Is spray foam a good way to insulate a house from the exterior?

Simon Levitt
Updated: Jan. 12, 2021


I have found this site very helpful - very appreciative to the experts who take the time to share their knowledge.

We need to replace the siding on our house. We will likely replace with a Hardie Board board and batten rain screen. However, the contractor can also remove the sheathing down to the studs and reinsulate first.

He reccomends 2lb closed cell spray foam in between the studs (applied from the outside). I guess this will spray to the back of our interior drywall.

I have no idea if we have an existing vapor barrier behind our drywall but doubt it - this is an older wood framed bungalow.

In addition, i dont expect our interior drywall forms a very great air barrier either - we have the usual holes from outlets etc but we have a lot of windows and other installations - all done seemingly DIY so I have no idea if they are well sealed to the interior drywall.

After research on this site and others, it seems that spray foam is the best way to go, as it will provide a vapor / air barrier in addition to insulation.  I don't really like the potential for harmful offgassing, but because we will not remove our interior drywall, I don't really know how we can air seal / vapor seal behind the drywall - this seems neccesary for other types of insulation.

Does anyone have experience with re-insulation from the exterior of the wall (so between the studs but from outside)? Is spray foam reccomended or is there something I am missing?

Any help is grately appreciated!


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