Insulating Cathedral Ceilings, First Aid for the Ailing House uexpress

Insulating Cathedral Ceilings

by Henri de Marne

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Q: Our house is 40 years old, with 2×4 walls and 2×12 ceiling rafters. All the second-floor ceilings are cathedral. All are insulated with fiberglass and a vapor barrier, and proper vents were installed. Per the photos, the roof is comprised of two shed roofs. I have two concerns:

— Increasing the R-value in the whole house.

— Most immediate, solving the heat loss/ice buildup on the north-facing roof. It gets little sun and there is a lot of heat loss that causes melting. The three skylights, three vents, crawl spaces over bathrooms and stairwell all show heat loss after a light snow as those places go bare first.

In theory, the roof was built as a cold roof as it has soffit vents at the bottom and top and proper vents along the span. However, the cold roof is not working for all the reasons mentioned, along with probable settling of insulation, incorrect proper vent installation as they are often crushed when insulation is installed, etc.

The question is: Would blowing in insulation between the rafters, while giving us a much greater R-value, still leave us with a warm roof because we removed any cold air flow we now have with the soffit and proper vents and lead to continued, though maybe less, melting and freezing of ice? If a new roof is involved, then of course the roofing material is a question: shingles or metal? I would think metal would allow the snow to slide off readily, particularly if the skylights were removed as barriers, but as it is a pretty long span, I worry about little avalanches sliding off and burying kids and pets. I will be interested to hear any thoughts you have. — via email

A: Your question involves only the roofs, so we wont discuss your desire to improve the energy efficiency of the walls.

Fiberglass, even with a properly installed vapor retarder and ventilation above it, is not the best insulation to use in a cathedral ceiling; closed-cell polyurethane is more effective. Some roofs with that type of foam sprayed to fill the rafter cavities function well as hot roofs, but it is rough on the shingles because they get very hot in the summer when the sun shines on them. Some shingle manufacturers cancel the warranty in such cases, but shingle warranties are difficult to collect and nearly worthless.

Some off-the-shelf foam baffles are flimsy and easily crushed by fiberglass, which, by the way, is unlikely to settle, but more likely to expand. The result is that you really do not have a cold roof, as there is little, if any, effective ventilation. I also do not see any evidence of ridge venting on your photos, in spite of your mentioning that soffit vents were installed at the top; there are no soffits at the top of your two roofs.

It would make no sense to blow in cellulose; it would crush the fiberglass and possibly make matters worse.

You have several options. Since the house is 40 years old, you must have installed a second roof, which may be close to the end of its life. This is probably the best option: Remove the existing shingles. Have a few sheets of plywood removed, most from the top of the roof, to see what is going on with the baffles and the insulation, and check the condition of the plywood, which may show signs of deterioration from years of possible condensation.

If all is well, and if the baffles are pretty much crushed, put the plywood back if it is in good shape, and cover the entire roof with 2-inch-thick rigid XPS (extruded polystyrene) foam. Screw 2-inch by 3-inch strapping vertically 16 inches on center through the foam and onto the existing rafters. Let the tails of the strapping hang beyond the existing fascia by 3 inches in order to install an off-the-shelf continuous venting strip on their bottom. Install new plywood, an ice and water protective membrane on the bottom 3 feet and a new fascia.

Shingles would be less expensive than a standing seam metal roof, but they would not last as long. Snow guards can prevent snow from sliding off the roof.

At the top of the taller roof, install Air Vents Peak FilterVent, an externally baffled ridge vent for such conditions. At the joint of the lower roof to the upper wall siding, install Air Vents Flash FilterVent, an externally baffled ridge vent for such conditions.

Another option is to add 1 1/2-inch-thick XPS or polyiso to the ceilings and cover the insulation with new drywall.

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