Vapor barrier - house wrap question!

[daybreak1998]

Howdy,

Everyone here is thinking this is a framed house structure. It's not. It's a post and beam building. House wrap should not be used.. The only items which should even be considered would be true foam board, or plastic or foil covered foam board. You do not want to have open fiberglass insulation next to the metal panels. The interior of the building can have its own atmosphere. You can even have a rain forest inside on those hot humid days. Buildings like this will always have some draft.

 

[Carl_NH]

I would do 1/2" celotex or similar board between the girts, then the R19 batts paper facing in, then cover that with the poly - a dual barrier so to speak.

The concern I would have is the batt insulation slumpinig in the 30" space - the 1/2" board would keep the insulation away from the siding and provide a air passage, then the R19 would be tighter along with the plastic in the space between the girts would be a very good insulation R value and reasonably tight.

The other alternative is to return the FBG insulation and do 2-3" spray insulation directly on the metal - this would be the best solution overall and eliminate the condensation aspects.

Carl

 

[BeezFun]

I always thought house wrap let inside moisture out but didn't let outside moisture in. The outside sheathing is metal and theoretically impervious. If you put tyvek or some such up against it won't that tend to concentrate moisture inside the sheet metal? Usually outside walls are T-111 or siding or clapboards or brick or some other semi-permeable material.

You're correct, but moisture and water vapor are two different things. Housewrap prevents the passage of water as in rain, but allows the passage of water vapor, as in humidity. Poly prevents both, kraft facing prevents vapor but not water, some paints prevent both, some paints prevent neither, interior plywood is poor at stopping vapor but exterior plywood is pretty good because of different adhesive. Designing this properly isn't easy, the manufacturers give really good guidance but very few people seem to look at it.

 

[BeezFun]

Howdy,

Everyone here is thinking this is a framed house structure. It's not. It's a post and beam building. House wrap should not be used.. The only items which should even be considered would be true foam board, or plastic or foil covered foam board. You do not want to have open fiberglass insulation next to the metal panels. The interior of the building can have its own atmosphere. You can even have a rain forest inside on those hot humid days. Buildings like this will always have some draft.

Post and beam buildings are used for everything from medical clinics to retail stores and offices. You happen to be familiar with them as barns, and barns are drafty, but they can be made as airtight as any other structure if that's a requirement. This fellow is trying to build something that isn't drafty and can be heated. I agree with you that I would not put fiberglass next to metal panels, but he has purchased the material and needs to find a solution that optimizes what he has. After thinking about this, I think I'd staple felt paper into each cavity against the metal. It passes water vapor easily, stops condensation on the metal from reaching fiberglass, it's cheap, and provides some thermal break between the metal surface and air in the wall cavity.

 

[BeezFun]

I would do 1/2" celotex or similar board between the girts, then the R19 batts paper facing in, then cover that with the poly - a dual barrier so to speak.
Carl

This is the worst possible combination of solutions. The foil on Celotex and the poly are both class I vapor barriers. So you've created a cavity in which moisture condensation in the fiberglass would never be able to evaporate in either direction. Here's a good article on vapor barriers. I think the pendulum swung too far trying to create air tight walls, that resulted in mold and moisture damage. Now things are swinging back to a middle ground that recognizes the importance of allowing a wall to dry out once it gets wet, because they all get wet at some point.

 

[inane2]

Here is some photos (from my phone) of the inside walls with the bookshelf girts and new metal up :

And the corner with the wood stove :

The posts are 5x5 and are on 12' centers. There is a short scab section of post on 6' centers between the actual posts. This puts the trusses 6' OC. There was a single 2x10 header. After toenailing in a 2x6 on the top 2x6 girt, we ripped a 2x10 down and placed it below the header. This formed a box (opens into the attic cavity). We already put in a single 15" wide batt in this area before we nailed the ripped 2x10 in place.

