The purpose of my Building Science Term of the Week feature is to provide a basic definition of a term you may have heard without going into all the whys and wherefores. Last week I probably went a little overboard explaining grains of water, so today I’ll give you just the basic facts about what’s sometimes called the ratio rule. If you want to go deeper, I’ll give you some links to other articles on this topic.
The ratio rule I’m discussing here applies to hybrid roof assemblies with two kinds of insulation, some of it above the roof deck and some below. Typically, the insulation above the roof deck is rigid foam, but it could be mineral wool or some other type. The diagram below shows the configuration. The ratio rule tells you what percentage of the total R-value of insulation you need to put above the roof deck.
The reasons for the ratio rule is that you can’t just go putting any combination of R-values above and below the roof deck. Well, you probably can do it in warmer climates (zones 1 to 3), but the colder the climate, the more you have to pay attention to the ratio of above- and below-deck insulation. Why? Because of the temperature of the roof deck in winter.
If you put all the insulation on top of the roof deck, you have no problem because the the roof deck stays nice and warm. If you put all the insulation below the roof deck (and do it properly), again, you have no problem. The roof deck stays cold, but you keep the water vapor from inside the house away from the roof deck.
So, insulation on top raises the temperature of the roof deck and insulation below lowers the temperature. The science behind the ratio rule is to keep the roof deck temperature high enough that it doesn’t accumulate enough water to cause moisture problems. Yes, there is actually science behind the ratio rule. But the numbers they settled on in the model code are a combination of science and politics.
The table above shows the ratios needed. If you’re doing the the code minimum insulation for a roof (the middle column, Rtotal), you don’t need to worry about the percentage. Just make sure you have the minimum R-value required (second column, Ra) above the roof deck. The percentages (last column) apply when you’re doing an insulated roof with above-code total R-value.
One more note about this kind of hybrid assembly. The model code around this issue is about air permeable insulation (e.g., fiberglass, mineral wool, cellulose), but the same science holds for air impermeable, vapor permeable insulation. That is, open-cell spray foam, as shown in the lead photo above.
That’s probably the shortest explanation of this topic you’re going to find anywhere. If you want to go deeper, my first recommendation is Joe Lstiburek’s article, Hybrid Assemblies. If you want even more (and are a GBA Prime member), Martin Holladay has covered this topic extensively in several articles. A good starting place would be Combining Exterior Rigid Foam With Fluffy Insulation. (And be sure to check out the comments, especially anything from Dana Dorsett.)
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and the founder of Energy Vanguard. He has a PhD in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.
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