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Solutions to inadequate insulation at truss heel

After recently reading "Prevent Ice Dams with air sealing and insulation" I know that I am not currently able to get the insulation I want at the heel of my standard trusses.

I will be building a double stud wall inside an existing structure, which will help increase the effective insulation at the inner top plate, but there will still only be approximately 9 inches of available space measured normal (perpendicular) to the roof sheathing where the trusses meet the (inner) top plate.

Zone 6a (Maine)

I have a number of thoughts:

First is a question: If one could guesstimate, is there a specific R-value at this critical heel at which ice dams become a notable risk, given: being in Maine, heating to approximately 65 degrees, air tightness being well addressed and the rest of the attic being loose fill cellulose at a target of r-60.

Secondly: In order to have, say, 2 inches for ventilation, I am left with only about 7 inches for insulation. I have read 2 inches for venting is preferable, but is that true still when it steals the inch from insulation?

Thirdly: What lengths should I go to to remedy this problem? My ideas being:

1) Install a different insulation at the heel that is perhaps more effective (rigid foam/spray foam) (rather than just continue the loose fill cellulose). I really don't want to bother getting a sprayer in...

2) Build a drop down of sorts (tray ceiling) to increase insulating capacity at heel. Is this something people ever do it retrofits? Seems relatively simple and effective though certainly more sheet rock work.

3) Scab down the bottom of the trusses straight across to increase space (same idea as tray ceiling, but entire ceiling would be lower instead of just the corner). If I did this I'd be tempted to jack the entire roof up, which I kind of want to do anyways (I know, I know), but that's irrelevant to the issue at hand...

Asked by Tyler Keniston
Posted Dec 5, 2017 5:45 PM ET


4 Answers

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I would cut foam board to fit until I got enough clearance for the loose fill to reach the wall's R-value. Seal with spray foam to keep in place and reduce wind washing.

Answered by Norman Bunn
Posted Dec 5, 2017 10:09 PM ET


Norman's suggestions -- using stacked rigid foam, with each layer carefully air sealed, and a one-inch ventilation channel between the rigid foam and the sheathing -- is the usual solution. This is better than what you've got. Better still would be spray foam (perhaps using a two-component spray foam kit).

The next time you need to install new roofing, you can consider turning your vented unconditioned attic into an unvented conditioned attic by installing continuous rigid foam above the roof sheathing.

I don't recommend Option #2 ("Build a drop down of sorts (tray ceiling) to increase insulating capacity at heel"). Researchers have found that the approach is ineffective. I wrote an article on this topic for the October 2008 issue of Energy Design Update. The article was called "Preventing Mold Growth In Cold Ceiling Corners." The topic has come up on GBA several times; here is a link to a previous thread on the topic, where you can learn more: R-value at roof/wall intersection.

Answered by Martin Holladay
Posted Dec 6, 2017 7:17 AM ET


Thanks Norman and Martin for the responses.

To be sure I understand, the foam would be preferable solely for its higher R-value, correct? (nothing to do with air/vapor flow resistance... given proper air sealing below)

I read the article you posted as pictures, Martin. I have to admit I'm a little confused by their results, specifically, why the 'EPS crown molding' isn't considered to be breaking thermal bridges created by the trusses. Would that continuous corner foam not be breaking the bridge across all the trusses? Is the issue that the bridge continues horizontally past the 6" of the foam to the ceiling drywall?

I'm failing to see how an interior soffit (or 'crown molding' if sized properly) would be any different than a sort of 'drop down truss' scenario as discussed here in another thread. Is there something I'm missing as far as the energy path here? Should I just trust the results of the study and not worry about the understanding? I'm just really not getting what's going on to make adding interior corner insulation ineffective. Could it just be detailing?

My situation is a bit unique in that I'm adding an interior wall to an existing structure (fortunately the structure is bare studs/sheathing currently). I attached a file detailing an idea to use rigid foam above the interior top plate since its non-load bearing. The space between will all be DP cellulose. I could then either scab down to the foam depth from the bottom chord of the trusses and loose fill, or perhaps even continue the foam across all the way... or leave it at the foam, creating that tray ceiling effect? Why wouldn't that work, is the question I can't figure out.

shop insulation truss detail.jpg
Answered by Tyler Keniston
Posted Dec 6, 2017 11:19 PM ET


I assume that the reasons that the researchers in North Dakota got such poor results from the "EPS crown molding" method were because (a) the EPS insulation wasn't very wide, and (b) the top plates of the walls weren't air sealed, so heat was leaking through the tops of the exterior walls.

If you can find a way to address these two problems -- by (a) using a wider, thicker band of rigid foam on the ceiling, and (b) by opening up the tops of the walls so that the upper section of each exterior wall can be sealed with spray foam -- then you're likely to get better success.

Answered by Martin Holladay
Posted Dec 7, 2017 7:23 AM ET

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