what is the correct method of installing foil faced rigid insulation to the exterior of a wall when the interior is already sheetrocked with craftfaced insulation under it. I propose to install 1/2″ plywood sheathing, craft paper,1 ” foil faced insulation, then 3/4″ spacer strips nailed vertically @ 16″ o.c. , with back primed cedar siding on the exterior.
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Foil-faced rigid insulation.
Are you talking about Polyiso, XPS or EPS panels, or do you mean compressed FG?
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....
Dinosaur The answer is Polyiso
Okay. It sounds like you've got the basics already figured out. What is the specific issue you're concerned about?
BTW, if you'll fill in your profile and give us an idea of where you're located, that can help us answer your questions. A lot of the answers to this stuff are regional/climate dependent.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
According to Rick Arnold Fine Home Building 181 Iso panels are considered exterior vapor barriors and should never be used when interior vapor barriors are in place. However when I called the tech department at R Max they said as long as I did'nt tape the joints, it should'nt be a problem. So as is so often the case, two differing opinions, and I was hoping to get some others. Thanks.
Rick is right, in principle, although a lot depends on the climate where you are.
In a temperate climate such as the northern US and Canada, most of the humidity that will try to infiltrate its way into the walls will come from interior human activities such as cooking and bathing. However in tropical or semi-tropical climates such as the southern US and the Caribbean, most of the humidity will come from the environment--outside the house.
The purpose of a vapour barrier is to keep as much humidity out of the walls as possible. Thus, in St-Croix, the VB would go on the outside of the wall while in Quebec it goes on the inside.
You don't want a vapour barrier on both sides of the wall because--no vapour barrier being perfect--some humidity will inevitably work its way into the wall and once it gets in, you want it to have an easy path to get back out again. If you VB both sides of the wall, it will turn into a mould-farm fairly quickly.
Assuming you're in a temperate zone, just leaving the joints untaped on foil-faced polyiso won't provide enough permeability to evacuate humidity trapped in the wall between the two vapour barriers. You would be better advised to use unfaced panels, or panels with permeable facing.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Have you ever taken dw off a wall with kraft face insulation after say 10-20 years?
R-Max may be assuming what I have seen in existing homes with kraft faced insulation in the walls. (1) the vp of the insulation was installed mostly wrong and very far from being complete, and (2) the paper itself has dried out to the point it has crumbled apart. With both those factors working agianst it being anything but a partial vp, the untaped rigid faom outside may be enough to let the wall dry in both directions.
Just another opinion, but like you said, it depends more on regional climate conditions.
I have to wonder about how adequate it would be to leave the polyiso joints untaped. As Martin Holladay has pointed out on this site now and then, a vapor barrier/retarder doesn't have to be perfectly installed to perform its duty. If the foam surface is 99% covered with foil, then the surface is 99% effective with respect to vapor diffusion. The 1% that allows some minor diffusion area will hardly matter.Now, if not taping the joints allows convective flow of air through it, then the vapor barrier has been bypassed. But that works both ways, allowing humidity in when it isn't wanted or allowing convective air flow within a FG batt, a definite no-no.
Good responce.
I'm in a mixed humid climate where we have slightly more cooling days than we do heating. i have not worked on any newer home that have been built with tighter envelopes, so I have'nt seen many of the issues I have seen discussed here.
Just trying to keep up and broaden my knowledge.
those factors working agianst it being anything but a partial vp, the untaped rigid faom outside may be enough to let the wall dry in both directions.
You're quite right about tarred or aluminised kraft VB's not being as impermeable as poly sheeting. Tarred kraft does tend to degrade and yes, installation standards 40 years ago weren't what they are today. VB's in older houses 'breathe' a heck of a lot more (which is actually good under certain conditions, but that's another discussion).
But in any event, that breathing means there will be even more moisture working its way into the wall from the inside (again, assuming a temperate climate)...so there would need to be a very permeable exterior sheathing to allow the wall to dry properly. Which kind of leaves out 2-part PU and most of the rigid foam panels....
In a case like that, I would probably opt for semi-rigid (compressed) FG with moisture-permeable facing as an exterior insulation add-on.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
I read the op post agian, and he is proposing the rigid foam as a sheathing.
I'm think that unless he is adding it as a layer over existing sheathing, it would be a could time to upgrade the insulation package and tighten up the whole exterior envelope. That would probably make a more significant contribution to the homes energy efficiency than the addition of the extreior foam board.
...he is proposing the rigid foam as a sheathing.
Not sure how you got that from his post, Dave; it appears he plans to lay insulation over ½" ply sheathing.
Actually, now that I've re-read his post myself, I'm not sure why he wants to put kraft paper over the plywood. 30# felt, yes. Natural kraft, why bother?
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
I haven't checked the specs in a while, but I seem to remember kraft faced insulation being defined as a vapor retarder, not a vapor barrier. The foil faced iso is in fact a vapor barrier (vapor impermeable). Given that one is a barrier you are essentially preventing drying to either the inside or the outside.
I'd just go with some type of rigid foam without the foil.
I'm wondering if kraft paper-faced insulation behaves like felt, which I've heard is something of a "smart"vapor retarder. I've read that felt becomes more permeable as it gets wet, so that as moisture builds inside the wall, and the felt starts to get damp, it starts to allow some of the moisture through. Which is a good thing, and one reason felt performs well.Steve