Can you spray foam a house from the outside?
I live in Calgary, Alberta. My house is a 1940s bungalow with a second storey added in 1988. We are replacing the vinyl siding with Hardie Board, and want to improve the insulation at the same time. Right now there is 1/2" rigid foam with foil directly under the siding. Someone recommended removing the sheathing and doing spray foam from the outside. It seems we could also drill holes and blow spray foam in without removing the sheathing. Now that the blowing agents are more eco-friendly, are either of these a good option?
How will it affect existing insulation in the walls? What are the risks for vapour build up if we have a mix of plaster and lathe in the older parts of the house, with fiberglass and interior vapour barrier in the new parts?
Yes you could do spray foam if you're removing the sheathing and exposing bare studs (and remove fiberglass batts to also fill the stud cavities). Your point about more eco-friendly blowing agents for spray foam is real, they are over 1000 times lower in green house gas emmisions and healthier, but there are only a few companies that do it so far, you can find them here -
New Blowing Agents for Foam Insulation That Don't Cause Climate Change
Spray foam insulation (or SPUF) would act as the air barrier, and as long as you have at least 2 inches it would also be a suitable vapor barrier, and it will offer you thermal protection with the highest R value per inch.
You possibly 'could' leave the sheathing on and blow foam through holes, but I would leave that call up to the individual contractor to see if they could pull that off with a consistent layer of foam.
Afterwards, we would install a house wrap house wrap / weather resistive barrier) as a drainage plane, then install siding with air space with vertical strapping, and finally the Hardie board cladding.
Terrific, thank you. I'm concerned about what happens to the foam when the house is eventually renovated or torn down. I'm also considering blown-in cellulose. A contractor suggested they could remove existing fibreglass by removing a few boards from the outside, and then blow in the cellulose. Does that sound like a good option?
Going with Cellulose instead of spray foam will add a bit of complication to the mix, but honestly it is a nice option for a few reasons - cellulose is a post-consumer recycled material that acts as carbon storage, it also means that when the house reaches the end of it's functional life, the wood framing members will be able to be recliamed, reused or recyled, whereas if they are covered in spray foam they will end up in a landfill.
We've never been fans of spray foam despite the fact that it is an excellent performing all-in-one product where you get the insulation, the air barrier and vapor barrier all in one; going with celllulose will mean attention needs to be spent to keep it dry, make sure the wall is airtight, and that you have a vapor barrier on the inside of the wall assembly.
If you're contractor has pitched you the idea then they obiously have a plan on how to carry it out, so I would propose you speak to them to find out what they have in mind, and post details here and we will be happy to weigh in.