Basement (Pt 3): Framing before Foaming

So now that the interior weeping tile drainage system has been installed, it is time to create a thermal break on the exterior walls to avoid water vapor in the interior air from condensing on the cold exterior walls and creating another water issue.  We have decided that the most efficient (cost, time, and quality) method is to use closed cell spray foam insulation from the floor to the rim joist. 

I had priced out simply gluing rigid foam insulation to the exterior walls, but the cost was nearly the same, and I would have had to deal with alot of custom fitting of small pieces to fill int he rim joist, as well as seal all of the gaps with pricey cans of spray foam.

I also priced out DIY spray foam kits, but for the amount that I would need for the job, it's around the same price as hiring it out to a professional insulation company.

I love doing new projects on my own and learning the tricks of the trade, but with as much time as I've spent down there this summer, if I'm not saving a good percentage of cost by DIYing, it's getting hired out.

After deciding on spray foam for the job, the first step is to frame the exterior walls.  The framing is kept about an inch away from the foundation walls, which allows for the foam to be sprayed behind the framing and encapsulate the framing.  This has dual benefits, but creating a vapor/moisture barrier between the framing and the foundation, as well as creating a thermal break.  If the framing is in direct contact with the foundation, even if water wasn't an issue, the cold foundation would draw heat out of the house through the framing, or the house would draw cold from the foundation in.  In either case, there would be a heat transfer that would change the conditions inside the house.

What about the bottom plate on the floor?

Heat, as well as vapor/moisture, will also transfer through the concrete basement floor.  I will cover the actual floor covering method that I plan on using in a specific flooring post, but this still needs to be considered in the framing. 

I haven't worked out every detail yet, since I haven't actually done the work, but it goes a little something like this:

1. Lay a 12" strip of dimpled membrane along the perimeter of the floor, folding 2" up the wall and locking into the dimpled membrane on the walls.  The seams will be taped with sheathing tape.  This blocks any water from the framing.
2. Lay a strip of 1" think rigid foam insulation along the perimeter of the floor, spacing 1" away from the wall to allow for spray foaming to the floor.  This blocks the heat transfer.  This strip should only be the width of the bottom plate
3. Lay the bottom plate, either metal framing or pressure treated (depending on what you use for your framing), maintaining the 1" gap from the wall
4. Pre-drill holes int he bottom plate / rigid foam / dimpled membrane, and attach everything to the floor with tap-con screws.  I may pull everything away from the dimpled membrane on the floor and fill the dimple with the pre-drilled hole in it with a sealant, to create a seal when the screw goes in.

I have done a crude mockup of what this would look like to illustrate the materials being used:

Luckily, I have a bit of the dimpled membrane I used on the walls to use in this framing step.  You may have noticed that I called for a 12" strip of dimpled membrane, with 2" wrapping up the wall, a 1" gap prior to the bottom plate, and a 2" to 4" wide bottom plate (depending on home much space I care to lose)...so what gives?  The extra 5" - 7" will be used to overlap whatever vapor barrier I decide to use on the rest of the floor as an underlayment.  I've confirmed with the manufacturer that the dimpled membrane can be used under the framing of non-load bearing walls, which none of the walls in the basement will be.