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What are best "best practices" for insulating cathedral ceilings

What are best "best practices" for insulating cathedral ceilings


Old 03-14-08, 10:44 AM
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Join Date: Nov 2002
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What are best "best practices" for insulating cathedral ceilings

It is easy to see why the home owner gets confused. The "best practice" for insulating a cathedral or vaulted ceiling depends on whether it comes from the fiberglass or the dense pack cellulose or the foam people. The fiberglass people give careful instructions on the proper venting, and the dense pack cellulose and foam people say no venting needed. A Google search for "insulating cathedral ceilings" gives all these varying recommendations including government studies over the past 10 years. Also these studies show that the color of the shingles to keep a roof cool is much more important than the ventilation at least for a cathedral ceiling. It is not clear that the agencies that recommend code have made specific recommendations based on the latest practice and studies. So what is the best way to insulate? Further, if structurally insulated panels are used, would anyone recommend that they have venting grooves?
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Old 03-14-08, 11:27 AM
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Location: VA
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Foam would be the best because of the high R value and air sealing capabilities.
Old 03-15-08, 07:58 AM
Join Date: Nov 2001
Location: USA
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Foam is a very good insulation when dealing with cathedral ceilings. Structural Insulated Panels (SIPs) is probably the best of all the foam products. The grooves that you are referring to are spacers. When plywood is placed over the roof side of the SIP these spacers provide a nailing surface and an air gap between the SIP and the plywood. Air can then travel through vents from the eaves to the ridge between the SIPs and plywood. This application is known as a "Cold Roof".

The reason for this is known as "Attic Ventilation". Which is a misnomer. Attic ventilation actually ventilates the underside of the roof. It does so to provide a by-pass for the low vapor permeability of roofing materials. Keep in mind that roofing shingles are designed to keep moisture from entering the house. This same characteristic of shingles also prohibits moisture that manages to get through the SIPs from leaving the house. So a more accurate term for Attic Ventilation is "Roof Ventilation".

While it is true that light colored roofing materials do lower roof temperatures what is clearly overlooked with these positions are the characteristics of insulation. The heat flow resistance of insulation results in retention of heat inside the insulation. When insulation is placed in close proximity of roofs, even with Roof Ventilation, those roofs will have a much higher temperature than roofs that did not have insulation in such close proximity. Furthermore these roofs will remain at high temperatures for a much longer time after the Sun goes down than the other roofs that do not have insulation in close proximity.

This situation becomes even worse when no ventilation is provided for cathedral ceilings. Usually the first sign is the excessive loss of shingle granules. The sole purpose of these granules is to reflect ultra-violet light. When you lose the granules the shingles degrade rapidly. This is caused by the exceeding the manufacturer's recommend temperature of their product. The lack of ventilation and the proximity of the insulation to the roof causes this high temperature. This temperature causes the shingles to expand more than what they were designed for, which causes the granules to pop off. There are quite a few other aspects to this that will not be discussed.

There are a few ways to insulate and ventilate a cathedral ceiling.
Old 03-15-08, 06:34 PM
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Join Date: Dec 2005
Location: VA
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Closed cell spray foam will be a radient barrier. Cool roof Cool attic
Old 03-16-08, 01:13 PM
Join Date: Jan 2008
Posts: 70
resercon- would this product also work both on the inside of the cathedral ceiling as well as on the roof?
What if a person just had the roofing board as both the roof and the ceiling- would this work?

We have 1" of foam covered with wood paneling on our cathedral ceiling. It does take out most of the summer heat. And we have no moisture problems. I see your response about heat buildup on the shingles with insulation directly under the roof. I am thinking of putting this product on the ceiling- would a combo ceiling and roof application work?

Last edited by rpatzer; 03-16-08 at 03:25 PM.

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