How do you best use polyiso insulation in a basement?
Polyiso foam board : fiber faced (white) (IKO Therm 3) or the older product, Organic faced (gray) polyiso. Both products have visible 2mm (+/-) holes in both sides of the facing material;
My Question regarding Polyiso (moisture and thermo) performance:
Central, Vancouver island, Canada.
Below grade 8’ total height, (2’ above grade, 6’below). Polyiso tightly butted together but not taped & in direct contact with/held against, inside of 8” thick concrete foundation wall by 2x4 stud framing, (bottom plate N0T in direct contact with concrete floor but iso is) Stud bays properly filled with Roxul/rockwool insulation, NO poly vapor barrier & ½” gypsum drywall, sealed with drywall sealer and painted with vapor barrier paint.
What problems do you see with this detail? Thank you
Polyiso rigid board insulation is a great product, but it does have its limitations. It is more expensive than EPS rigid insulation, but it does have a higher R value.
A downside of polyiso is that the performance starts to drop as the temperature drops, so we don’t love it as an option as the exterior surface of insulation in a cold climate. But Vancouver Island is a fairly warm climate relatively speaking, so I wouldn’t be as concerned there about the loss of performance during cold weather. You can read more about that performance drop and other rigid insulation panels here –
Rigid insulation panels: which ones are best for different applications
EPS or Rockwool are typically our preferred products for basements due to their resiliency when exposed to moisture, where Polyiso can be negatively affected by exposure to moisture and there is often a lot of that, or bulk water infiltration in basements.
The part that concerns me the most within the wall assembly that you propose, is that while the stud wall won’t be touching the concrete (that's a good thing) the polyiso panels will be, so that leaves them potentially exposed to either wicking moisture from the concrete, or soaking up bulk water. If you go with polyiso I would recommend taping up the bottoms to prevent moisture infiltration.
What made you choose the polyiso, is it the higher R value? Have you considered EPS panels?