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Cathedral Ceiling Insulation and Ventilation

Many of us have cathedral ceilings at the cottage and I've found that proper installation is more important than most of us might think. In a building that sees occasional heating, water vapour can cause problems like mould and rot more than in a building that is constantly heated. I'm not a carpenter by trade but I like to build and learn as much as I can about a project before I start. Here's what I've learned about cathedral ceilings in a northern climate.


Why do we have to treat cathedral ceilings differently?

Other than the obvious visual difference, a flat ceiling differs from a cathedral ceiling in that it usually provides some air space throughout the attic above the insulation that sits on the ceiling. Cooler air comes into the attic through the soffit vents and warmed air exits through the roof vents. Roofing and ceiling finish in a cathedral ceiling will seal off the space in between each of the roof joists, leaving only the bottom opening of each bay exposed to air flow. Air flow is what moves heat and moisture out. It can also cool the space directly under the shingles, extending their life. A hot attic isn't going to let much of the heat from inside the building escape.

In order to introduce air flow through the joist bay, we need a place to let air in and a place to let air out. A vented soffit at the bottom of the roof will let air in, while a continuous ridge vent at the top will let the air out. Whirlybird-type attic vents won't work here, since they will only let the air out of one joist bay. A continuous ridge vent will let air out all the way down the ridge of the roof and keep the rain out as well. You can install a ridge vent on top of an already shingled roof but keep in mind that you need to allow air to enter from somewhere. Installing a ridge vent could actually cause more problems if you don't have some air flow.

But that's not all there is to it! Air flow is good, but what happens when we stuff a bunch of insulation in that bay and choke it off? Not only have we cut off the air flow, we now have fiberglass batts up against a cold roof. What happens when you head out for a weekend at the lake, turn on the furnace and have a great time with the family inside for a couple of days, turn off the furnace and leave for a week? Here's what happens in your roof: the bottom of the insulation warms up to 20 degrees Celsius but the top stays at -20. The temperature in between the two surfaces is a gradient but at some point it reaches zero, the point where water (and water vapour) freezes. Add to this the fact that the humans inside the building are emitting a lot of water vapour just by breathing, let alone the cooking and showering. Now you have moist air moving through your insulation and, at some point, freezing the insulation. Now you have moisture being held by the insulation. In spring, it will melt and maybe even drip through your ceiling finish and onto the floor but probably won't completely dry out without proper air flow. Sounds like a recipe for mould, reduced effectiveness of your insulation and even rot.

The solution, obviously, is to open up that air flow and to keep that moist air out of the insulation. To keep the air space open, install some baffles like Raft-R-Mates - styrofoam baffles that can be stapled into place and will hold the insulation away from the roof and provide 1" of air space. Sure, you're giving up an inch that could have been used for insulation but properly vented R-28 is going to be more effective than wet, choked off R-32 so in this case, less is more.

Notice that these are a one-size-fits-all type of thing, designed to fit between 12" and 16" bays. If you feel that the entire width of the bay should be open, which isn't a bad idea, you can build your own baffles as seen here:

That takes care of opening the air space but we still need to keep that moist air off the insulation. This is done with standard 6-mil poly vapour barrier and accoustical caulking sealing any seams and joints to the vapour barrier on the walls. Vapour barrier is another science in itself, I won't get into it here.

A 2x10 roof joist is 9.25" but the baffles are going to take up an inch, leaving 8.25" for insulation - R-28 for fiberglass batts. You might want more insulation than that in your ceiling and one option is to install styrofoam against the bottom of the joists, before your finished ceiling. Styrofoam is it's own vapour barrier but I'm still not sure if I would skip using a poly vapour barrier since it is easier to work with and seal. There is a rule about having a vapour barrier embedded in your insulation though - it can be no more that 1/3 of the way in from the warm side and this is not measured in distance but in R-value. In the case of a 2x10 roof joist with R-28 batts, this means no more than R-14 can be on the warm side of the vapour barrier. You might be thinking about those hot, humid days now and how there's no way of keeping that humid air from travelling through your joist bay. On the other hand, unless it's raining outside, inside the building is more humid than outside so this approach is trying to deal with the biggest moisture source. Air flow through the top of the joist bay will dry out the rest.

In the end, this is a multi-step solution and all the steps work together. Provide a place for air to come in via soffit vents or even air space though an adjacent (vented) attic. Install ridge vents for the air to exit. Make sure you have some space blocked off for that air to travel through your roof system. Finally, do your best to keep water vapour out of your insulation in the first place.