Tomorrow And Tomorrow And Tomorrow Wall Vk

[Melissa Alvarado]

Powderskiwhile some--or many--basement walls without foam seem to perform well, from a building science perspective it's risky. It's much better to install continuous foam insulation on the walls before framing the walls. In zone 6, 2" of foam plus 3 1/2" of cavity insulation (fiberglass, etc.) is safest but you can probably reduce that to 1 1/2". Or you can do more foam and, optionally, eliminate the cavity insulation.

Conscientious builders are not using XPS because its blowing agents are potent, persistent greenhouse gasses. EPS or polyiso are better choices. For basements, I specify foil-faced polyiso, which has much lower environmental impact than XPS and has higher R per inch. Others advocate for EPS, which allows some inward drying, though I don't think that's an important consideration.

You can fasten the foam to the concrete with construction adhesive, just make sure it's made for foam. Or you can use Hilti IDP fasteners, which are like plastic nails with big heads and a grippy shank. All seams should be taped.

I've had no problems with LVT over bare concrete in basements but I'm in a bit warmer climate (zone 5). Provided there is a vapor barrier under the slab, and as long as you heat the basement and maintain interior humidity in the winter low enough that there is no condensation at the temperature of your slab, covering it with LVT is not a problem.

So sounds like the way to go is 1) Pull up the bottom plates, 2) install R-13 2" polyiso on the walls before framing (since polyiso is cheaper here for the same R-value.) That's not quite R15, but I can't get 3" polyiso by tomorrow.

How do I attach the foam to the concrete wall? Using PL300, even though there is foil on both sides of the foam board? And what tape for the seams on the outside? Do I need to tape the bottom of the boards where they touch the floor?

Lastly, the rim joists currently have 6 inches of rock wool pressed into place. Is that enough, or do I cut small pieces of foam board to fit on the inside of the rock wool? If so, should I use Great Stuff spray foam to seal the sides of those small pieces?

PL 300 says it adheres to most building materials, so it should work for foil-faced polyiso. If the foam is flat, that might be all you need. Polyiso is often not perfectly flat, necessitating mechanical fastening or bracing the panels while the adhesive sets. I spec Hilti IDP fasteners, which require a hammer drill for installation. I'm not sure if you will find them on short notice. In a pinch you could use Tapcon screws with fender washers under the heads. You just need a few per panel.

A high-quality tape that most lumberyards carry is 3m 8067 All Weather Flashing Tape. It's not low-VOC (I found the hard way) so make sure you have ventilation when installing it. Or you can use foil tape, probably found in the ductwork department.

For your rim joists, if there is nothing on the exterior to prohibit drying, such as continuous ice and water shield, your main concern is limiting how much indoor air can reach the mineral wool, and by extension the rim joists. You can use foam blocks sealed with canned foam, solid wood sealed with tape or other approaches.

Based on similar questions asked recently, it sounds like I am more concerned with LVP on floors than others. My family has a house in the western Maine mountains with a walk-out basement and if we put impervious materials on the slab, water accumulates, especially near the walk-out portion which is not well insulated from outdoor temperatures. Moisture accumulation = mold risk. Do you have good insulation details at your walk-out section?

In a few areas I will still need to frame very close to the concrete wall and insert foam blocks into the stud cavities. (There's a width constraint on the wall in a few locations.) I should use canned spray foam around the perimeter of those insert foam blocks, right? Do those blocks need to be sealed with tape on all sides? Thank you again!

BTW, regarding flooring, the walkout section is quite dry. The only moisture I've seen is on the back of the foundation, with a small vertical crack running near the corner of a pillar. I'm hoping to inject epoxy into that crack, but I think I have to wait until warmer weather comes.

Before putting down impervious flooring, it's a good idea to tape down a few square feet of plastic for a week or two (or longer, if you can wait)--that will help confirm that the slab really is dry and not just that the surface is evaporating to the interior. In general, with a walkout basement the uphill side is at more risk of moisture from the exterior; at the walkout side the bigger problem is a cold slab condensing interior moisture. But there are exceptions.

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