Fiberboard exterior insulated wall
Hello
We are working on our wall assembly and I am wondering if there are any issues/thoughts with the below proposed wall. My understanding is that minimum exterior insulation R value %’s are not an issue if that exterior insulation in vapour open, as opposed to foil faced rigid foam for example.
We are proposing Gutex fiberboard insulation on the outside, then foil faced insulation between the studs (we have a source of cheap seconds foil faced polyiso thus its appeal)
Exterior to Interior – climate zone 4c.
1) 60mm (2 3/8″) gutex wood fiber insulation R 8.5
2) Ply sheathing – taped as air barrier
3) 2X4 studs on 24″ centres
4) Polyiso foil faced between studs, about R 24
5) Smart vapour retarder – is this necessary if the polyiso is foil faced and a vapour barrier anyway?
6) Drywall
Thoughts – we may run a service cavity outside the vapour barrier and attach the drywall to that on certain walls
Thank you
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
If you can get polyiso cheap, I'd use that on the exterior since it's easy to tape the seams, but the foil facing wouldn't allow any drying for the sheathing in your particular assembly. I'm not familiar with the Gutex product, so can't really comment on that specifically, but if it's vapor open, it would be a little safer than polyiso here since you'll have foil faced polyiso on the interior.
Polyiso between studs is what we like to call "cut n' cobble". It works, but it's pretty labor intensive to install. You may find the time involved to do that type of installation isn't worth the cost savings of the cheap polyiso, but that's up to you -- on a technical level it should work fine. You'll want to foam the perimeter of the polyiso to seal things, I would used canned foam in the gun for that. The foam gun is MUCH easier to use, and more economical, on larger jobs like this.
A vapor retarder wouldn't really be necassary here since the foil facing on the polyiso panels is a vapor barrier. With a vapor open exterior insulation, your wall can dry to the exterior so you should be good. Just make sure to do a good job of air sealing the interior of the wall.
Bill
Thank you Bill - you might be right I think I'll consider exterior polyiso, however the area is small and its only one storey so the labour aspect isn't a big deal.
Best regards