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Another vapor barrier question

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bill1952

SAWHORSE
Joined
Aug 12, 2021
Messages
2,116
Location
Clayton NY
My current project converting garage to habitable space. Because of 12" ICF stem walls, building an inner wall so cavity is about 12". Existing "outside" wall has Kraft paper faced fiberglass. I plan 8" batts in inner wall with a 6 mil poly vapor/air barrier on inside. Can I leave the craft paper? Doesn't seem with poly it matters. I could slash it but feels a little silly.

Would you allow the craft paper to remain?
 
Thank you steveray.

I should add the craft paper is not stapled - either face or inside studs to please the drywallers - but simply friction fit.
 
It shouldn't make a difference if the poly and kraft paper are both on the same side of the insulation (facing the inside for a cold climate) Having a vapor barrier (or retarder) on both sides of the insulation is a problem.
 
Paul Sweet said:
It shouldn't make a difference if the poly and kraft paper are both on the same side of the insulation (facing the inside for a cold climate) Having a vapor barrier (or retarder) on both sides of the insulation is a problem.
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Thanks but from inside poly, 8" of fibreglass, then craft paper and 3 1/2 more inches. Adding to existing.

Thinking I should slash craft paper.
 
Bill,

For what it is worth, and the costs, I would replace the 8" fiberglass with mineral wool. I know this is about the vapor barrier, but the existing fiber with paperback , backed up by a Rockwool/mineral wool insulation will cover your concern and imo would be a better direction.
 
tbz said:
Bill,

For what it is worth, and the costs, I would replace the 8" fiberglass with mineral wool. I know this is about the vapor barrier, but the existing fiber with paperback , backed up by a Rockwool/mineral wool insulation will cover your concern and imo would be a better direction.
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How does the rockwool solve my concern about possible double vapor barrier? I get rockwool is superior to fiberglass. I do want to use the fibre glass I have on hand - can't beat the price. With 12" of insulation, I'm probably beyond payback anyway. Will look at rockwool cost difference. I'm on a tight budget - like $25-30/sf - albeit that is mostly materials only.
 
Paul Sweet said:
It shouldn't make a difference if the poly and kraft paper are both on the same side of the insulation (facing the inside for a cold climate) Having a vapor barrier (or retarder) on both sides of the insulation is a problem.
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Question for you then: how would you deal with a cooler/freezer room? We had a situation where a stick-build, fibreglass insulated wall turned to mush (before my time) because the vapour barrier was on the inside, which in summer, meant condensation from vapour on the outer elements of the building.
 
Inspector Gadget said:
Question for you then: how would you deal with a cooler/freezer room? We had a situation where a stick-build, fibreglass insulated wall turned to mush (before my time) because the vapour barrier was on the inside, which in summer, meant condensation from vapour on the outer elements of the building.
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If there is condensation on the outer elements of the building enclosure, there was insufficient insulation in the assembly. Additional insulation should have been added to move the dewpoint into the assembly and away from any condensing points of interest.

Solution: use factory manufactured panels designed for cooler and freezer rooms and air gap them to the building enclosure where possible.
 
bill1952 said:
My current project converting garage to habitable space. Because of 12" ICF stem walls, building an inner wall so cavity is about 12". Existing "outside" wall has Kraft paper faced fiberglass. I plan 8" batts in inner wall with a 6 mil poly vapor/air barrier on inside. Can I leave the craft paper? Doesn't seem with poly it matters. I could slash it but feels a little silly.

Would you allow the craft paper to remain?
Click to expand...
I would allow it as the building official. As the builder I would slash them. If there ever were an issue in the future, I would beat myself up over being too lazy to slash the paper, but that might be just me.
 
Think of vapor retarders as speed limit signs. Your poly is the slowest on this street. Kraft paper increases in pearmenace as the relative humidity goes up and starts out higher (faster) than poly. There is no issue as far as a vapor retarder is concerned. I assume the kraft paper is against the wall else you may have a fire concern to address.
 
Bill1952
I would go ahead and slash the kraft paper, just to be safe.

Wooly,
I think that additional insulation meets the intent of the code, which is not to have the kraft paper facing an air space where a fire can spread. However, some building officials will take "substantial contact" literally.

Inspector Gadget,
I would never think of using fiberglass insulation for a cold storage space. I would only use a closed-cell foam.
 
Paul Sweet said:
Inspector Gadget,
I would never think of using fiberglass insulation for a cold storage space. I would only use a closed-cell foam.
Click to expand...

