Basement Remodel Insulation

[Rossn]

Hi Dana - given the moisture and drainage issues, and the need to install dimple mat around the stem wall, I ultimately ended up installing a full perimeter foundation drain with dimple mat to create an air gap on the exterior wall. Also, re-grading on one side and a surface drains system for the gutters. Of course, it was a bigger project than anticipated, but is now complete. For 3/4 of the perimeter, the dimple mat is air sealed at the top, as the radon system is connected to the perimeter drain. For the other 1/4 of the perimeter (where brick is below grade) there isn't the same level of detailing of the dimple mat sealing.

The question now is: Now that the stem wall should remain pretty dry, what can be simplified/changed about the wall assembly that we had originally landed on (see image, below)?

Notes:
- From an interior perspective, the flat-framing is in-place, and the poly-iso is behind the framing, but not yet attached to the wall (so it could be shifted up or down).
- One of the original/primary concerns was that the (seeming) untreated sill plate which sits directly on the stem wall (no gasket) could experience dry rot, given moisture in the stem wall. With the work completed, there could still be humidity in the air-gap in the wall, though it seems like the work performed has mitigated this risk
- The former wall design included an interior smart vapor barrier and high degree of air sealing from the interior; spider all the way to the exterior upper wall, which has thin a paper-board type sheathing with an interior foil face that has micro-perforations.
- In our initial discussions, there was discussion of using a 2" layer of closed cell insulation on the exterior wall, but we then backed off this approach due to moisture considerations.
- I do have access to an in-situ concrete moisture meter now, and can get a reading on the stem wall, though I need to get revised readings.

 

[Dana]

With surface drains and a dimple mat that goes down to the footing the stem wall isn't going to become saturated. The rock wool vapor diffusion vent at the bottom of the wall is no longer serving any purpose. Bring the polyiso lower toward the slab, skip the rock wool. The cut edge of the polyiso should still stay off the slab as a precaution. An inch of EPS between the slab and the cut edge of the wall polyiso is sufficient.

With the perforated foil on the sheathing and MemBrain on the interior side of the assembly the structural wood is protected from both interior and exterior moisture drives- it's fine.

 

[Sponsor]

 

[Rossn]

Thanks, Dana! I have already cut the height of the polyiso to be somewhere between the top of the stem wall and top of the sill plates, depending on the evenness of the wall. There is no capillary break between the footer and stem wall. I do like the idea of a gap at the bottom from a basement flood perspective. 1) Would it be reasonable to spray spider in that 5.5" gap, or would that still have an issue in a flood type situation?

2) Given the moisture from the stem wall is now regulated/minimal -- is there a way to get away from the tedious air sealing that was going to be required between the interior of the house and the interior of the wall (across the Membrain)? This seems like it will be a pretty big hassle. As I recall, the purpose of this was to prevent condensation from forming on the inside of the exterior wall, based on air exchange in the wall.

 

[Dana]

Air sealing vs. vapor retardency. A square inch of air leak can move as much moisture as a few thousand square feet of vapor diffusion through painted wallboard. Which do you think is more important?

 

[Rossn]

Yes -- that's exactly why I'd like to avoid having to detail an air barrier at every outlet, baseboard, ceiling/floor joists (recall the wall is furred out), etc. What options do I have for avoiding having to do this highly detailed air barrier, which may be very time consuming and also may not be perfect?

While neither of us are fond of CC foam, 1.5+" of CC foam on the exterior wall + cellulose or spider fill? Anything other options?

 

[Dana]

If you follow Certainteed's instructions for detailing MemBrain as an air barrier you'll find it's really not all that bad. If you use air-tight electrical boxes it's even easier. (In Canada they sell cheap vacuum molded "vapour hats" for air sealing electrical boxes to 6 mil polyethylene, which is cheaper than air-tight electrical boxes.)

