Is It Bad to Have Insulation Against the Roof Sheathing?
We want insulation in our homes, but can insulation cause a problem? Improperly installed insulation can.
Crawling around attic is not always easy, nor is it my favorite thing to do. You have to find your footing on a ceiling joist that is covered by insulation. Since joists can run in different directions in the various spaces of the attic, you may find it harder to locate a place to balance. Another hazard is ceilings at different heights. I think this is why I see home inspectors stating that the viewed the attic from the landing. That is not always possible.I had a bicycle and boxes blocking access to the main part of the attic in one home. I feel like a gymnast getting around these obstacles. Standing on the landing of one home would have caused me to miss a problem with the insulation. Finding a stud to walk along was not easy, but the view did help.
Once away from the landing, I could see what had been done with a cathedral ceiling. On the landing, the view of the remainder of the attic was limited by duct work and equipment. Moving past the ducts, I saw that there was insulation between the cathedral ceiling below and the sheathing for the roof. Frequently, this space is not insulated, which is awful for energy efficiency. However, this set up causes a different problem. In the photo, you see the attic framing leading up to the peak of the cathedral ceiling below. What you have could be called an insulation sandwich: sheetrock for the ceiling; insulation; sheathing; and roof covering (which in this case is composite shingles). This “sandwich” is attached to the framing that supports the sheathing and the ceiling. The problem arises with moisture. If there is a roof leak, the insulation will be damaged by the water, leaving you with less insulating value.
The problem with the insulation against the sheathing expands. The moisture is held in place by the insulation, which allows for damage to the framing and other building components. This may sound like I am suggesting not to have any insulation in place. The solution is not to ignore the insulation, but to add one component: an air space. By leaving an air space between the insulation and the sheathing, we can have air flow over the framing. If there is a leak, the air flow (ventilation) will help dry out the moisture.
This ventilation offers another benefit. Have you ever been on a roof on a hot day? Hot, isn’t it? The heat penetrates the framing of the home, and that heat makes its way into your conditioned space. This is called thermal bridging. The air flow dissipates moisture and heat. The heat is taken by the air flow to the vent for the attic, thus cooling the attic (lowering your utility bill).
What is the solution? Having baffles installed between the insulation and the sheathing will create the air space. Retro fitting baffles does not work to well. Pushing them down from the top does not work, since you cannot ensure that you will have them go all the way down. Removing the insulation to install the baffles could work, but again not the best option for ensuring full insulation with an air space. Foam insulation may be the best idea, but this can be costly. The old insulation could be removed; baffles can be installed, and the foam can be sprayed in. This can be hard with roofs which are not a straight plane down. I think the best time to have the job done right is when you are putting on a new roof (this includes the sheathing. Insulation with the baffles is easier to install. If you have a cathedral ceiling in your home, you may want to walk around your attic.
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