Best way to retrofit / insulate a 1883 house
Hello,
I am considering ways to insulate a home with balloon framing, 4 1/8″ X 3″ studs, lath and plaster, 7/8-inch horizontal plank sheathing and clapboards. The clapboards come off easily and many have to come off anyway for a major multi-level porch project on the west, weather facing wall.
The thermal bridging is worse than most as the studs are 3 inches wide and every 16 inches as well accounting for almost 20% of the wall (probably more including windows doors etc.).
Was considering the Dow (Ox paperboard now) SIS 1-inch sheathing on the outside of the studs to replace the planks AND fill the cavities with spray foam or hard foam strips. I know the hard foam strips will be kind of a pain to install but I do not want to rip off the clapboards and sheathing of the entire wall and try to time the spray truck weatherwise.
If I go section by section I can take my time get 2 layers of 2-inch hard foam in each bay and cover with the SIS.Then when the wall is all SIS I can reattach the clapboards. This should result in R-30 in the cavities, eliminate the thermal bridging, and an extra R-5 on top of everything. Also the infiltration is BAD now and that should be gone.
Questions:
1. Does the filling of the cavity with hard foam or Spray foam eliminate the possibiilty of condensation on the back of the SIS?
2. Would be be good to try and seal the inside (back of the lath) with a better vapor barrier than the SIS preventing water from getting trapped.
3. What is the best way to let rain that gets behind the clapboard run down behind it and out, can this be done with 1/4 inch?
4. I could use the 1/2″ SIS and have more room for the drainage behind the SIS still getting most of the thermal bridge problem. would this be better?
5. Should I reconsider the whole thing and use a different approach? I am kind of leery trusting systems that were invented 4 years ago (SIS) on a historic home that is in great condition from 1883.
Thank you for any input.
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Replies
Getting a good air seal with cut'n'cobble rigid foam is nearly impossible with uneven antique framing.
Even with some magic insulation giving it a center cavity value of R500 it still only adds up to R17 average (not counting the SIS) with a 25% framing fraction after thermal bridging (and your fraction is probably even higher, despite the balloon framing.) Before adding in the SIC, open cell spray foam would yield a whole-wall R (after thermal bridging) of about R10, closed cell foam would yield about R12, no better.
With the fat studs and high thermal bridging robbing performance, going high-R on the cavity fill with closed cell foam is a waste- adding 3/8" of fan-fold XPS underlayment between the SIS and siding would have a similar overall performance of going closed cell vs. open cell. Re-flashing the windows and using XPS underlayment as the drain plane works great!Open cell spray foam is a better option, and likely to be cheaper than any cut'n'cobble approach. With open cell foam the studwall will run ~R10 with the thermal bridging factored in, plus whatever the SIS gives you (which is thickness-dependent.)
Dealing with the window & door flashing is a critical detail- buildings of that vintage often had NONE, and only the rapid-drying of empty stud bays kept them from rotting.
Whether that stackup works without an interior vapor retarder depends on your climate. Where are you?
Rob,
Like Dana, I want to know: where are you? Climate matters.
Focus on a continuous layer of exterior foam -- the thicker the better -- and forget insulating the cavities, like Dana says.
Upstate NY Otsego county, thanks.
Rob,
Here's a link to an article on installing exterior rigid foam: How to Install Rigid Foam Sheathing.
Rob,
More answers:
Q. "Does the filling of the cavity with hard foam or spray foam eliminate the possibiilty of condensation on the back of the SIS?"
A. By "hard foam," I assume you mean a rigid foam like EPS, XPS, or polyiso. The answer to your question is no. If there are air leaks between the rigid foam and the studs, or if you use open-cell spray foam (which is vapor-permeable), then moisture can reach the back of the SIS panels and condense. SIS panels don't have a high enough R-value to be used in a cold climate; here is more information: Calculating the Minimum Thickness of Rigid Foam Sheathing.
Q. "Would it be good to try and seal the inside (back of the lath) with a better vapor barrier than the SIS preventing water from getting trapped?"
A. No. Outward vapor diffusion in winter is almost never a significant factor in wall performance. More information here: Vapor Retarders and Vapor Barriers and Forget Vapor Diffusion — Stop the Air Leaks!
Q. "What is the best way to let rain that gets behind the clapboard run down behind it and out? Can this be done with 1/4 inch?"
A. The best way to let rain get out is to have (a) a good water-resistive barrier (WRB), and (b) impeccable window and door flashing integrated with the WRB, and (c) a ventilated rainscreen gap between the WRB and the siding. Yes, a 1/4-inch gap is sufficient.
Q. "I could use the 1/2-inch SIS and have more room for the drainage behind the SIS still getting most of the thermal bridge problem."
A. The 1/2-inch SIS has an R-value that is too low to keep the interior face of the sheathing above the dew point in winter. You need thicker exterior foam.
Q. "Should I reconsider the whole thing and use a different approach? I am kind of leery trusting systems that were invented 4 years ago (SIS) on a historic home that is in great condition from 1883."
A. SIS alone is not enough. You need thicker exterior foam.
Thank you for responding, from what I can see in the references I should have R-10 exterior insulated sheathing in Zone 6, This with a proper rain channel behind the clapboard would push the wall out too far and resut in lots of extra trim work and probably diminish significantly the historic look of the house. Also there are at least 3 layers of wallpaper on all of the interior walls and that is against one of the main rules for exterior Insulation according to one or more of the links given. It would be a shame to not do anything for energy while ripping up this wall. I am wondering about having 2 inches of hard foam blown into the cavities AND the sides of the studs, then cutting strips of the 1 inch SIS (OR other product, 1 1/4 inch max) to put onto the outside of the studs. Then I could attach the clapboards through the strips to the studs and be done, this way the Insulation is against the inside where the moisture barrier is, the back of the clapboards can 'breath'. I could put flashing and even end cut solution on the sill. for that matter I could also put end cut solution (or similar protection) on the outside of the studs where they are incontact with the Insulated sheathing. This would be the place where water would likley gather? I am assuming that the 3 inch wide strips where the clapboard is on a stud could drain/dry laterally? Of course the flashing would have to be update and done very well around all windows and doors.
Am I grasping at straws? What could be done with out removing layers of wallpaper and without pushing farther than 1 1/4 past the studs pre clapboard?
If no serious insulation is possible without risking problems at least I could seal it up risk free as infiltration is the elephant here?
As far as losing strength by removing the existing 7/8 plank sheathing, I think it has minimal shear strength put I would put up steal diagonal braces, I will talk to my engineer about that.
another idea to attach the clapboards would be a second set of studs/nailers that have 2 inches of insulation behind them(on the interior side) and stick out 1 1/4 past the old studs, then the SIS board strips on the old studs would have 1/4 inch gap to the clapboards.
Really want to do as much Energy saving retro to this wall as I can without risking troubles.
thank you
Rob,
I don't know why you are worried about wallpaper. Unless it is vinyl wallpaper -- a type that is relatively rare -- then you needn't worry. Paper is vapor-permeable.
If you can't install rigid foam on the outside of your wall, it sounds like you need to insulate your stud cavities. A wide variety of insulation materials could be used to insulate your stud cavities; the most likely candidates are dense-packed cellulose and spray foam.
Your idea of creating a ventilated rainscreen gap between the exterior sheathing and the siding is a good idea (whether your furring strips are made from ripped SIS panels or more ordinary materials).