I recently finished writing the chapter on the interaction of structure and control layers in my book. The last part was on the various ways to insulate the top of a house. The most common, of course, is to have a vented attic with insulation on the attic floor. But when you visit discussion forums like those hosted on Green Building Advisor and the Journal of Light Construction, you’ll find a lot of talk about putting the insulation at the roofline. And you can end up really confused.
When you move the insulation to the roofline, things get more complicated. You can put all the insulation on top of the roof deck, all the insulation on the underside of the roof deck, or some on top and some below. You can put it all below the roof deck but use two different kinds. Your insulated roof can be a cathedral ceiling or it can be above a conditioned attic. You can vent the insulated roof assembly or build an unvented insulated roof.
But what’s the best way or the safest ways to insulate the top of the house? I spoke with Kohta Ueno of Building Science Corporation about this topic and put that question to him. Here are his top three methods:
1. Vented attic
This one’s the most common and the least likely to cause problems . . . if you do it right. A lot of houses with vented attics in cold climates have problems with condensation in the attic or ice dams on the roof, but those are failures to air-seal and insulate properly. When you get the air leakage and conductive heat loss low enough, those problems disappear.
What to do with the heating and cooling equipment and ducts is an important issue to address, though. If you’re going to do a vented attic, you’ll want to keep all of that out of the attic. It’s possible do this well with deeply buried ducts, and my contractor friends out West do this a lot. But in a humid climate, burying ducts increases the risk of condensation.
When you go this route, you get another benefit, too. Vented attics are the least expensive way to insulate the top of the house.
2. All exterior insulation on the roof or some on top with fibrous insulation beneath
Second on Kohta’s list is putting all or some of the insulation on top of the roof. This keeps the roof sheathing protected from moisture damage because it stays warm. You can use either rigid foam or mineral wool on top of the roof deck.
If you go with a hybrid assembly, Kohta recommends using fibrous insulation on the underside. You could use open-cell spray foam there, but cellulose or fiberglass would be much less expensive and do the job just as well when installed properly. The key with a hybrid assembly is to follow the ratio rule to make sure you have enough insulation on top to keep the sheathing above the dew point.
With insulation on top of the roof deck, you don’t want any venting at the roof deck. You can put ventilation channels above the topside insulation if you’d like, but don’t vent below the insulation. It will reduce your R-value.
3. Closed-cell spray foam insulation beneath
The third way is to use closed-cell spray polyurethane foam on the underside of the roof sheathing, which creates a conditioned attic. Closed-cell is the best type of foam to use here. It works in every climate because it’s a class 2 vapor retarder, which will protect the roof sheathing from moisture damage. Just be sure to put enough insulation on the roof, get it sealed and insulated properly over the exterior walls, and provide a bit of conditioning for the attic.
There you have it. Insulate the top of the house using one of these methods, and you can be confident that it will work well if you do it properly. When you get into other assemblies, the risk goes up. That doesn’t mean you can’t do them. I have open-cell spray foam on the underside of my roof deck. You just need to be extra careful.
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog and is writing a book. You can follow him on Twitter at @EnergyVanguard.