Rigid foam over T1-11
This is my first time posting, so I apologize for the length!
I am slowly starting to re-side my house and I want to do it in a way that boosts efficiency which naturally leads me to exterior rigid foam. But I also am concerned about my resource use and cost, so I want to retain as many original features as possible. I live in Western Mass, I don’t use A/C but am looking to reduce my reliance on my wood and oil consumption in winter and make the house more comfortable.
The existing set-up:
1st floor- Old shiplap sheathing, degraded felt and cracked/warped cedar shakes. Largely uninsulated 2×4 walls. Boards are sound, just not tight.
2nd floor- Newer construction (2000), T1-11 siding directly mounted to studs of 2×4 walls with fiberglass. T1-11 is in decent shape, a couple sheets starting to warp.
(detail pictures of existing siding/sheating are attached in photos)
My plan:
– install exterior air supply for the wood stove
-blow-in cellulose in uninsulated walls
-air sealing done with blower door
-Keep existing shiplap sheathing and t1-11 to limit waste.
– tyvek house wrap
– 1.5 inches rigid foam (can’t go too much further because of roof overhang),
-tape all seams
-Horizontal furring strips beveled on top to shed water
-keeping the existing windows (they are rather modern) using the schematic here to seal them
– Reverse board and batten – Battens screwed to the furring strips and the boards outside the battens- the hope being that the vertical battens can create a rainscreen behind the siding, saving wood and reducing how far siding builds out.
My Concerns / Questions:
-I am picking up the first load of hemlock in the next couple days… are there glaring oversights in this plan?
-I am concerned about the texture of the T1-11 and the large cracks in the shiplap. I have read how dimpled house wrap reduces insulation values, will the much more pronounced textures of these surfaces negate any benefits of using the rigid foam as an air sealer and insulator?
– I am worried about using the exisiting windows. I plan on following the schematics here: https://www.greenbuildingadvisor.com/detail-library/construction-details-remodeling but it seems like there’s room for things to go wrong since the two gable sides of my house have minimal overhang. I’ve considered building out the overhang, but that would significantly add to the time/cost since I have solar panels on the roof that would have to be removed to thread in new shingles.
The comments/answers on this site have been so helpful in my planning and growth in learning about building science. Thank you so much to all of you who contribute to this amazing resource. I am a science teacher and I just love reading the articles and comments from everyone, it has opened a whole new world of construction considerations to me.
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Replies
>"-I am concerned about the texture of the T1-11 and the large cracks in the shiplap. I have read how dimpled house wrap reduces insulation values, will the much more pronounced textures of these surfaces negate any benefits of using the rigid foam as an air sealer and insulator? "
For the shiplap sections using a fully-adhered house wrap such as Henry Blueskin VP100 or Vycor Env-S dramatically improves the air tightness compared to just sealing the foam layer.
For the T1-11 section the vertical channels present a significant thermal bypass, but laying a bead of polyurethane caulk between the T1-11 & Tyvek at both the top & bottom would block that convective bypass, at which point both the Tyvek and foam can be detailed as air barriers. The remaining texturing of the siding isn't really much of a thermal bypass, and is actually useful for drainage/drying. In fact, it's useful to use a crinkle type housewrap (eg Tyvek Drainwrap)between the foam T1-11 unless re-working the window flashing to direct bulk water to the exterior side of the foam layer. (If you're completely re-working or replacing the windows, directing the bulk water to the exterior side of the foam is generally safer.)
Dana, what do you think about stuffing a bit of backer rod in the ends of each gap and using the blue skin to hold it in place? That might be quicker/cleaner than the sealant in some cases. The sealant would probably have better longevity though.
Bill
Dana & Bill, thanks so much for your input!
-Regarding the T1-11 channels, I had previously considered using a sealant as you mentioned, but I wasn't sure that it would be sufficient, so I am glad to see you agree. It seems like backer rod couldn't hurt either, but maybe in conjunction with the sealer... both options seem sound.
-The questions I am left with are, do you think I should be using the adherent blueskin for both the T1-11 AND the shiplap sheathing? Their website really sold me! I also really appreciated what Dana mentioned about keeping the dimpled surface on the housewrap because it is going to be the main drainage plane for the windows. My fear is that if I use the Blueskin (which seems like it only comes in a non-draining style?) on the shiplap (and maybe T1-11), then I am running the risk of trapping water behind the foam since I am using the existing windows.
Should I use Blueskin and a second, outer layer of tyvec drainwrap to facilitate water flow? Objectively, it seems like overkill, but it is still cheaper and faster than replacing all of the windows... Then again, putting drainwrap directly against blueskin seems like it would reduce breathability and defeat the benefit of not needing penetrations to hold the blueskin in place.
>"The questions I am left with are, do you think I should be using the adherent blueskin for both the T1-11 AND the shiplap sheathing? "
Unless bucking out the windows and/or window flashing to the exterior side of the foam, I'd personally be inclined to air-seal the T1-11 using caulks & tapes, and use DrainWrap or similar on that section, but go with Env-S or VP-100 on on the plank sheathing part. Putting DrainWrap over Blueskin doesn't seem worth it, a lot of extra expense for a marginal benefit.