We live in a townhouse and our basemnet only has one wall with exterior exposure. We have purchased 1" Thermax foil-faced rigid insulation and are planning on putting it on our bare poured concrete basement wall sometime this summer. With an R-Value of 6.5 per inch, will I need to put drywall over top to achieve the R-Value required by code? We bought the thermax because it's rated to be ok without drywall over top from a fire safety standpoint. Our basement is a daylight basement (not walkout), but rather half above ground. We were not planning to run the Thermax down to the floor, but rather only on the area that is above ground. If the Thermax alone isn't enough from an r-value perspective, then I really can't understand why it would be advantageous over regular rigid insulation since you need to drywall over it anyway. We have a heating/cooling duct in basement even thoug it's unfinished...the point of the Thermax is to try and retain some heat during the winter and minimize condensation on the walls.

Thanks, JBuzz

The insulation should extend for the full height of the basement wall, and all foams are required to be covered by at least one layer of gypsum board.

Hi Richard,

If you take a look at the Dow spec, it says that Thermax doesn't require the gypsum board like most (if not all) other rigid insulation types. As for the area being covered, I thought the building science guide says that it's ok, and even preferable to not bring it all the way to the floor given that the bottom half of the wall is already underground and therefore insulated by the soil on the other side. I though I had all of this straight but I trust your knowledge and experience. Thanks

Found the link:
BuildingScience Info

This message has been edited. Last edited by: Buzz,

It isn't what Dow or Building Science says, it's what the building code says. Unless you fall under some strange code, it's full height, and cover the foam.

Gotcha! In that case, perhaps Thermax wasn't the best choice because I don't think it has vapor permeability like regular foam. I think that's why they say not to bring it down to the floor and to use other foam if you ever choose to finish the area. What passed code here was the builder nailing a huge fiberglass batt to the wall...and oddly enough they nailed it to the bottom half of the wall, leaving the above ground portion completely uninsulated.

You're a smart man...any idea why code would call for it to extend the full height of the wall? I can see why covering it makes sense from a fire safety standpoint.

This message has been edited. Last edited by: Buzz,

The building code considers a basement "conditioned space" even if there are just hot water pipes running through it. The reason for full height insulation is that the earth outside the foundation wall is approximately 55 degrees, maybe a tad less, and there would be heat loss through the uninsulated wall to the earth...hence the insulation.

Do I think it's nuts? Sure I do...but unfortunately, it is a code requirement, and so we are bound to conform to it.

By the way, "Building Science" is a private company, and I'm sure they post what they do in good faith, but much of what they post in many areas is contrary to the building code, so what is their point? If they really thought their ideas were worthy, they would campaign for code changes. Anyone can submit a proposal for a code change, and if it is backed up by firm data, it would have a chance to be adopted.

Last Question (I promise):

When we put up the foam, should it go on top of the wall as well, butting up to the sill plate, or do we stop at the top? Thanks

Buzz,

I agree with your decision to use Thermax, however the code should have dictated how thick to buy it, if it is possible to return it and buy thicker - code compliant foam then I would strongly suggest it.

As for your question about where to end the insulation - I'll tell you how we do it, it's more time consuming and expensive but I believe it's the most effective. Run your foam up the wall with the top within about 1/2" from the underside of the floor joists, use the same foam to do the rim joist, and the top of the foundation wall (facing down). All of these pieces should have about 1/2" gap all the way around - once they are glued in place then come back with a can of spray foam and foam all those 1/2" gaps you left. If done correctly, there will be nowhere for air to penetrate and you will have consistent insulation all the way to the underside of your subfloor.

I had a really hard time finding a place nearby that could order it for me and all they could get was 1". I think I have enough that I could feasibly double it up. Since I don't think the foil facing is vapor permeable at all, I don't think that would create a problem. I could use adhesive to put the second layer on top of the first. Does that sound advisable or am I asking for problems? Sounds dumb, but does the R-value of the concrete count?

Yes, it counts, but it offers very little in the way of R-value.

I have no opinion about laminating foil-faced foam.

No opinion? Come on Richard...I can't make a move without your opinion. I promise I won't quote you.

I can't think of why it would create a problem, but I don't know the first thing about vapor barriers, etc. I called Dow and was told it's fine, but the lady didn't speak with a lot of confidence so I'm still leary.

Today, there is nothing on the wall and we see a little condensation in one area when the temp gets down to <20 degrees. Since I don't really understand what r-value practically translates to, if I put one layer of the 1" on, would I really be helping, or does adding 6.5 r-value mean that I could feasibly get condensation on the insulation or drywall on top?

I guess I lied when I said I was done asking questions.