Hybrid Insulation and vapor barrier

Reply

  #1  
Old 12-06-09, 09:28 PM
New Member
Thread Starter
Join Date: Dec 2009
Location: Annandale, VA
Posts: 7
Hybrid Insulation and vapor barrier

Here's the situation. We're doing a reno on a master suite on a 2nd floor house with siding. We did closed cell poly foam for a 1.5" inch base layer followed by mineral wool insulation the rest of the depth of the stud wall.

The bathroom has 2 exterior walls. Having just started using the mineral wool and understanding that alot of literature suggests that it is natural a vapor barrier, I'm a little concerned with trapping moisture in the wall if we add a standard 6 mil poly behind the drywall. Additionally, it seems to me that the 6 mil poly and foam would act as 2 vapor barriers. Anyone done this type of install?

Thoughts?

Scott
 
Sponsored Links
  #2  
Old 12-07-09, 06:02 AM
Member
Join Date: Oct 2008
Location: New England
Posts: 9,992
Hi Scott, your concerns are correct. I don't have all of the specifications, but when faced with potential multiple vapor barriers, fewer is better that more. In fact, none will often work well if you have a good air barrier.

You have to consider both the air barrier and the vapor barrier, they often appear to be one in the same, but you have to be careful. Example, sheetrock, well sealed at floor and electrical boxes makes a great air barrier, but is not a vb. Where as 6 mil plastic stapled up over the stud wall is a great vb, but due to the staples is not necessarily a good air barrier.

In your location you have a mixed climate, some heating and potentially some ac. The general rule is to place the vb on the warm side of the insulation, inside in winter and outside in summer. But moving it back and forth doesn't work very well, so you would opt for inside. Only the deep humid south actually goes for the outside.

Now, what do you do since you have already placed the closed cell on the outside. Don't feel bad, it is done all the time, ie when adding new siding with polyisocyanurate under it, so it can work. The correct application requires that the foam insulation be at least a certain thickness. That thickness is climate specific. I need a min of 2" way up north, so I suspect your 1 1/2" is adequate for VA.

As for the mineral wool acting as a vb, I have no data. If you post mfg I would like to read up on it. If it is a vb and in contact with the foam, AND the drywall is well air sealed, omit the 6 mil plastic and you should be in good shape. The disclaimer is, I don't have all of the data and I'm not there so putting a stamp of approval on everything is difficult.

Hope this helps
Bud
 
  #3  
Old 12-07-09, 06:43 AM
New Member
Thread Starter
Join Date: Dec 2009
Location: Annandale, VA
Posts: 7
Manufacturer's specs

Thanks for the reply Bud. The product in question is Thermafiber's SAFB 2" (we have 1960's 2x4 exterior walls). The pdf detailing the product is here http://www.thermafiber.com/images/uploads/pdf/safb.pdf.

The website itself does not seem to imply that their product is a vapor barrier in fact they make an exterior application product that is foil faced that seems to be the same otherwise (vs the unfaced that we have).

Recognizing that the internet is truth by consensus (dangerous...), I've read some materials that suggest that mineral wool insulation is naturally a vapor barrier. I'm just thinking that between the closed cell poly foam, the mineral wool, possible 6 mil poly that we might be trapping moisture in the cavity in the bedroom. Add to that the impermeable Schluter membrane that we'll use in the bathroom behind the shower along the exterior wall and the whole thing makes my head spin!

Thanks for your help!

Scott
 
  #4  
Old 12-07-09, 07:11 AM
Member
Join Date: Oct 2008
Location: New England
Posts: 9,992
Looks like expensive stuff, primarily for sound absorption and fire rating, but a fairly good r-value. They do list different facings, but what you have doesn't appear to be a vb. http://www.thermafiber.com/images/up...lationspec.pdf
Any product that meets fire standards becomes too much reading for me.

The bathroom is a tough call, with all of the moisture. Were the drawings for this prepared by an architect? If so, I would let him/her put their neck on the line. If you get really concerned about the bathroom area, I would remove the foam board and use something that can breathe and then use the poly on the inside, just under the drywall.

Here's a link to read, but you have probably already been here. Energy Savers: Vapor Barriers or Vapor Diffusion Retarders

If you need more info or help, I can probably find it.

Bud
 
  #5  
Old 12-07-09, 03:21 PM
New Member
Thread Starter
Join Date: Dec 2009
Location: Annandale, VA
Posts: 7
Findings

Allright Bud,
Here's what I got from some research (with help from manufacturers). Thermafiber mineral wool is not a vapor barrier. It is naturally an extremely good vapor retarder.

The closed cell poly we used would have to be applied to a thickness in excess of 1.75" to be considered a true vapor barrier. Since we applied to a thickness of only 1.5", we have an extremely low Perm rating but not completely a barrier.

Because we have to consider the wood of the stud itself in this whole equation, we would need to install the vapor barrier to protect the wood from moisture from the inside with the idea that there will (hopefully) be some very, very limited degree of vapor exchange with the outside. Otherwise, I'll have a sponge that will bite me in the ass later on.

I'm hoping this all works out well. With the closed cell poly installed and the mineral wool, outside it's 35 degrees and inside it's warm and toasty, and you can't even feel cold on the wall surface. Additionally, it's quiet as can be -- went from hearing road noise to silence. Good stuff.

Thanks for your help. Much appreciated.

Scott
 
  #6  
Old 12-07-09, 05:07 PM
Member
Join Date: Oct 2008
Location: New England
Posts: 9,992
Just curious, what is the cost for that stuff. It's got to be high, all good things are.

Bud
 
  #7  
Old 12-07-09, 06:41 PM
New Member
Thread Starter
Join Date: Dec 2009
Location: Annandale, VA
Posts: 7
Pricing

The pricing was actually very reasonable. I bought it from Kamco Supply Corp (you're from New England if I'm not mistaken, I know they've got a hub up there -- check out Kamco Supply Corp)

We got it for less than the price of comparable fiberglass. Here's the breakdown:

Fiberglass R-15 kraft faced 3.5", 58 sq ft, $43.60, $.75 a sq ft

Thermafiber 3" R-12(roughly), unfaced, 53 sq ft, $26.23, $.49 a sq ft ( we used 2", but it works out the same per sq ft)

The fiberglass beats it a bit for the size based off of r-value, but that's not the whole tale of the tape. The fiberglass is not as thermally efficient in terms of it being an air barrier or vapor barrier. If fiberglass gets wet it loses r-value, mineral wool does not. Mineral wool has about 3 times the sound deadening quality that fiberglass does. Fiberglass melts in a fire, mineral wool can withstand 2000 degrees before igniting (and is authorized as a fireblock in a ton of municipalities).

In our situation we combined the benefits of closed cell poly (air tight and extremely vapor efficient, r-6 per inch) with the mineral wool (extremely air, vapor, and sound efficient, r-3.8 per inch) to come up with:

1.5" closed poly (r-9) $1.07 sq ft
2" mineral wool (r-7.6) $.49 sq ft
total r of approx 16.6 at $1.56 a sq ft

About twice as much as the fiberglass, but hopefully worth it. The mineral wool has been a staple of acoustic experts and commercial big building for years. It's starting to make a cross over into residential use. Evidently, it's been in a fair amount of use in Canada, but hasn't quite caught on yet with the big box stores. I recommend it!

Scott
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes