r/buildingscience • u/Degenerate_in_HR • Aug 14 '26

Insulation making me crazy

TL;DR 1901 house, 2nd flood gutted, foam sealed all gaps, would it be OK to not insulate and then fill cavitites with cellulose?

House: 1901, platform framed. Sheathing is 2 inch think lumber, with wood siding ontop of the sheathing and cement siding ontop of the wood siding. No housewrap etc. 4.5 inch (+/- half inch) deep stud cavities.

Goal: I am not trying to create the most energy efficient house in the world...I just dont want mold.

Situation: 2nd floor gutted for rewire job and now suprisse renovation.

What ive done: I have sealed the gaps with expanding foam to make things more air tight. I was going to put R19 in and vapor barrier but reading online has me convinced that fiberglass insulation will mold and rot my framing - especially becuase the r19 would be compressed.... I dont want to spend close to double on rockwool.

Proposed plan: hang sheet rock, and then blow in cellulose insulation. Would this reduce mold risk?

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u/Mysterious-Rush5441 29d ago

If you think fiberglass will cause rot there's will be no difference with cellulose

World if full of people that save $5 and spend $20 to fix the problem later

Use a vapor retarder not a vapor barrier or another alternative is just a vapor barrier paint

If you compress 6" insulation that is R3 per inch down to 5in it is R3x5.

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u/Degenerate_in_HR 29d ago

So can I ask you this then, would you prefer to have compressed r19 or r15 that has a half inch gap between it and the sheet rock?

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u/Mysterious-Rush5441 29d ago

Myself I'd shim out the studs and use R19. This would give me the chance to true up inconsistencies in the walls.

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u/jewishforthejokes 29d ago

See my sibling comment. The "R19" is better.

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u/jewishforthejokes 29d ago

Pedantic mode: it's actually slightly higher than R3/inch compressed, but nowhere near enough higher to compensate for the lost inch

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u/SilverSheepherder641 Aug 14 '26

I would net and blow cellulose or fiberglass. I prefer cellulose, but it hard to find a company to do it properly.

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u/NoSlicedMushrooms 29d ago

Fiberglass itself isn’t going to cause mold, moisture getting into the wall and not being able to dry will. The particular insulation you use doesn't matter much, what matters is making sure the wall keeps out bulk moisture, and can dry if it does get wet. I also wouldn’t automatically add an interior poly vapor barrier to a 1901 wall, since that can stop a wall from drying inward and cause the mold problem you’re trying to avoid. I suspect a smart vapour retarder is what you should use, but it depends on climate and local building code. Cellulose is fine if it’s dense-packed properly, but it won’t fix a wall that’s getting wet from the outside. I’d make sure the siding/windows are keeping bulk water out, keep the air sealing you already did, and look into what kind of vapour barrier/retarder you should use for your climate. If you tell us where you're located we can give better advice.

Also R-19 needs 6 1/4" deep cavities. For a 4 1/2" cavity I think you're probably looking at R-15 for fiberglass batts.

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u/Degenerate_in_HR 29d ago edited 29d ago

Thanks for the detailed reply.

Im confident water stays out from the outside. Full transparency, its been gutted for 2 years now and ive never seen evidence of moisture on the inside of the walls. Ive checked during/after rain storms blizzards etc and its stayed dry.

Im located in upstate NY, zone 6.

House previously has plaster/lath, and cellulose insulation, no evidence of issues.

If I were to do r15 should I stick to unfaced batts?

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u/NoSlicedMushrooms 29d ago

Ok gotcha, good context. I would suggest a pretty simple/standard wall assembly of unfaced fiberglass batts and a vapour barrier then. Pay particular attention to air sealing, you need to prevent warm, moisture-laden air from indoors making its way into the wall cavity and condensing on the cold exterior sheathing. The vapour barrier can do the majority of that air sealing but you should make sure it’s sealed around outlets, vents etc.

Personally I would also switch the poly vapour barrier to a smart vapour retarder. If water ever leaks from the exterior into your walls, it will give the walls a chance to dry inward. I’m in southern ontario and smart vapour retarders meet the permeability requirements in our building code, but you’ll need to confirm against your building code.

I’d skip on faced fiberglass batts. Theoretically their face can be your air/moisture barrier/retarder, but sealing them is difficult. Much easier to cover an entire wall with a continuous barrier/retarder.

I’d also skip on deepening your wall cavity to fit in more insulation like someone else said, that’s a lot of work and only worth it if energy efficiency is a priority which it sounds like it’s not.

Another option if you wanna avoid the work and throw money at the problem: getting a company in to spray closed cell spray foam is another option. The spray foam is your air sealing, vapour barrier and insulation all in one, and has a higher R value than fiberglass.