 

[Rossn]

Thanks for passing along the instructions. I spent Friday afternoon trying to locate someone in the Denver area who installs spider. In speaking with JM, it sound like they really don't sell spider in my area, which is a bummer, given I had put a lot of stock into that. I recall spider had a few advantages beyond just the net-free install. Will other blown fiberglass (with netting, or intello -- as you mentioned previously) work as-well, and without the airborne fiber concern? Are there any brands that are better (health & environment than the others). I also have to insulate the ceiling/floor joists, due to the radiant ceiling.

On the airsealing front, the reason I'm sensitive to things that take more effort (airsealing from the interior) is that I'm one guy working on this huge project, and every two days of labor I add pushes out my project a week. There is also a gap between my walls (shown below), that I would have to figure out how to seal (maybe foam-in-place). Beyond that, now I may have an additional little one on the way, and am really time-bound on this project, as I currently have one bedroom in the house that is not in the construction zone. So, if there is a reasonable alternative to airsealing the entire interior (there's about 175' of wall to seal), I'm all ears.

 

[Dana]

If there are no air leaks there is no airborne fiber. Behind taped up Intello the airborne fiber becomes a non-issue for Optima or L77 once the fugitive fiber that got out during installation is cleaned up.

 

[Rossn]

Is there a particular reason these two products are better than the rest, or are they just two fiberglass options?

 

[Dana]

Most of the newer fiberglass blowing wools for walls are pretty much the same- same performance, comparable pricing. Spider has the same performance, but has the advantage of a moisture activated adhesive, which limit the potential for airborne fiber particles getting out of the wall and into the indoor air. With any properly detailed air sealing they're pretty much the same.

 

[Rossn]

When they built this house in the 60's, they made their own forms of plywood, then used those forms for the subfloor upstairs. For forms use, there are a number of 3-4" slots showing as you look down the joist bay... maybe every foot. Since I'll also be attempting to insulate the ceiling (for radiant ceiling), that is where I was thinking I'd like to prevent exchange of fibers to the upstairs. Due to plumbing, electrical, blocking, and a number of sistered and in-between joists that have been added for stiffness, standard batts aren't a good option. That's where I was hoping for spider to come through. With loose fill fiberglass, i'd think some of those fibers would be migrating upstairs through the carpet or older hardwoods.

 

[Dana]

When they built this house in the 60's, they made their own forms of plywood, then used those forms for the subfloor upstairs. For forms use, there are a number of 3-4" slots showing as you look down the joist bay... maybe every foot. Since I'll also be attempting to insulate the ceiling (for radiant ceiling), that is where I was thinking I'd like to prevent exchange of fibers to the upstairs. Due to plumbing, electrical, blocking, and a number of sistered and in-between joists that have been added for stiffness, standard batts aren't a good option. That's where I was hoping for spider to come through. With loose fill fiberglass, i'd think some of those fibers would be migrating upstairs through the carpet or older hardwoods.

Air sealing the slots & seams with can-foam , caulk, or housewrap tape- whatever is most appropriate for the hole/slot/seam would prevent fiber fragments from getting out.

The classic airborne fiberglass shards disaster scenario is unsealed recessed lights in a vented attic, where even wind pressures will drive air in & out through the fixtures. With a basement ceiling it's much less likely, but air sealing it ensures that copious quantities are never present in the indoor air.

 

[gregm]

When you mount the polyiso to the wall you can use foam-board construction adhesive both to hold it in place prior to putting up your furring, and to seal the top gap between the foundation (or foundation sill) and the top of the foam. Standard construction adhesives use solvents that would eat into the foam.

Is it OK to use standard latest calk to adhere and seal foam board? My contractor did that in one section instead of using the foam board adhesive. In my case it's 2" XPS going on the header of a wall to build it out to the drywall.

 

[Dana]

Is it OK to use standard latest calk to adhere and seal foam board? My contractor did that in one section instead of using the foam board adhesive. In my case it's 2" XPS going on the header of a wall to build it out to the drywall.

Latex caulk isn't going to hurt anything, but it's guaranteed to fail in short years. Polyurethane caulk or foam board construction adhesive are the sealants of choice for rigid polystyrene. Some butyl caulks are OK too.