DIY Home Improvement Forum banner

1 - 11 of 11 Posts

·
Registered
Joined
·
14 Posts
Discussion Starter #1
Hello all, and thanks in advance for any advice offered.

I am about to tackle a notoriously cold bedroom above my garage once and for all. The current batt insulation up against basement ceiling (or under bedroom floor - depending on how you look at it) that is held in place by wire just isn't cutting it. This insulation is not enclosed (no gypsum board over it).

I found a solution I like that was posted by what appears to be an expert on the subject:

https://buildingscience.com/documents/insights/bsi-064-bobby-darin-thermal-performance

However, I want to keep the 2" loss of ceiling height that both options in the article require.

I'd like to go with one of the two options as shown in my images. Is one better than the other?
 

Attachments

·
Registered
Joined
·
10,763 Posts
I prefer the top image because air sealing is a top priority and as you fit and seal the 2" rigid into place you will accomplish a lot of the needed air sealing. Also detail the perimeter rim cavity with the rigid to catch any leaks. Seal where the sill plate meedts the foundation, caulk or foam.

The reason for air sealing is to take advantage of the warm over cold situation and eliminate convection, which is the purpose of batt insulation. Essentially the insulation will work better than its rating.

I assume the 2" loss you mentioned is because they wanted the rigid below the joists which would be better but a 2x10 joist is already r-11 so not terrible. (r-1.25 per inch).

A layer of drywall below is mandatory for both an air barrier and code. At least 1/2" but may require 5/8", check local codes.

When it is all open also seal all penetrations. A fire rated can foam should work.

Bud
 

·
Registered
Joined
·
14 Posts
Discussion Starter #3
Thank you Bud for taking the time to respond with your advice. I'll be sure to follow your guidelines. Yes, you are correct about the 2" loss (if attached below the joists).

So if I'm understanding you correctly, in addition to the air sealing advantage, the r-value would also be greater with the top image installation?
 

·
Registered
Joined
·
10,763 Posts
The total r-value of the two arrangements are probably the same. The advantage that provides an added measure of r-value is the horizontal (vs vertical in walls) layering with warm over cold. Warm air above cold just stays there. In an attic we have warm air at the the top of the ceiling under whatever insulation and that air is pushed out by invading cold air.

By totally air sealing those joist cavities there will be no invading air.

Unfortunately it isn't part of the documented testing but there are examples if one searches, which I have. But no one has provided a number we can apply, only that it works better.

Bud
 

·
Registered
Joined
·
531 Posts
Have you given any thought to how you will attach the rigid foam? It's not always as easy to cut the stuff as you imagine, so you probably won't end up with perfectly straight pieces that fit snugly to the joists. One way to do it is to use canned spray foam to seal and adhere the EPS foam sheets. Depending on the size of the job, it might be worth it to get a pro foam gun that screws on to the larger cans of foam. The cost per oz of foam is a wash, but it lets you start and stop at will without clogging up the gun.
Fiberglass is a lousy air barrier, as in it isn't an air barrier at all, so stopping air flow alone will make a big difference here. Air seal it well.
And, it can't be said enough...Attach 5/8th in Type X fire rated drywall to the ceiling!!! That really should have been there from day one and is a code violation to not have it between garage and living space.
 

·
Registered
Joined
·
10,763 Posts
As a tip from a few months ago, sharpen the edge of a 2" putty or drywall knife and use that for cutting the foam. I immediately tried it after it was posted and it works great, good depth and a stable blade.

Bud
 
1 - 11 of 11 Posts
Top