Building Science Fundamentals: Roof, Part 1: Ventilation
By Dr. Joseph Lstiburek
Dr. Joseph Lstiburek talks about the not-so-controversial ways to maximize the efficiency and airflow of your roof and attic.
There’s been so much stuff said about roofs that you sometimes lose perspective. I’m going to start off by saying what might seem controversial but really shouldn’t be. This is a vented attic, and it’s probably one of the most unappreciated building assemblies we have in the history of building science. It’s beautiful. It’s hard to screw this up. For 20% of the effort, it gets us to 80% of optimal performance, and it works in hot climates, in mixed climates, the Arctic, the Antarctic, the Amazonian rain forest — it works absolutely everywhere. The value proposition of a vented attic, meaning the money that you invest in building one of them — it’s hard to argue with the benefits. But for all kinds of reasons, we manage to screw it up. The single most important thing you have to remember about a vented attic is that the ceiling plane — the gypsum board layer, the drywall layer — needs to be airtight.
1) The ceiling plane MUST be airtight. Absolutely airtight. Above the airtight ceiling plane, the only thing that should be seen is insulation and air, nothing else. Not last year’s Christmas decorations, not your high school prom dress, not the tuxedo you were married in and can no longer fit in. Nothing but lots of insulation and air. Just an airtight ceiling and nothing else.
2) If you’re going to vent the roof, then VENT THE ROOF. If you’re actually going to vent the roof, let’s be serious about venting the roof. Wash the underside of the roof deck with air. That means the entire perimeter of the roof needs to have air inlets, meaning continuous soffit ventilation. It’s dumb to have baffles every third or fourth bay, the entire underside of the roof deck should be washed. Where the air leaves isn’t as important — whether it’s a ridge vent, or mushroom caps, or gables. What’s important is that you have continuous air entry at the perimeter of the roof down low.
3) Put more vents down low than up high. This is where the code tends to have it wrong. You want more entry points at the perimeter than exit points at the top. People say you want to balance the lower down ventilation with the upper ventilation, and a lot of people interpret the codes to say that if you get it unbalanced you want more ventilation up high. That is absolutely wrong; you don’t want more places for the air to get out than to get in. The reason is, if you construct a house with a leaky attic ceiling and you have lots of ridge vents or you have lots of vents up high, the makeup air is going to be pulled from the house rather than being pulled from the outside. That scenario is a disaster. Attics should be ventilated with air from the outside, not the inside. That’s why I hate these whirligig turbine vents — because they depressurize the attic, and if your attic ceiling isn’t perfectly airtight, you suck air conditioned air or heated air out of the house. It’s even crazier when the powered attic fans can actually suck on the roof and they’re controlled by a thermostat. How stupid is that? Of course the attic is going to be hot. You turn them on and they suck all the air conditioned air out. No powered attic ventilation; more vents down low than up high; wash the entire underside of the roof deck. But all of that is secondary to having the ceiling plane airtight. This last tip is more important in cold climates than anywhere else. But where the ceiling insulation hits the perimeter wall, you don’t want the amount of ceiling insulation on the top plate to ever be less than the R-value in the wall itself.
4) Put more insulation on top of the wall than inside it. In other words, if you have an R-20 wall, you want at least R-20 on your top plate. A higher R-value is better, but a lower R-value is not. If you have an R-30 wall you want at least R-30 on top of your top plate. A reasonable rule of thumb is: Thou shalt never, according to Joe’s Rule of Thumb, have less R-value on the top of your top plate than in the wall. It would be nice to have even more, but not less. Notice: nowhere in this discussion did the term “vapor barrier” come up. If you really want to have a vapor barrier in the ceiling, limit it to climate zone 6 or higher, but that’s really not important compared to the airtightness of that ceiling plane. The building code calls for a vapor retarder in climate zone 6 or higher. It’s okay to put one in, but if you don’t, take a Valium and relax. You don’t want to go through a lot of brain damage in a renovated house to try and add a vapor barrier underneath insulation in an attic. What you really want to do is make that ceiling plane airtight, make it airtight, declare victory and be done. Don’t mess around with permeability’s and calculations and whatever. To recap, airtightness on the ceiling; washing the underside of the roof deck; unbalanced ventilation should be in favor of the lower vents because you don’t want to depressurize the attic; no to powered attic ventilation or the whirligigs; and you don’t want to squeeze the insulation at the perimeter so it’s less than the R-value of the wall. That’s it. You can build that everywhere in the world and life is good.
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