Expert: Ted Barnhill (Principal, Design45 LLC) - 2/14/2010

Question
Hello!

I live in a 20 year old middle unit town home in MD. It is a 2 level + basement where the rear of the basement is walkout, but not the front. I am not the original owner. The recent spells of cold weather have made it more apparent where the cold spots in the house are. Most noticeably is in the basement. The basement is "completed" but seems to be an after thought construction job.

Facing the front of the house, the utility room is on the right side in the basement. There is a bump out for bay windows that are on the 1st level above the basement. In the utility room, I can see the bump out cavity above the concrete wall that extends out of the front of the house 2 feet and is about 7 feet wide. There are some folded up bats in this cavity. I moved some out of the way and can feel a draft and see daylight. Temp inside the utility room is approximately 55 degrees whereas the temperature in the cavity is 29 degrees. The height of the cavity opening is approximately 1 foot.

How can I go about improving this situation?
I could TRY to squeeze myself a bit in to the cavity in between the floor joists and spray foam the walls of the cavity. I also have some left over pink foam boards. After I spray foam the walls of the cavity and put the folded bats back in can I seal off the cavity with the foam boards?


Any alternatives / suggestions would be greatly appreciated.

I do have pictures if my description leaves you dizzy. :)

Thank you!

-Giles

Answer
Giles,

It sounds as if you are describing a cantilevered floor beneath your bay window.  This is a notoriously tricky place to air seal and insulate properly, especially with batt insulation.  This may go without saying, but you should not be able to see daylight or feel a draft.

The best solution is to have a spray-foam installer fill the joist cavity in your cantilever with closed-cell foam spray.  They have long wands and will easily be able to get at the entire cavity.  This will completely seal the space, and provide outstanding R-value.  This is a very small foam job, so you may run into minimum setup charges; call around.

If you want to do it yourself, you can try to attack it with cans of foam as you suggest, but it can be difficult to get into all of the corners.  As your main goal is to seal it air tight, you can also attack it from the outside, by removing whatever finish/soffit is under your cantilever, and then properly attaching sheathing and an air barrier paper like taped tyvek.  You may not have much room to work here either, so you'll have to pick you battles.  I do not recommend using rigid foam above the wall to create your airtight seal, as the leaks within the cantilever will continue to make your floor above cold, and may provide an avenue for moist air to intrude, and over time, cause mold.

I hope this helps.  Best of luck staying warm!

Ted Barnhill
Design 45, LLC