Q: Can I install radiant barrier if I have spray foam up on the rafters of my attic? How does AtticFoil work with spray foam?
We get this question a lot – someone is usually in the process of new construction and they want to incorporate both the AtticFoil radiant barrier AND some spray foam in their attic space. First, I’d like to say that the two products compliment one another very nicely. Foam isn’t going to perform like radiant barrier because (like traditional insulation) foam slows conductive heat gain/loss, but it doesn’t actually stop it. Radiant barrier foil is different than spray foam because AtticFoil’s reflectivity of radiant heat is 97%, meaning you stop the heat from ever even entering the home.
That being said, The foil is most effective at this when it’s closest to the outside layer of the home. For this reason, adding the foil with the required air gap it uses, can be tricky. There are several ways to go about this. First, you could use a traditional Cathedral Ceiling Method for Installing Radiant Barrier. In that case, you’d have some spacers added to the bottom of the roof deck, then the foil, then an intermediate layer like foam board, then spray foam over the foam board. The reason for the intermediate layer is because in many cases, the spray foam does not attach well directly to the pure aluminum radiant barrier.
Another, more economical option, is seen in this scenario pictured below:
The roof decking is being laid on (or replaced) and you can clearly see that the foam in between each rafter cavity leaves a small air gap between the foam and the deck itself. AtticFoil radiant barrier only requires a 1/2 to 3/4″ air space in order to work effectively; in this situation, you could run the foil across the top of the rafters and staple it down before new decking way laid on top. Another option would be to attach the foil directly to the bottom of the decking (or use a pre-manufactured radiant barrier decking) and then lay the decking down, with the foil facing the small air gaps. This is a perfect example of using radiant barrier and spay foam together and this is an ideal installation because the foil will serve as the first line of defense, reflecting 97% of the sun’s radiant heat from entering the layers below the foil and then the little bit of heat passing through the foil will covert to conductive heat and the spay foam will significantly slow down the transfer of the conductive heat into the attic and/or home as your second line of defense.
Q: What about adding the foil from the inside, below the rafters filled with spray foam?
For blocking heat (ie. summertime benefit) the foil is not going to offer much benefit being placed that far inside the envelope. There might be some benefit for heat retention in winter, since the foil would be right near the heat leaving the interior of the home, but generally people are adding foil to roofs/attics with spray foam because they are trying to lighten the heat load in the summer, even if they also want the benefit of keeping heat in during the winter.
Bottom line: for a situation where you want to stop heat gain and/or you are looking for year-round benefit from the radiant barrier foil, it’s best to place the foil closest to the exterior of the home/building.