Granted... the only site-built cooler I've dealt with was closed-cell foam. The file I referenced was before my time, but I have heard about it. I suspect the building was built either without a permit or without inspections, but that's another discussion for another day.
 
tmurray said:
I would allow it as the building official. As the builder I would slash them. If there ever were an issue in the future, I would beat myself up over being too lazy to slash the paper, but that might be just me.
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Thank you tmurray and others. I just wanted to be sure it wasn't a major mistake. I'll slash craft paper before the 8" batts. Looking at various details for sealing poly vapor barrier, around electrical boxes, etc. I'm somewhat focused on a really tight vapor barrier to minimize the chance of condensation in walls and attic.

Thanks again!
 
bill1952 said:
I'm somewhat focused on a really tight vapor barrier to minimize the chance of condensation in walls and attic.
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What's the full cross-sectional build-up of the wall from inside to out, with the perm ratings of each layer? How did you determine that a poly vapor barrier just under the wallboard is a measure that will do more good than harm? [Honest questions as I'm not familiar with your climate zone, assuming the project is in NY state.]

Joe Lstiburek at least seems to think that allowing drying to the interior is usually more important that preventing water vapor ingress from the interior. It may not cover your wall assembly, but see e.g. https://buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

Cheers, Wayne
 
wwhitney said:
What's the full cross-sectional build-up of the wall from inside to out, with the perm ratings of each layer? How did you determine that a poly vapor barrier just under the wallboard is a measure that will do more good than harm? [Honest questions as I'm not familiar with your climate zone, assuming the project is in NY state.]

Joe Lstiburek at least seems to think that allowing drying to the interior is usually more important that preventing water vapor ingress from the interior. It may not cover your wall assembly, but see e.g. https://buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

Cheers, Wayne
Click to expand...
In colder climates, the interior vapour barrier serves two purposes, obviously as a vapour barrier, but also a plane of air tightness. The location is important in winter as it prevents the movement of moist warm air in the building from migrating into the envelope where it cools and deposits moisture. If there is no poly vapour barrier used in colder climates, an airtight drywall approach needs to be used.
 
I understand that. The only question was the craft paper from the original wall, and building and insulating a second wall parallel and 12" (or 8 1/2" really) in side that with poly air and vapor barrier. As noted, the craft paper is as casually installed as possible, with flanges not stapled and much piecing. Friction fit. Very discontinuous barrier of any name, and I'll slash it besides.

I'm thinking instead of 8" batts on inside, horizonal 5 1/2" between stud walls and regular 3 1/2" friction fit in inner wall. Saw this - FHB or JLC? - while searching double vapor barrier. Iirc R25 is required here and I should be over R40.
 
tmurray said:
In colder climates, the interior vapour barrier serves two purposes, obviously as a vapour barrier, but also a plane of air tightness. The location is important in winter as it prevents the movement of moist warm air in the building from migrating into the envelope where it cools and deposits moisture. If there is no poly vapour barrier used in colder climates, an airtight drywall approach needs to be used.
Click to expand...

Never seen a client try the drywall approach. I'd figure a critical failure point would be electrical outlets. We have enough challenge with electrical outlet sealing in standard construction as it is.
 
Inspector Gadget said:
Never seen a client try the drywall approach. I'd figure a critical failure point would be electrical outlets. We have enough challenge with electrical outlet sealing in standard construction as it is.
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There are some air/vapour tight electrical boxes that are used.

The only place I've seen it used was the lower mainland of BC.
 
Inspector Gadget said:
Never seen a client try the drywall approach. I'd figure a critical failure point would be electrical outlets. We have enough challenge with electrical outlet sealing in standard construction as it is.
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Good thing they are required to be air sealed now...2021 IRC

N1102.4.6 (R402.4.6) Electrical and Communication Outlet Boxes (Air-Sealed Boxes)​


Electrical and communication outlet boxes installed in the building thermal envelope shall be sealed to limit air leakage between conditioned and unconditioned spaces. Electrical and communication outlet boxes shall be tested in accordance with NEMA OS 4, Requirements for Air-Sealed Boxes for Electrical and Communication Applications, and shall have an air leakage rate of not greater than 2.0 cubic feet per minute (0.944 L/s) at a pressure differential of 1.57 psf (75 Pa). Electrical and communication outlet boxes shall be marked "NEMA OS 4" or "OS 4" in accordance with NEMA OS 4. Electrical and communication outlet boxes shall be installed per the manufacturer's instructions and with any supplied components required to achieve compliance with NEMA OS 4.
 
I find three or maybe four methods. One is wrapping a piece of poly around box and sealing it to vapor barrier. Another uses a large plastic "pan" that goes behind and around a box. Another uses a tight plastic box with a flange on its face and (at least one) double stick tape plus a gasket at cable port. Last, spray with closed cell foam (keeping out of box). One or more may require caulk around cable entrance.

Can they all comply with N1102.4.6?
 
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