Soffit framing after air barrier is complete
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I have decided to use a Larsen truss wall detail on a small house that we are building. I was intrigued by an air-sealing detail I heard mentioned, perhaps on this site. The idea was to frame the house and most of the roof but to add the eave and rake overhangs after the building has been made airtight. Does anyone know of a drawing of this sort of detail?
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Do you mean a detail like page 8 of this PDF?
http://www.southmountain.com/smclibrary/articles/2011_01_JLC.pdf
The "tack-on-overhang" technique is also mentioned in this blog
https://www.greenbuildingadvisor.com/blogs/dept/musings/airtight-wall-and-roof-sheathing?page=1
I think Garth Sproule brings up a "flaw" in the Rosenbaum Roof...near the end of the comment section
...what happens to the air barrier when the roof cladding is replaced?
one possible improvement would be to add an "over-roof" and overhang at the same time
...similar to the attached snippit of a Euro-Roof
that was not the one i was thinking of but i do remember reading that article. i was just searching around this site and came across the discussion that ensued after one of mr lstiburek's articles on vented roofs. no need to continue that discussion here but i was really impressed with the detail with the 2x8 service core in the ceiling that was topped with air-tight sheathing. this would work quite well with my design but there were valid concerns over the raised heel truss detail. i plan on hand framing the simple gable roof but won't my larsen trusses exacerbate the issue of getting enough insulation above the top plate?
...low slope roof, maybe 4/12
Erik, I am confused....are you planning to do a vented attic or a compact vaulted roof?
vented attic
Attached is the Airtight Attic Floor detail with raised heel truss
concerning Larsen Truss(and similar)...
Have you looked at Lucas Durand's blog?
http://ourhouseuponmoosehill.blogspot.com/p/details.html
And the Riversong House at GBA?
https://www.greenbuildingadvisor.com/homes/thick-cocoon-cellulose-protects-superinsulated-house
and Thorsten Chlupp's Sunrise Home?
Erik,
If I understand the situation right, it might be difficult to get enough insulation that far out over the wall.
Have you looked at Sunrise home?
It uses ceiling joists and an airtight deck under energy heel trusses.
The attic actualy acts as a sort of hopper for the cellulose filled walls below
John,
I didn't see your post when I made mine.
Weird...
lucas, i like your detail but it looks like it's got 'engineer' written all over it. when i sketched out your detail using my roof/wall/larsen truss, getting 20" of blown cellulose above my top plate might not work with the roof overhangs i want/need. i guess a fella could use some sort of foam product directly above the top plate thusly lowering the rafter heel detail
Erik, it's true that I have an engineer's stamp on my plans.
Although if I were to do it again I might have just spent more time talking with my inspector.
I'm not sure what type of insulation you're planning to use in the walls, but it shouldn't be a problem to get as much as you need using the drop ceiling approach - just frame the walls a little taller for more insulation.
Instead of the ceiling insulation covering the top plate, the top plate extends above the ceiling insulation (the wall insulation going all the way up to the top plate).
But personally, I like the way Thorsten has done it in Sunrise Home.
I am so glad you asked this question. I was trying to figure that detail out as well. In the south you have to have substantial overhangs. The house I am planning on building will have porches on both sides, but I need overhangs on the gable ends. I can't seem to find building details on how to add them without thermal bridging.
We are planning on probably 2 layers of 2" polyiso on the roof and walls. Metal roof. Upstate SC.
I did find this under "A Real Chainsaw Retrofit" - a scabbed on rake over exterior foam https://www.greenbuildingadvisor.com/blogs/dept/musings/real-chainsaw-retrofit.
at the risk of hijacking my own thread... are any of you learned folks, as my father would say, on the twitter?
Erik, no. I'm a little "behind the curve" when it comes to keeping up with that stuff.
Have you got any drawings/sketches of your plan you can share?
You guys are funny. "On the twitter" and Lucas Durand being "behind the curve" when he is at the forefront of green building, has a great blog about building a superinsulated house, etc. http://ourhouseuponmoosehill.blogspot.com/