Insulating the area around the top plate and soffit
![](https://secure.gravatar.com/avatar/bb908a26b2b7177a3b1ccc8e06fc0dbf?s=52&d=https%3A%2F%2Fimages.greenbuildingadvisor.com%2Fapp%2Favatars%2F200x200%2Fj.png&r=g)
When I peeled the one layer of fiberglass batt away from the soffit, I found I had less than 5 inches from the top plate to roof, thanks to the 2×4 roof trusses. And the roof is pitched to where I have to move almost 3ft. away from soffit for enough room to get 17″ to roof, the level I would need to get R 49. That seems like a lot of area that would be under-insulated, especially the maximum R 13 of space at the top plate.
Probably the best solution would be spray foam, but it isn’t in my budget. I feel even after I properly air seal the attic, and vent and dam the soffits, blown-in wouldn’t provide sufficient R value in the areas that will be under-insulated because of the shallow joist, and roof pitch. Made worse if the blown-in is installed with gaps.
This worries me as we had condensation issues on the northside of house this winter in the areas that had gaps in the insulation near the top plate/soffit. The northside of the house faces an opened area. In the winter, the winds blow cold and hard on this sunless part of the house.
Good idea on the rigid foam, or just go with straight blown-in insulation? Any opinions on any of this would be appreciated.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb1.png)
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb2.png)
Replies
John,
Here is a link to an article that addresses your question: "Insulating Tight Spaces at the Eaves."
So, I see that I share the same problem with many other homeowners, and this question was asked numerous times.
It looks like my budget will have me using rigid foam panels and the cut & cobble method to get more insulation over the top plate.
I'll cut the 2" thick XPS rigid foam to fit in between rafter bays (22.5"), and 24" long, which will cover the top plate and get me far out enough to where the cellulose will be deep enough on its own.
I was considering using a 100% silicone window/siding caulk to seal the accuvents and smaller gaps, and spray foam for the larger ones. Is there better caulks to use for air sealing in an attic?
After reading numerous articles here on GBA, it's obvious the money and time i'm investing in the attic will be meaningless if I don't properly air seal.
The biggest bypass culprit is the utility chase. It runs the length of the wall between the one bathroom, and the kitchen. It is quite the gaping hole. The plan was to cap and seal this, but should I have insulation blown into the cavity, first?
John,
Q. "I was considering using a 100% silicone window/siding caulk to seal the accuvents and smaller gaps, and spray foam for the larger ones. Are there better caulks to use for air sealing in an attic?"
A. Silicone will work, but so will other types of quality caulk like polyurethane caulk. You should avoid inexpensive latex painter's caulk.
Q. "It is quite the gaping hole. The plan was to cap and seal this, but should I have insulation blown into the cavity, first?"
A. My vote is to do a careful job of air sealing, and then to install deep insulation above the new air sealing materials. For more information, see "Air Sealing an Attic."
And you may see what we did with very limited space as well. Ended up doing perimeter closed cell foam inboard 4' per recommendations from Martin.
https://www.greenbuildingadvisor.com/article/one-mans-quest-for-energy-independence-part-1
John,
You say your budget won't cover spray foam, but have you had it quoted? It's not all that much foam and it might cost less than you think, at least to get the 1' or so triangle right out by the top plate. CCSPF is roughly $1 per board foot in most markets, installed, so you could do a back of the envelope calculation and see if you're even in the ballpark.
Rigid foam isn't all that much cheaper than spf, especially when you consider the installation costs.