Installing an Exterior Air Barrier
A week into trying to find the right answer. 10,000 articles, a good 300 videos. read several dozen from so called the pros of 25+ years. And every one of them pros are contradictive of each other. Anywho. I am stressed out there is no answer. I live in Southern TN, Right off the TN river. I live on a tall hill, Plenty of breeze. We are hot and humid almost year round. I am building a small 1 bedroom mother-in-law house. 2×4 frame. Undecided on fiberglass or spray foam. I am wanting a air tight, water tight home. For we get blasted at times with high winds and rain often, or it is just plain hot and humid outside. My first question is the use of like 6mill visqueen for outside air barrier?. My wall cladding will be a cement board. Wanting to use a 1/2 Polystyrene foam board. . So should I wrap the outside walls first in the 6mill plastic and seal it all well,,,, then put the foam on then the cement board? Do I really need a air gap between the Hardie vertical panels and foam board, Hardie says it can go right on the foam board. I am open for anything right now. I am about to give up.
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Replies
No. Visqueen (polyethylene film) has no place in a modern wall assembly, except possibly on the interior in very cold climates. Just use a standard housewrap (Tyvek, Typar, etc.). Use the exterior sheathing as your air barrier by taping the seams, using sealant where necessary, etc. There are many details on this site for information. If you get a lot of wind-blown rain, then the air gap behind your siding does make the house more resilient, allowing moisture to drain quickly and to dry out from behind the siding. It is not required by code or manufacturer's requirements but it is a best practice.
I agree with Peter-- and will second that Visqueen has no place in a modern wall. A polyethylene film would be a bad idea in a warm climate where air conditioning is used. The film in the location you describe it would get cold from the interior and allow for condensation in the wall assembly to occur potentially leading to damage and rot. Do not use it.
Tyvek is an air barrier, but it's vapor permeable so moisture can still go through it. That's a Good Thing since it allows for drying of the structure. Polyethylene is a VAPOR barrier, so it blocks the migration of moisture, which can lead to rot and mold if moisture gets trapped in your wall assembly.
If you're already planning on using rigid foam, you won't really gain anything from using spray foam in your wall cavities. I'd use fiberglass or mineral wool (mineral wool is my preference) batts in the wall cavities, then detail your rigid foam and exterior sheathing as your air barrier. You can put Tyvek over the rigid foam for some extra protection, then your siding. The gap you hear talked about is a "rain screen gap", and helps with drying. Rain screens make structures more durable and are generally recommended, especially considering how it's usually pretty cheap and easy to add one on a project like this -- just use furring strips.
Bill
The paint on your siding will last much longer with an air gap behind it. Just make sure you detail to keep bugs out.