths in TABLE R702.7.1. For zone 5 that's R7.5 for 2x6 walls, which have 5.5" of cavity depth. (For marine zone 4C, which is NOTnot your climate zone they prescribe R3.75 minimum.) If one presumes R20 for cavity fill, the total foam + fiber for zone 5 would be R27.5, and a foam/total R-ratio of R7.5/R27.5= 27%, and that's valid even at the cold edge of zone 5. You're on the warm edge of zone 5 and have pretty much an 27% ratio, so even on the coldest-in-winter part of the wall, the above-grade section it will not accumulate wintertime moisture. Below grade the wintertime temperature averages at the foam/fiber boundary will be warmer than above grade. A 27% R-ratio is a pretty robust for wintertime dew point control even on fully above grade walls in your climate. You could push that by quite bit without a mold problem or wet rock wool. If the house were in Saranac Lake it would need 2.5" of EPS and only 5.0" of rock wool to work.

What's the problem with drilling though the tiles for TapCons? Is your concern all about the asbestos?

The total amount of material is miniscule, and the amount of friable fiber in the air would be infinitessimal, even if you didn't use any mitigation procedures. But if you would worry about it, spraying a blob of shaving cream where you want to drill and drilling through the blob, then mopping it up with a paper towel for disposal in a labeled air tight 6 mil plastic bag would meet OSHA protocols. For the amount of asbestos you are actually disturbing that's probably extreme overkill. If even that would bother you, glue the foam to the floor with foam board construction adhesive (it's available at box stores), and glue the subfloor to the foam, and don't drill.

Vinyl asbestos floor tiles are at least as vapor retardent as 6 mil polyethylene, so there is nothing to be gained by adding another vapor barrier layer.

#15 felt is not a vapor barrier- it has variable permeance (1 perm when bone dry, 5+ perms when subjected to high humidity). Putting it on top of the subfloor as a flooring underlayment is fine, and it would not trap moisture in the subfloor. (Using low permeance flooring such as vinyl can create a moisture trap though.)

 

[ajs317]

That's all perfectly clear--thanks for the detailed response. Yes, I was concerned about the asbestos with the drilling particularly with the recommendation that I see to vacuum out the holes, as I don't have any kind of special vac filter equipment. If that's not necessary, I may give the shaving cream method a try.

 

[ajs317]

A few additional questions (with pictures!) that have come up as we get the basement stripped of all the finishings:

- There are a few places in the foundation wall where a fairly large hole was cut out so that an outlet box could fit into the block wall and sit flush with drywall that was furred up to the wall (i.e., no stud wall). Should I fill these holes before installing the insulation or will putting the rigid foam over them be all I need? If so, how best to fill them?

- In one corner of the basement (see pic attached), the place where the main drain assembly exits out and down to the septic tank consists of a seemingly open hole in the foundation. There is currently a bunch of old cloth and other debris down in this opening so I’m not yet sure how far it goes but maybe all the way down to the subsoil. Should I fill this in somehow? Or let the ends of the floor insulation extend out over the hole?

- The basement has a winding staircase that basically comes down into the center of the basement. In the portion of the basement where the stairs are, the floor joists run from the foundation sill across and then are supported at the stairs by notches at the end of the joists that sit on top of what looks like a 2x3 nailed flat against the framing of the stairs (see the second picture). This doesn’t seem like a good setup to me at all, and I can’t imagine it’s to modern code. To make things worse, right above the floor joists that are supported by this notched connection is our first floor bathroom with a bathtub that we use regularly for our kids. Is this something that I should correct, maybe by building a short load-bearing wall right in front of the stair assembly for the joists to rest on? The only thing that makes me feel better is that this setup is clearly original to the house and has stood the test of time for 65 years.

 

[Dana]

To better restrain potential bulk water and soil gases it's better to fill any large holes in the CMU. Mudding in the holes they punched for the electrical box with cheap premix concrete (with a bit of bonding agent mixed in) troweled reasonably flat would be fine.

Open sumps and holes in the slab are a path for soil gases to enter the house, and some attempt to seal them in a gas-tight fashion is worthwhile. The southern Hudson Valley is a high radon zone, and if you ever needed to install a slab depressurization fan, the large open holes become a short-circuit- the fan pulls basement room air down the sump, under the slab and out, depressurizing the basement, not just the slab, which is NOT what you want.

It's best to seal over areas that might need service access some day with something that can be removed and replaced. If you installed some angle-irons onto the foundation (sealed with a bead of polyurethane caulk) with the top-facing flange level with the slab you could caulk in a sheet of 3/8" plexiglass cut to fit, caulked at the perimeter to the steel/wall and the slab. Removing it later might be a destructive process, but a once in 25-50 years replacement isn't a terrible cost.

A 2x3 nailed ledger and notched ends is a valid way to hang joists onto beams. If it looks like the ledger could be pulling away from the beam you might throw in a few 3" lag screws every few feet for insurance. It appears the beam is not fully spanning, but if ends are anchored in a doubled-up full span joists it may be structurally adequate. If not, a steel lally column (cut to length, no need for an adjustable type) can be added to the otherwise floating end of the beam. A concrete filled steel post is pretty easy to box in, easier than building a structural studwall, and adds a lot of structure. If you're unsure of the slab thickness there, pour yourself a ~1' x 1' x 3" (or taller) footing/pier on top of the slab to rest the lally column on. If the slab is at least 4" thick that's usually not necessary, but you'd want to remove the tile where the column rests, since the tile will crumble over time if the house settles or moves a bit. If it were a bigger beam supporting a load bearing wall above a bigger footing would be in order, but I suspect a square foot of 4" concrete is plenty in this case.

About a decade ago I added a lally column to a less than perfectly supported beam at a stairwell to my 1920s vintage house as insurance. In my case the slab is an unreinforced 1.5 - 2" rat-slab that would be prone to cracking under the load, so I went with ~3" tall 12" x 12" concrete footing. There are full span beams original to the house in other parts of the basement that are supported in the middle with steel lally columns on similar sized above-slab piers/footings.

Don't sweat the asbestos hazards when drilling or removing small amounts of asbestos reinforced floor tile. The incidental one-time risks are DEEP in the statistical noise- there are more important hazards to worry about. The standards are designed to protect workers who would otherwise incur an accumulating occupational exposure if disturbing large amounts of materials multiple times per year over several years, and even then the risks are quite low, many orders of magnitude lower than what asbestos workers were exposed to in the 1950s. Even if you were stripping the whole floor without taking protective measures the risk would be impossible to measure, but that's when it would make sense to take at least some precautions.