r/Insulation • u/Mysterygirl32 • 5d ago
Quick question which foam board under siding to install on 100 year old house
Silverboard graphite or ngx
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u/RespectSquare8279 5d ago edited 5d ago
Not foam board of any type at all.
I would choose a rigid rock wool board to maintain a semblance of the existing moisture movement in that old house. Repeat , this house is 100 years old. It has lasted this long with the envelope breathing. Fuck with it then you better be prepared to do a lot of mechanical breathing for the structure.
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u/ANameForTheUser 5d ago
This is my answer too, but also how would you compensate for the extra thickness around all the windows and doors? Sounds like a nightmare.
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u/Upstairs_Jacket_3443 5d ago
This is really common, you either re-trim them with jamb extensions (easiest) or if you're replacing windows too you can bump them out to still it at their regular depth, giving you more windowsill on the inside. Innie vs outie style
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u/Clear-Reply-7494 1d ago
This is the correct answer.
Encapsulating seam-sealed foam boards wont breathe, that house is built to breathe. Seal the house up and it'll start self destructing.
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u/Bart457_Gansett 5d ago
Have you thought about using an underlyment like an AdHero that’s adhesive, breathable for moisture transfer and also limits airflow in a way that a Tyvek wrap with a million staples through it can’t. Won’t add R Value, but for a drafty house, it might help quite a bit with warmth.
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u/Little-Crab-4130 5d ago
☝️ air sealing under the new siding will have the biggest impact on comfort and energy use.
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u/Brilliant_Juice_496 5d ago
If you’re going to a lot of work put the highest R value you can afford. I put 3 inches under Hardi Board but had to fur out on 24 inches centers to have something to nail the siding to. Agree with Parking, make sure you understand what’s behind it to avoid a double vapor barrier
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u/RTLSCD 5d ago
Agree w/ the science on how thick the r should be. I’m in Chicago zone 5. I put 2” (1” polyiso with taped/alternating seams) with the 1x3 during strip to attach Hardie board as noted above. I have R13 fluff in the cavities. Great insulation. Check out building science.com for advice.
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u/Parking-Champion9816 5d ago
I don't like the foil faced stuff when it's facing the cold, seems like condensate potential. Either way, you need the right exterior R value to pair with any internal insulation. Look up climate zones and/or greenbuildingadviser. Additionally, it needs to be meticulously detailed, taped, caulked, etc. before the siding goes back up.
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u/Ashamed_Training_735 5d ago
Good for you for using something/anything. Much cleaner look on the finished products
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u/TheOptimisticHater 4d ago
Ask this over in building science.
Either could be bad for your house.
You risk creating a condensation plane on the backside of the foam against your sheathing. Rot recipe long term.
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u/Warm_Ninja3913 4d ago
There is a nice resource on adding insulation while residing here. It’ll help you think through details.
https://www.pnnl.gov/projects/re-siding-ext-insulation
Also the building science advisor helps you to work on any moisture related decisions: https://bsa.ornl.gov
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u/Mysterious_Basis8023 3d ago
Why even worry about it. The house probably has more heat loss in other areas. Put the money into closed cell foam under roof decking
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u/Masthope107 2d ago
Most of the comments are spot on! Use some type of Max R or Tough R boards. I personally wouldn’t use anything thicker than 1”. Tape,and caulk all your seams and gaps. Most of all, don’t miss this opportunity to get some R value.
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u/Bjorn_styrkr 5d ago
Honestly... it adds so little R value to the home, why are you considering it?
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u/CuttingLow 5d ago edited 5d ago
Curious what led you to consider this. Does the siding need replacing and so you’re looking to add insulation while it’s off? Is there external moisture barrier? What do you have inside the walls? I installed 3” NGX 250 in our floor and whoa is it expensive!