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Advice on Vapor Barrier for 2nd story roof (interior) and lower story floor.

Advice on Vapor Barrier for 2nd story roof (interior) and lower story floor.


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Old 04-18-13, 08:34 AM
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Advice on Vapor Barrier for 2nd story roof (interior) and lower story floor.

Hello all. I'm redoing a circa 1900 home in central wisconsin. I've read a lot of confusing information about vapor/moisture barriers. The following is my situation and I'm soliciting the advice of the good people of this forum. I'm using the term 'vapor barrier' for everything although I understand that there is some difference between barrier and retarder that escapes me.

1) My roof has 3 layers of cedar shake topped by shingles. Looks like there is felt under the shake. I'm guessing the roof is 30 years old, but it doesnt leak so I'm not messing with it now. I've pulled all the lathe & plaster off the interior walls and pulled out all the blown in insulation in the 2nd floor ceiling. Based on the way the ceiling lays, its going to be much easier to put in batts instead of blown in insulation. My question is whether I should worry about putting in a vapor barrier between my trusses and the drywall, or can i just put in unfaced batts and then put my drywall up? If I need a vapor barrier, should i use felt, or can I use Tyvek housewrap? I have access to an endless supply of free Tyvek. Should I put up the batts (unfaced), vapor barrier (tyvek or other), and then my drywall? Or do I not even need to worry about the vapor barrier since I have felt under the shingles already?

2) Home has a full field stone and poured concrete basement under the majority of the home, except for one section that has a 4' crawl space with gravel floor. I've put in 3/4" subfloors already and am getting ready to put in laminate and tile flooring on the first floor. Do i need a vapor barrier for this floor or not? Again, can I staple some Tyvek housewrap on top of the 3/4" OSB that I layed as subfloor before I drop down the underlayment for the tile? For the section that I plan to put laminate, I'm putting another 1/2 plywood floor overtop of the 3/4" OSB to get the floor up to equal height with the tiled area (using 1/2 cement board underlayment under the tiled area. If not Tyvek, what else would work, felt again? Also, should that vapor barrier (if needed) go under the 3/4" OSB, or is it OK to sandwich it between the OSB and either the cement board or the plywood I play to lay down? One thing to note, when I pulled up the old subfloor, I noted that there was frost under subfloor boards, so it is definately getting moisture from somewhere.

Thoughts and comments welcome!
 
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Old 04-18-13, 08:45 AM
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Welcome to the forums.
1. Vapor barrier, if used, goes the warm side of the wall. Typically, this is laid over top of the floor joists in the attic with the insulation then on top of it in a retro fit like you're doing but with nothing on the ceiling, attaching it to the underside of the ceiling joists (we're talking about the same pieces of wood, ceiling/floor just based on where we are compared to them at the time) is fine. FWIW, Tyvek is not a vapor barrier. Now, that aside, this is the perfect time to get up there and air seal so you have as little air migrating from your house to your attic as possible.
2. Are you talking about laminate on top of the basement concrete slab? Sorry, that wasn't quite clear enough to me.
 
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Old 04-18-13, 09:50 AM
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Thanks for the response. Some follow up:
1) So if I were to use kraft faced insulation on the ceiling, the paper side would go towards my drywall. If I wanted to use unfaced batts of insulation, could I not put the insulation between the joices and then put some vapor barrier between the drywall and the joices? What sort of vapor barrier is best to be used, felt?
2) sorry for not being clear. No, the flooring is going on first floor, not in the basement. The basement is not finished. I layed the 3/4 OSB subfloor on the on the wooden floor joists 6' above the poured concrete floor. I dont have any plans to put anything on the concrete floor in the basement.
 
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Old 04-18-13, 09:59 PM
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My question is whether I should worry about putting in a vapor barrier between my trusses and the drywall, or can i just put in unfaced batts and then put my drywall up?
Trusses? In 1900? I'll guess you're referring to the 2nd floor ceiling/attic floor joists.

Install insulation between the joists with a continuous vapor barrier against the bottom of the joists, immediately behind and in contact with the ceiling drywall. Kraft-faced fiberglass isn't the best material, especially in your climate, and that paper is not a vapor barrier. Kraft paper coated on the back with asphalt is a vapor barrier. I would consider installing mineral wool batts with 6 mil. poly over it for the vapor barrier.

Air seal the ceiling as you install the drywall, as Mitch suggested. Above the ceiling, make sure your attic is fully and adequately ventilated. Don't forget to leave, or create, an opening to let you get into the attic anytime you need to.

So you want to insulate the basement walls and condition that space, or insulate the floor to isolate the basement? It sounds like the floor. If so, Install insulation from below, between the joists. Use insulation hanger wires to hold it in place. A vapor barrier, if you install one, needs to be on the top of this insulation, in contact with the bottom of the subfloor.

Tyvek is not a vapor barrier. It is moisture-permeable and blocks wind.

To determine the amount of insulation you need in each area, you can enter the data for your house and location in the ZIP-Code Insulation Program.
 
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Old 04-19-13, 06:28 AM
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So you want to insulate the basement walls and condition that space, or insulate the floor to isolate the basement? It sounds like the floor. If so, Install insulation from below, between the joists. Use insulation hanger wires to hold it in place. A vapor barrier, if you install one, needs to be on the top of this insulation, in contact with the bottom of the subfloor.
Yeah, isolate the basement. So, can i put the vapor barrier sandwiched between my subfloor and a 1/2" plywood? I dont really want to rip up the subfloor again and put something between the floor joists and the 3/4" OSB subfloor.... or do I not have a choice? Also what are my vapor barrier options? Do i need 6 mil plastic or does roofing felt work?
 
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Old 04-20-13, 03:27 PM
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can i put the vapor barrier sandwiched between my subfloor and a 1/2" plywood?
The vapor barrier needs to be in contact with the insulation and the bottom of the flooring. IOW, under the subfloor.

For mineral wool batts or any other unfaced insulation, use 6 mil. plastic. Just cut it about 4" wider than the joist bay and staple it to the bottom of the subfloor. Or, if critters can't get in there, install fiberglass batts faced with asphalt-backed Kraft paper, paper side up, and fold the stapling edges down as you go. Hold it up with hanger wires and don't worry about stapling anything.
 
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Old 04-22-13, 03:24 PM
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Is there a vapor barrier plastic under the gravel? If not, I would use this;http://www.fortifiber.com/pdf/data_s..._aquabar_b.pdf
At less than 1 perm rating, it is much better than felt paper (5-30 perms variable with wetness) and less than 1/2" ext. plywood (0.70 perms). I would also install a vapor barrier under the gravel (12 mil.) to keep the framing dry.

The laminate should have its own foam vapor retarder...

No poly for your Zone 4---

"In addition to an air barrier at the ceiling line, a Class II vapor retarder (see sidebar) should be installed in Climate Zones 6 or higher (see Map 1).

Class I vapor retarders (i.e. vapor barriers – see sidebar) can be installed in vented attic assemblies in Climate Zones 6 or higher (see Map 1) but should be avoided in other climate zones as top side condensation can occur in summer months during air conditioning periods.

No interior attic assembly side vapor control is required or recommended in climate zones other than Climate Zones 6 or higher (see Map 1) for vented attic assemblies (note the distinction, this is not the case for unvented attic assemblies as will be discussed later). With vented attic assemblies moisture that diffuses into the attic space from the conditioned space is vented to the exterior by attic ventilation." From; BSD-102: Understanding Attic Ventilation — Building Science Information Bold is mine.

Gary
 
 

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