Is it a good idea to use spray foam and fiberglass together? Let's talk about it.
Welcome to Foam University. Today we're going to talk about when and if you
should use spray foam insulation and batt insulation together. Before we dive
into this please go below hit that like button for me, thank you.
So, fiberglass and spray foam insulation being used together. You've probably
heard this referred to as a flash and batt system. So what this simply is for
those of you who may not know, it's a quick layer that's why they call it a
flash like a flash coating of spray foam on the outside of the stud cavity and
then you would put a layer of fiberglass batting on top of that spray foam,
towards the inside the stud cavity. Now this is usually presented to you as a,
you know potential cost effective way to to insulate the exterior walls. Well the
fact is is this may not actually be cost effective. The reason I say that is
by the time you put an adequate amount of spray foam insulation on the outside of
the wall cavity, when you put that fiberglass on top of it, the money that's
spent on that fiberglass probably is not actually giving you enough savings,
energy cost wise to actually pay for itself. So in a way you're likely best
just to leave the spray foam as it is because like I said, that fiberglass will
likely not save you enough money to pay for itself being applied. So keep that in
mind if you're considering this as a cost effective alternative. Now we just
mentioned you know putting that spray foam on the outside, typically you're
going to be talking about closed cell spray foam, and this is where the next
potential issue comes into play, because you need to put the spray foam in thick
enough to actually achieve an air seal. Now that thickness will vary depending
on what type of brand of foam you're using but generally it'll be about two
inches at least. So let's say you need to put two inches of spray foam in there, if
it's being recommended to just put a quick flash coating of one inch of spray
foam that's generally not going to give you an air seal. What can happen at
point is moisture can come through the foam and now you're trapping moisture
between the foam and the fiberglass. Over time that fiberglass can become
saturated and caused all kinds of moisture issues inside your walls. So
keep that in mind and like I said that kind of goes right into the
cost-effectiveness because you will have to bring that spray foam out thick
enough to achieve an air seal. Once you've done that, once that spray foam is
to the point of achieving an air seal that's probably enough insulation to
give you a thermal resistance also. So again it's probably not actually
cost-effective to add fiberglass on top of that. Now is there ever a time when it
does make sense to use fiberglass and foam insulation together? Yes, absolutely.
There is a slight alternative option when you're talking about a flashing batt,
and what that is is using foam board instead of spray foam on the outside of
that stud cavity. Foaming or can foaming or caulking or silicone around that foam
board to make sure you have a good air seal on the outside of the cavity and
then putting that fiberglass batt on the inside. This is a good alternative way to
do somewhat of a flashing batt but it's a DIY method and this can be a good
cost-effective way to insulate an area, specifically if you're looking to
renovate or remodel a small room maybe a bathroom or a single bedroom something
like that. Using that method of foam board, some type of sealant and then
fiberglass can be a lot less expensive than having a professional company to
come out and spray foam everything, but again giving you that building
envelope and establishing that air seal as well as thermal resistance. So again
is there ever a time and place where using foam and fiberglass together can
make sense? Absolutely but it all depends on the application. Wondering how all
this works with code? Check out this video here also look at the link in the
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