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Putting in a Dormer

Answered by Jeffrey ~ June 8, 2010 ~ No Comments » | Respond to this question

This spring and summer I'm finally converting the attic and putting in new kitchen cabinets. The big project is the dormer. From what I've gathered online, the framing and rafters are covered the same way you'd do your roof, with plywood, felt, shingles, and flashing. How do you dry-in the room, insulate it, and still maintain adequate ventilation?

Bill C. ~ Sacramento, California

Jeffrey Anderson

Hi Bill, When you say dormer I'm assuming you are referring to one of the large central dormers found in a lot of California homes, and not the two smaller dormers found in Cape Cod styled homes. You are correct, the roofing on these types of dormers is done in the same fashion as the rest of the roof. The dormer is framed in with roof rafters, and that is covered with roof sheathing, felt, and your roofing material.

I am not sure how the area is arranged now and how it is insulated. If the area isn't finished at all up there, there may be blown insulation laying on the ceiling of the floor below the dormer area. You are probably going to try and get as much headroom up there as possible, so in some cases your ceiling sheetrock may be nailed directly to the framing that makes up the dormer roof. Your rafters are more than likely 8 or 10 inches deep, and that is the area where you will be installing your new roof insulation.

I recommend that you go with a batt insulation in those areas, and I would choose an R-30. R-30 batt insulation is available in thicknesses from 8 inches to 9 1/2 inches. If your rafters are 8 inches deep, then you need the 8 inch batts. Roof rafters that are 10 inches can use the 9 1/2 inch batts. In the areas of the attic where you are going to install vertical framed walls due to running out of headroom, so basically around the perimeter of most of the room, I would install R-19 insulation, or you could even install R-30 in those areas if you wanted the extra R factor.

Your concern with ventilation is good, many people don't think about that. There should be vents in your exterior soffits that are framed in along the bottom of your roof rafters. The vents could be round, or they could be rectangles, or the entire soffit could be vented. If you live in a very old house, there might not be any vents at all. There should also be roof vents. They can be on the ridges of the roof, or they might look like boxes sitting on the rear of your roof about 1/3 of the way down the slope. These boxes or ridge vents work with your soffit vents to ventilate your roof. If you don't have any vents in your soffit or on your roof, this remodeling project might be a good opportunity to add them.

When you install your new batt insulation you want to make sure you don't completely block off the area between each roof rafter with insulation. There is a device that goes by different names, I call it a baffle, and it is made out of Styrofoam or sometimes cardboard, and it is installed between the insulation and the roof sheathing. It keeps a channel open between the soffit vents and the roof vents, and allows ventilation in the area. It is very light and easy to install, and just keeps the insulation from closing off the area. You should be able to purchase them from insulation companies around Sacramento, or if you are going to have an insulation contractor do the insulation tell them you want baffles used.

Once the insulation is installed, and you have any inspections required, you can sheetrock the space just as you would any other area of your home.

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