The 5x5 posts worked out great as this allowed my to take the bookshelf girt and sit it back some on the existing girt and toenail into that as well. Toenailing here and at the post ends really stiffened the board up as I was concerned with the 12' span.

There is 8' between the bottom of the ripped 2x10 down to the bottom of the top (non-treated) 2x4 nailed onto the achored treated 2x4. I plan on closing this in with vertical sheets of 7/16" OSB.

Again, the stove will have white metal in the corner. Behind the stove will be 6 feet of metal. The other wall that isn't visible in the stove will have all 12' in metal. This is the front wall and goes to an overhead door.

 

[BeezFun]

Looks very nice. I thought about this and think I'd staple felt paper in the cavities to cover the metal. That will prevent condensation or rain from getting to the insulation, but will pass water vapor so the wall can dry out. It's also cheap and easy to put up. Don't seal the bottom edge of the corrugated metal, the corrugations are the only way water has to drain down behind the wall. If the top edge is open as well that will give you pretty good circulation in the wall cavity to keep things dry.

How do you plan to support the bats of insulation? They are running horizontally and there is nothing to staple the top edge of the bottom roll to in each girt cavity.

 

[inane2]

Thanks BeezFun. Thanks for sharing those interesting links as well.

As you mentioned, the installation of the top row of batts will be fine. I can staple the top edge to the top girt. I can staple the bottom edge of the bottom row but that won't cut it I'm sure. I really don't want to rely on friction and end up with sagging insulation.

I looked at lowes and the longest wire support hangers they had were 24". Do you think using two of those diagonally in both lower corners would suffice? Man, I wish they has used full posts 6' OC!

Also, I've been looking at tar paper. It's cheap, around $15 per roll and it is 36 inches wide which would more than cover each cavity and give me some leftover to staple on top and bottom.

I already have new metal for the roof. What's your take on installing low-e radiant barrier over the purlins when I'm up there replacing the metal? It may be this weekend or next spring before I get to that project.

http://www.low-e.com/products/products_view.php?Product=Low-E

It looks like nice stuff to work with. 1/4" thick, no mess or itch. I will have a 14" ridge cap and will blow insulation into the attic on top of the metal ceiling.

Thank you!

 

[BeezFun]

I looked at lowes and the longest wire support hangers they had were 24". Do you think using two of those diagonally in both lower corners would suffice?

Not really, I think you'd be better off running a horizontal 1x or 2x through the middle of each cavity and staple the bottom roll to that. You can keep it from sagging by nailing through the OSB to catch it. You'll need to use a nail gun or screws to catch it. Not a great solution, but best I can think of. Your other option is to run the insulation vertically and staple the top edge of the facing to the horizontal element you have. Snug the insulation pieces up against each other.

Also, I've been looking at tar paper. It's cheap, around $15 per roll and it is 36 inches wide which would more than cover each cavity and give me some leftover to staple on top and bottom.

Yes, I think that will help improve your situation for not much money.

I already have new metal for the roof. What's your take on installing low-e radiant barrier over the purlins when I'm up there replacing the metal? It may be this weekend or next spring before I get to that project.

It's a good product but since you are installing an insulated ceiling, I think adding the barrier is probably not enough of a benefit to justify the cost and labor of installation. It might reduce the temp of your attic space marginally, but your insulation addresses that more directly. I think better value for that money is to install something like a solar powered roof or gable fan, and make sure you have enough ventilation space either along the soffits or on the gable ends. Or I'd add more insulation to the ceiling.

It looks like nice stuff to work with. 1/4" thick, no mess or itch. I will have a 14" ridge cap and will blow insulation into the attic on top of the metal ceiling.

Make sure you put up something before the metal ceiling to reduce air flow into the insulation, and avoid direct contact with metal. That would be a good application for plastic sheeting to act as a vapor barrier too. When you put it up caulk the seams and staple right through the caulking to make a nice airtight joint. Run it down the walls a few inches before you put up your OSB. Caulk and staple it along that new top plate you've created.