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Jim F 04-05-2010 10:09 PM

Insulation question
When I demo'd my bathroom and laundry walls I found unfaced fiberglass covered with a thin poly sheet. Behind the shower stall I found unfaced. This I learned was due to the vapor barrier properties of the shower stall. I am wondering why the builders would have used poly covered unfaced for the rest of the construction as opposed to craft faced. I can sort of understand it in the bathroom due tothe high moisture but the laundry area is not high moisture. Was it cheaper to do it this way? Or does the poly have advantages over paper?

Bob Mariani 04-05-2010 10:54 PM

yes it does. Several reasons. If you staple the paper to the sides of the studs you leave a gap between the insulation and the drywall. This allows for a convective loop (air moves up the wall) that decreases the effetive R-Value of the insulation. if you nail it to the face it bunches up a bit leaving bumps that make life difficult for the drywallers. So the best way to resolve these issues is to use unfaced and then poly the walls. Then i glue the sheetrock to the poly at corners and top and bottom edges. Also the poly is glued in these same places as well as to all electrical boxes. Now you have a better air sealed wall and lower energy costs.

Jim F 04-05-2010 11:44 PM

Thanks. I have found a lot of shortcuts in this addtion but the insulation seems to be well installed. So when is it appropriate to use kraft faced?

jklingel 04-06-2010 01:20 AM

good question
Jim: First, I am not a pro, but I ain't no dummy/beginner, either. I really don't know what the point of the foil-faced is. Some people say it is a vapor barrier, but I don't see how it is, in any way, shape, or form. I've never seen a perm number on it, but how can it be a vapor retarder (the correct term) with all the gaps along its edges? Maybe it helps somehow w/ air movement, but, again, its got a lot of gaps. IMO, vapor retarding and air movement can be handled much better with either poly, vb paint, plenty of ventilation.... whatever your climate requires.

Bob Mariani 04-06-2010 04:13 AM

kraft faced (w/paper) is used as a vapor retarder as jklinkle posted. This means that the perm is high enough to allow some vapor to pass via diffusion. As such is is good in mixed climates where a true vapor barrier is not advised. This is the issue. the change in temperature from the outside to the inside creates an equal change in pressure. Air will move from a high (warm) area to the low pressure (cold). So in the winter in mixed climates (most of the US) the warm air is attempting to get to the outside. Insulation is designed to resistive (R-Value is thermal resistivity) this movement or slow it down. With enough insulation the warm air does not reach its dew point before hitting a cold surface. If it does the water vapor condenses and you have a problem.
But in the summer the transfer is in reverse..... so it needs to move into the structure. This is why you see so much confusion with vapor barriers (poly)
In areas where you do not use AC... use a vapor barrier on the warm side.
In mixed climates.... no poly but kraft paper is good. (with the exception of installation issues and the fights with the drywallers
In areas where heat is rarely used ... poly on the outside wall is best

However none of this address the wall as a whole. Best approach is a wall with rigid or spray foam on the inside and the outside. This stops the thermal bridging from the studs. It provides an inside and outside air barrier. thus no airflow in or out, which is what works best.

jklingel 04-06-2010 12:04 PM


Originally Posted by Bob Mariani (Post 424598)
Best approach is a wall with rigid or spray foam on the inside and the outside. This stops the thermal bridging from the studs. It provides an inside and outside air barrier. thus no airflow in or out, which is what works best.

Bob: Agreed on stopping air flow. However, if you used closed cell foam both in and out, won't that give you 2 vapor barriers, which is a big no-no? thanks. j

Jim F 04-06-2010 12:47 PM

The concern with two vapor barriers is if you have two sandwiched together and moisture finds its way in between it will cause mold. That explains why I found unfaced fiberglass behind my fiberglass shower wall. Hopefully there is enough breathable space between the inner and outer walls to prevent this. It does get pretty complicated from a novice standpoint I must admit. Thai is my main concern with reinstalling the insulation the outer walls and especially behind my new tub surround ans backer board. I want make sure not to overkill with the vapor barriers.

jklingel 04-06-2010 01:01 PM

bathrooms are ugly
High moisture areas (bathrooms, pools, perhaps kitchens) are tricky when it comes to insulating and vapor retarders/barriers. I am going to have both an HRV and spot ventilation in my kitchen and bathrooms; BIG CFM fans in the bathrooms. The best vapor control is to have no vapor in the first place. BTW: If you have not heard a Panasonic Whisper Green (??) fan run, it is because you barely can. I just installed one in a bathroom retro, and it is as quiet as a mouse wetting on cotton. Plus, they have some great features; anyone should read up on them if a fan is in their future. john

Bob Mariani 04-06-2010 01:27 PM

with foam at both ends of the wall there is no way for water to enter. This is why it is not an issue.

jklingel 04-06-2010 02:57 PM

Bob: That is IF the foam is perfect. I know it is good stuff, and most of the installers are probably pretty conscientious, but I'd not opt for that option myself. Murphy's Law would zap me. But, everybody's gotta make their own call on that. j

Bob Mariani 04-06-2010 03:03 PM

I do believe that I stated that. However if using spray foam it will be perfect. If using rigid foam it is easy to make it perfect using spray foam at all edges an penetrations and tuck tape at all seams. And be sure to seal with spray foam the tops and bottoms of the walls if there is another level (floor) there.

Jim F 04-06-2010 05:04 PM

I currently have drywall and insulation removed in some areas of my outer walls. The insulation had to go because mice got in there and nested (through a large square hole cut for the old shower drain). In these areas I am looking at the outer sheathing with the siding nails sticking through.

What your saying Bob is that if I put a 2" rigid foam against the outer wall, and another 2" behind the drywall and seal it up good I will be good to go. That gives a total of R20. So do I still need to fill the middle 2" space with unfaced fiberglass?

Bob Mariani 04-06-2010 05:09 PM

no. you would fill the interior wall cavity with spray foam or fiberglass batts or dense packed cellulose. The outside could be 1" to 2" foam board. This must be sealed well. This will then act as a vapor barrier, insulation and a thermal break to stop the conductive heat loss through the studs.

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