What is the best way to insulate a 2x8 wall?


What is the best way to insulate a 2x8 wall?

Michael Sweglar Published: July 8, 2020, 10:43 a.m.Last updated: July 8, 2020, 4:47 p.m.

I have been reading so much on insulation ....and out of the foam boards I think eps might be the best to use,  but not sure if the foil face would be good, may not be breathable...might be better to just do eps board and buy attic foil to put over that .....this might be the cheapest way to get some insulation on the out side of the building for the thermal break .....rock wool would be good if I could find 4x8 sheets and do a price comparison. So I would have 2x8 studs then osb then some thermal break , breathable foil , 3/4 space then sheathing. Inside I was thinking of 2" of spray foam or 2 layers of 4" foam board not sure which kind would be best ......or spray foam and fill the rest with rock wool....it would be nice to talk to someone on this situation to get a good understanding of what to do with limited money....is there a contact number 


Responses (3)

Michael Sweglar
Michael Sweglar July 8, 2020, 4:56 p.m. Reply

Ok on the  is what I was thinking not breathable.....there is a product that is foil that is breathable ... so I was thinking I would put eps or rock wool on top of the osb board then put the attic foil on top of that ...this is to be used instead of tyvek and works the same....is there a 4x8 1" rock wool board out there I have not found any...thanks

Emmanuel Cosgrove
Emmanuel Cosgrove July 9, 2020, 1:55 p.m. Reply

Rockwool has, or at least Roxul had, 2 inch thick 2x4 panels of comfortboard that we used once by attaching them with strapping. If it were me I would lean towards some sort of mineral wool product over EPS, since once you start putting EPS and OSB together the permeance really drops fast. And same with the foil – a ‘breathable’ foil likely has perforations, which would make it *more* breathable but I would look at the product specs before I stuck it on the outside of building. There is a surprising amount of building products and techniques that are allowed and even encouraged by a lot of building codes that actually make no sense whatsoever and lead to a lot of building failures.

This page here has a lot about permeance and it will help you understand the risks -

How to choose the right house wrap weather barrier

All that to say, when it comes to managing vapor movement through walls I would be careful on the outside with too many products that inhibit the ability of moisture to escape. That’s what we like about mineral wool, it’s the safest bet. Where are you located Michael?