T1-11 and rigid insulation installation
I’ve read often but never posted before. I’m building a small addition which is sheathed in 7/16″ OSB. 3/4″ rigid insulation and T1-11 will be added over that to match the existing house. Cedar shingles will be used for the main entrance and gables.
I know this has been addressed before and I gather that the best practice (short of not using T1-11) is to caulk well under the window nail flanges, then again around the flanges and under the trim. The sills will have a pan flashed out over the T1-11 and under the trim.
I’ve read that the wood siding should not be in direct contact with the foam insulation. What should I put between? I could use vertical furring strips over the studs on the end wall, but not on the side walls where the wall thickness must match the existing house? Is 15# felt aceptable? “Cedarbreather” or something equivalent?
The current house has thermo-ply over the studs, then rigid insulation, then T1-11. It is 20 years old and so far has fared pretty well. There is minor delamination on the south side and minor rot against the deck in the back. I have not found any problems behind the T1-11 that I have removed.
Any suggestions or comments that steer me towards the best course of action would be appreciated.
Thanks!
(the window openings are framed in but not cut through the OSB in the attached picture)
Replies
Greetings Arth,
That's an interesting addition.
This post, in response to your question, will bump the thread through the 'recent discussion' listing again which will increase it's viewing.
Perhaps it will catch someone's attention that can help you with advice.
Cheers
We have a rambling house built mostly in the 40's and early 70's with all T1-11 siding (in most places over fiberboard sheathing I think, in the old section over old "fake" (milled) log siding). Lots of 6'x2' 1970's Anderson casements. Wall construction is mostly 2x4, some 2x6, both with R11-R13 fiberglass. If we ever have to replace/redo a wall section, should we add any insulating boards outside? Any impact on windows/etc?
I'm supposing that this building is on the west coast by your hour of posting and the minimal insulation?View Image
Philadelphia suburbs - gardening zone 6b/7; min temps in winter in the 0-5 degree range, circa 5500 heating-degree-days, SE-facing ridge just west of Valley Forge Park (on Valley Forge Mtn).Roof is probably even worse insulation-wide - 3" (maybe 4") Homasote tongue-in-groove cathedral ceilings over big 4x12 beams 48" on center, with 1-2" of fiberboard insulation on top of that (circa 1970-1973). (that's the two ends - bedroom wing (2-story shown) and great room (other end; max ceiling ~25', *lots* of floor-to-ceiling glass, clerestory windows (facing SE)). Only nice thing is no thermal bridges (snow takes a long time to melt).At least I've sealed most of the blatant air leaks, and we collect a fair bit of passive solar in the winter, and we have (per another thread) a modern woodstove in the sunroom.
Do you plan any other insulation than the 3/4inch foamboard over the OSB?
Can anyone jump in here to help this first time poster?
hope this helps.http://www.owenscorning.com/around/insulation/project/extrapower.asp
Thanks! I imagine adding R3-R5 to the walls (and eliminating the bridging) would help noticeably (and reduce air leaks. Probably not worth it until there's some other reason to tear off the T1-11, though. And with all the glass the effect of sheathing is limited.
This site has lots of good data, with specific recommendations for different climate zones. Here's a link the the "mixed climate" data, which looks like the appropriate one for Philadelphia, PA.
In particular, see page 9 for rain screen recommendations, page 19 for an energy analysis, and page 22+ for specific building sections.
http://www.buildingscience.com/designsthatwork/mixedhumid/DTW_MixedHumid.pdf
Jon
Thanks very much to everyone who posted replies. I think the topic got a little confused. Rez - if the question about location and additional insulation was directed at the original post, the answer is R-13 in the wall cavities. The rigid insulation is 1" R5 XPS (on the new section). The roof is framed with 12" I-Joists 24" OC and will be about R38 with 10" high density bats and XPS. The building is located in Harpers Ferry, WV. Instead of adding or enlarging a single room, this addition adds one room and enlarges several others. The purpose was to create a new split level entry (to replace a small circular staircase) and effectively rotate the house 90 degrees. I'll attach part of the plans so it will make a little more sense and post some pictures when it's finished. Thanks again!
I see I erred when viewing jesup's post and attributed it to you. Apologies for my inattentiveness.
It seems your post is mainly concerned with the wall covering/wood to foam question as the technique will also affect the window installation.
I could point you to some web sites that might contain some data that could be used but you have some variables that need a more customized approach which I am not capable of giving from lack of experience.
Hopefully someone will be along to fill in.
Cheers
Edited 1/25/2007 11:33 am ET by rez
What kind of insulation? If foil faced polyiso, it is open celled and I would not worry so much about it myself. it is the closed cell pones like the Dow blue that would keep the back side of the T 1-11 damp more.
But a rainscreen setup is always better, especially in your wet climate.
So in that case, I would use 1/2" foam board and 1/4" vertical furring over the studs where you need to meet the old plane. What is that wall made of anyway?
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The existing house has 2x4 walls, 1/8" structural (red) thermo-ply (a foil faced fiberboard i.e. heavy-duty cardboard), 3/4" Dow XPS (the blue stuff) and T1-11 siding. Everything is held together with wide crown staples so the T1-11 is directly on the insulation. So far, there doesn't appear to be any sign of moisture problems behind the T1-11. The house was built in 1985.I'm using 7/16" OSB in place of the thermo-ply on the addition. The 8' 'extension' walls (the walls that extend out from the original house and must be the same thickness) have 1/2" polyiso behind the T1-11. The new front wall has 1" XPS. The majority of the siding on this wall is cedar shakes over furring strips and 30# felt. There is also some T1-11 on this wall.The issue I'm still trying to figure out is how to detail the window installation. Should the nailing fin go outside the T1-11 (this is the way the original house was built - so far, no leaks), outside the OSB and under the insulation (with odd exterior casing and trim details) or should I have trimmed out the rough openings with 2x6s that extend out to the plane of the T1-11? It's pretty much a moot point now as the siding is already up. The windows will go in when they arrive - probably next week sometime.Thanks for the good advice.
Since you have no signs of problem as originally detailed, I would stay with that scheme.
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