Hello,

What is the proper way to frame and prepare a 2x4 wall against a foundation wall that is below grade?

Here is what I've heard, but please correct me if I'm wrong.

1. use Drylok of similar product to seal
2. glue 2" rigid insulation to foundation wall (seems pretty thick to give away that much space) Can you use thinner insulation?
3. Seal rim joists with insulation
4. build wall and secure 1/2" from insulated foundation wall

 

1. Pointless. If you have a moisture problem, it needs to be addressed outside with grading, gutters and downspout extensions.
2. Depends on your area, it needs to be thick enough to not allow condensation to form. I would think 2" would be necessary in IA.
3. Yep.
4. No gap with the foamed wall unless you need it because the wall is not straight or even.

 

Awesome thanks!

Is rigid insulation fine for rim joists?

Also, what type of caulking do you put under the pressure treated base plate before securing to floor?

 

Why start construction when you know you have a problem. Wood that stays wet will rot.If you can't have a higher foundation, grade the ground adjacent to the foundation so the foundation is above grade and the ground is sloped away from the foundation. Good luck.

 

No problem with the foundation or water. Just curious abut framing against a foundation wall.

 

Stickshift is right that Drylock won't solve any problem that results in liquid water coming through foundation wall.

XPS insulation is R5 per inch. In Idaho, minimum recommended r value for basement walls is 10-15 depending on zone 5 or 6. So 2 inches of foam is the minimum you should consider.

All joints in the foam should be taped. You should caulk between the foam edges and side walls and the basement floor and between the foam and firestop above it. There should be no way air can get from the inside to the basement wall or you will get condensation and water behind the foam.

You need a firestop above the foam and all penetrations in the firestop need to be sealed with firestop foam or caulk. There are several ways to accomplish firestopping. You can use an extra wide 2x top plate, you can put type X drywall or plywood against the bottom of the joists, or you can fill the area above each stud cavity with mineral wool insulation.

If you use solid foam against the rim joist, it has to be caulked or foamed in place so no air can reach the cold rim joist.

As SS said, there's no need to leave a gap between the stud wall and foam, but often it makes it easier to get a straight and plumb wall. It's a rare block wall that doesn't stick out a bit here and there. But putting the wall tight to the foam helps support the foam. You'll find that some inspectors will want to see the foam mechanically supported, either with screws into the block or by the wall. But even if you space the wall out, you can use a few shims to support the foam.

There's no need to caulk the bottom plate. You can use construction adhesive to help secure it if you want.

 

Carbide - OP is in Iowa, does that change the thickness of the foam?

 

Doh! Never did learn those #$@$ state abbreviations!

But Iowa has same recommendation 10-15.

 

Depending upon which zone he is in, the wall could be a combination of rigid and fiber insulation. A lot depends upon what the local code officials will want to see, but r-5 with 3.5" Roxul woulf give a r-20 wall. Where r-5 might be cutting it close, 1.5" rigid might be allowed.

Also, if you fit rigid insulation into the rim cavity it may require a thermal barrier, like 1/2" drywall.

Bud

 

Well I would recommend fiber in the stud wall anyway; it's cheap and easy to do and will increase comfort in the space.

But (there's always a but)...you have to make sure you won't get condensation on the surface of the foam, since air will diffuse through the fiber.

Let's say the inside temp is 70F, and temp of wall is 50F. With R5 foam and R15 fiber, the temp at the surface of the foam will be 55F. At 50% relative humidity, the dew point is just about 55F. If the RH was any higher it would surely be a problem. Of course RH in the winter is usually below 50%, but if OP runs a humidifier.....

And the problem can actually be worse in the summer, because the wall will still be cool but the RH may be a lot higher. If the space is conditioned, probably not a problem.

With 2 inches of foam and 3.5 of fiber, the temp at surface of foam will be 58. Gives a little more safety margin.

Of course, with no fiber, surface of foam stays essentially at inside room temp, so no problem with condensation at all. But now you're using more energy to condition the space.

Bottom line, adding fiber in the stud wall will save a little energy and make the space more comfortable, but I don't think I'd go less than 2" foam.

 

Oh, and Bud's right about thermal barrier in front of foam on the rim joist. Easiest is probably to fill the space with Roxul.

 

I don't understand how you can accurate predict what the temperature of the insulation is (58F in the example).

In MN, just north of Iowa, our normal soil temperature at 4' is 54F. - Don't confuse the number for frost depth because that is just an indicator and code must be written in an enforceable way and are concerned with something like a single occurrence in about 100 years or so.

The surface temperature of a concrete wall is quit uniform and Mother Nature or some mysterious person does not draw a line on the inside at the grade line. The concrete has high conductance and the heat travels easily up and down as well as horizontally, resulting in a relatively uniform wall surface temperature. In fact, the conductivity and thermal storage actually can help reduce energy costs in the summer if you air condition and the basement is a part of the thermal envelope of the home.

Dick

 

Of course you're right, I can't, and I should have made it clearer that this was just an example I made up to illustrate that it's possible to get condensation on the inside of the foam under the right (wrong?) conditions. And to explain why increasing the thickness of the foam layer reduces the chance of this happening. I didn't mean to imply this was an accurate prediction of what the OP would experience in his basement. That said, I chose numbers that I think would not be unusual to see in similar circumstances.