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by CareyBot

Q: I read your recent article on adding insulation to a bare-beamed ceiling and I have a question about insulating the ceiling between the beams.

From what I read, it seems that no ventilation is needed between the roof sheathing and the R-13 batt insulation. Is that correct and, if so, is it because it’s a flat roof?

I have what I think is a similar situation. I have an attic dormer loft that we would like to finish. It has 2-by-6 roof rafters and a ceiling height of just over 6 feet at the peak. It would be great if I could put the R-13 insulation in and drywall it without allowing for ventilation between the insulation and roof sheathing. But I’ve heard that that can cause moisture problems that could lead to rot.

This loft area that we would finish off is about 10 by 16 feet. We would use this area as a sitting room to read or have a glass of wine. There is a window that looks out onto the ocean, where we can see the sunset.

A: Let’s see if we have this right. An attic loft equipped with a dormer overlooking the Pacific Ocean. You’d like to convert it into a sitting room so you can have a drink while watching the setting sun. What’s not to like? Although we’d probably opt for a nice single malt rather than a glass of California chardonnay.

Insulating your loft area is straightforward and something you could certainly do yourself. We do wonder whether the rest of the attic is insulated. If it’s not, you might consider doing the whole megillah. For now, we’ll just stick to the dormer area.

Your point about our failure to mention ventilating the rafter bays in our article is well taken. Without air movement, batt insulation could breed condensation. That’s not a good thing.

In that column we responded to a reader’s question about insulating an open-beam ceiling without disturbing the existing roof covering. We suggested that he frame 2-foot-wide openings between the 4-by-8-inch beams to accept fiberglass batts, then screw drywall to the framing. After the "rock" is finished, the result is a newly insulated and attractive ceiling. But we inadvertently left out the step of drilling a series of 1 1/8-inch holes in each block and installing metal grills at the wall and ridge of each bay to allow for air circulation.

Berkeley, Calif., architect Tim Rempel also picked up on our omission. He suggested that rigid foam or isocyanate foam insulation instead of fiberglass batts for this application. In retrospect, we agree that a better solution is to use rigid polystyrene or polyurethane insulation board.

The rafters of your dormer are probably no more than 24 inches apart, so they will support 1/2-inch drywall. This eliminates the need for additional framing. Using rigid insulation board eliminates the need to provide ventilation for the rafter bays. That’s the route we suggest you take.

Rigid insulation board carries an R-value of between 4 and 8 per inch of thickness, depending on the type of board you use. R-value is a measurement of the resistance to heat flow from conditioned space to unconditioned space. Fully insulating the rafter bays of your dormer with rigid foam board should provide R-20 to R-40 of protection. Foam insulation board is lightweight and easily cut with a handsaw. It can also be scored and snapped with a utility knife in the same manner as drywall.

To install it in the rafter bay, simply cut the panels to fit and press them into place. Seal the edges of the boards with caulk and the joints with tape. Aluminum-coated tape used for heater ducts works well for sealing the seams.

If you can’t find 5 1/2-inch-thick board to fill in the space, it’s OK to stack pieces. For a more detailed discussion of rigid insulation, go to links.sfgate.com/ZZM.

Once the foam board is in, drywall the ceiling in the customary manner. This step is critical because the foam board is flammable and the drywall serves as a fire barrier.

When your attic sitting room is done, we wish you many a cozy sunset. We’re envious.

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