I want to insulate the outside of my house and I'd like to know which type of insulation you think is best.

Anonymous Nov. 15, 2017, 10:34 a.m.

I live in Southern Alberta and will be replacing my exterior vinyl siding with wood, I also want to add insulation at the same time. It's a 2x4 wall with pink insulation and a vapour barrier behind the drywall,  and only vinyl on the outside. There are a lot of different kinds of foam and I don't want to choose the wrong one, which do you recommend and how much will I need? Thanks, Michael. 

Responses (1)

Ecohome Nov. 15, 2017, 11:05 a.m.

When I drive by houses doing exactly what you are planning, sometimes I cringe at what I see happening. There are many different types of board insulation and it’s very easy to add the wrong type, or probably a surprise to some, but the wrong amount.

Some rigid board insulations are a vapour barrier, some are not. Some are air barriers, some are not. XPS foam insulation at about 1 inch thick will act as a vapour barrier, EPS foam insulation will start to act as one at about 2 inches, and polyisocyanurate (the one with the foil on both sides) is a vapour barrier at any thickness (foil is waterproof). You can read about them all here.

Installing insulation that acts as a vapour barrier on the outside will not serve you well, except if you add enough of it, to the point where about 2/3rds of the total insulation is on the outside. At that point what you are doing is building a R.E.M.O.T.E wall, read more about that here. What that means, is if you have 3.5 inches of fiberglass in the stud cavities, you can stick about 7 inches of EPS foam on the outside with no problem, because you will then be keeping the interior of the wall warm enough that the ‘dew point’ is no longer a problem.

What is really important is that you don’t affect the wall’s ability to dry. For that reason, we would recommend rock wool rigid board insulation (known as Roxul prior to re-branding). It’s sort of a failsafe insulation in the sense that air goes through it, moisture goes through it and water drains through it. You can add as much rock wool as you want, or as little as you have to if there are other considerations like property line setbacks or such, because it won't alter the basic function of the existing wall.

After that, install a vapour permeable drainage plane, and vertical strapping to allow moisture to vent and water to drain, which won’t happen if you install strapping horizontally. We have a page on how to install siding where that is all explained in detail.