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Cathedral ceilings that breath
By Steve Maxwell
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If ever there was a time of year that proved the need for home ventilation, winter is it. And this is especially true with fully enclosed ceiling areas, sometimes called cathedral ceilings. I know this is a trouble spot in many Canadian homes because of the number of emails I get from readers struggling to understand why water is dripping from their ceilings on warm, bright winter days.
More than 10 years ago I developed an approach to cathedral ceiling ventilation that I now know works well over the long haul. It encourages the movement of air between rafters and sheathing in attic-free roofs framed with lumber as narrow as 2x6. And it does this while also offering the opportunity to add reasonable levels of thermal insulation, too.
The source of the water problem is simple. Without a ventilated attic space, any moisture that sneaks into a cathedral roof framework turns to frost within the insulation there. Several weeks of very cold weather leads to large accumulations of frost that melts on the first warm day. Avoiding this problem -- and the related problem of a roof that ices up during winter -- is what a ventilated rafter design does for you. The issue of ventilation is poorly understood, even by some professional builders. That’s why it pays for you to understand the basics, even if you’re hiring someone else to do renovations.
My approach to cathedral ceilings that breath requires three elements:
1. Site-cut foam spacers and baffles that maintain a 1 1/2-inch deep ventilation channel underneath all roof sheathing.
2. Fiberglass insulation and vapour barrier filling the remainder of the inter-rafter space.
3. Another layer of rigid foam placed over the vapour barrier, fastened to the underside of the rafters with nails and metal washers, followed by drywall or wood paneling.
Step#1: Install Spacers and Baffles
Although it's possible to buy ready-made ventilation baffles to fit between rafters, I prefer to cut my own on site because they work better. Ready-made baffles don’t offer a ventilation channel across the full width of rafter cavities, a feature that I believe is key. See the labeled photo for a glimpse of the baffle and foam spacer that work together.
Step#2: Fiberglass Insulation and Vapour Barrier
At this stage you’ve got rafter cavities that are partially filled with foam, and now’s the time to fill them completely with fiberglass batts. If you’re dealing with 2x8 rafters, go ahead and install R-20 batts; 2x6 rafters should still be able to accommodate R-12 batts, plus the foam baffles that are already there. Apply a polyethylene vapour barrier over the fiberglass, with all joints sealed using acoustic caulking.
Step#3: Add More Insulation
The insulation you’ve installed so far won’t be prone to condensation build up, but it doesn’t offer enough thermal performance, either. That’s why you need to layer more rigid foam on the inside edges of the rafters, over top of the vapour barrier. This may sound like suspicious advice, but it is within code. Specs state that it’s acceptable to have a vapour barrier as far as 1/3 of the way into an insulation layer, based on R-value numbers, not material thickness. This means, for instance, that as much as 1 1/2-inches of foam (for a value of R-7.5) can be safely installed on the inside face of a cathedral ceiling framed with 2x8s and insulated to R-25 (that’s 6 inches of fiberglass, plus the 1-inch thick baffle). You could add even more insulation than this, and probably should, though you’d have to move the vapour barrier further towards the interior surface of the insulating sandwich to do it safely.
If you install gypsum board, intentionally leave a 1/4-inch caulking gap around all framing elements that penetrate the cathedral ceiling. It’s not unusual to have exposed collar ties or beams in buildings of this sort, and these breaches of the air barrier membrane will lead to moisture accumulation within the ceiling unless you take precautions. When you’re all done drywalling, lay a bead of caulk in the gap you left, tooling it with a 3/4-inch diameter dowel. Polyurethane caulking is best because it remains highly flexible, yet can be painted.
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Cathedral ceilings that breath
By Steve Maxwell
|
|
|
If ever there was a time of year that proved the need for home ventilation, winter is it. And this is especially true with fully enclosed ceiling areas, sometimes called cathedral ceilings. I know this is a trouble spot in many Canadian homes because of the number of emails I get from readers struggling to understand why water is dripping from their ceilings on warm, bright winter days.
More than 10 years ago I developed an approach to cathedral ceiling ventilation that I now know works well over the long haul. It encourages the movement of air between rafters and sheathing in attic-free roofs framed with lumber as narrow as 2x6. And it does this while also offering the opportunity to add reasonable levels of thermal insulation, too.
The source of the water problem is simple. Without a ventilated attic space, any moisture that sneaks into a cathedral roof framework turns to frost within the insulation there. Several weeks of very cold weather leads to large accumulations of frost that melts on the first warm day. Avoiding this problem -- and the related problem of a roof that ices up during winter -- is what a ventilated rafter design does for you. The issue of ventilation is poorly understood, even by some professional builders. That’s why it pays for you to understand the basics, even if you’re hiring someone else to do renovations.
My approach to cathedral ceilings that breath requires three elements:
1. Site-cut foam spacers and baffles that maintain a 1 1/2-inch deep ventilation channel underneath all roof sheathing.
2. Fiberglass insulation and vapour barrier filling the remainder of the inter-rafter space.
3. Another layer of rigid foam placed over the vapour barrier, fastened to the underside of the rafters with nails and metal washers, followed by drywall or wood paneling.
Step#1: Install Spacers and Baffles
Although it's possible to buy ready-made ventilation baffles to fit between rafters, I prefer to cut my own on site because they work better. Ready-made baffles don’t offer a ventilation channel across the full width of rafter cavities, a feature that I believe is key. See the labeled photo for a glimpse of the baffle and foam spacer that work together.
Step#2: Fiberglass Insulation and Vapour Barrier
At this stage you’ve got rafter cavities that are partially filled with foam, and now’s the time to fill them completely with fiberglass batts. If you’re dealing with 2x8 rafters, go ahead and install R-20 batts; 2x6 rafters should still be able to accommodate R-12 batts, plus the foam baffles that are already there. Apply a polyethylene vapour barrier over the fiberglass, with all joints sealed using acoustic caulking.
Step#3: Add More Insulation
The insulation you’ve installed so far won’t be prone to condensation build up, but it doesn’t offer enough thermal performance, either. That’s why you need to layer more rigid foam on the inside edges of the rafters, over top of the vapour barrier. This may sound like suspicious advice, but it is within code. Specs state that it’s acceptable to have a vapour barrier as far as 1/3 of the way into an insulation layer, based on R-value numbers, not material thickness. This means, for instance, that as much as 1 1/2-inches of foam (for a value of R-7.5) can be safely installed on the inside face of a cathedral ceiling framed with 2x8s and insulated to R-25 (that’s 6 inches of fiberglass, plus the 1-inch thick baffle). You could add even more insulation than this, and probably should, though you’d have to move the vapour barrier further towards the interior surface of the insulating sandwich to do it safely.
If you install gypsum board, intentionally leave a 1/4-inch caulking gap around all framing elements that penetrate the cathedral ceiling. It’s not unusual to have exposed collar ties or beams in buildings of this sort, and these breaches of the air barrier membrane will lead to moisture accumulation within the ceiling unless you take precautions. When you’re all done drywalling, lay a bead of caulk in the gap you left, tooling it with a 3/4-inch diameter dowel. Polyurethane caulking is best because it remains highly flexible, yet can be painted. |
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