Question

Should I leave an air space between my attic insulation and the wood roofing?

Asked by D.L. Bennett, Arlington, VA

When insulating under an inverted V–shaped roof, is it important to leave an air-space between the insulation and the wood roofing? I want to use denim insulation that fits more closely to the wood than paper-covered insulation. Would you recommend using some sort of spacer? There are no attic vents at the eaves—if that matters.

Answer

Mike Binder

Answered by Mike Binder

Chevy Chase, MD

Meditch Murphey Architects

March 5, 2009

An air-space between the insulation and the bottom of the roof sheathing is desirable.

  • In the winter, humidity from the house's interior will get past the insulation and come in contact with the cold roof sheathing, where it will condense into water.
  • Without a proper air-space to allow the insulation and roof to dry out, this water will collect, potentially leading to rot and mildew. Mildew can be very dangerous to your health.

There are special attic vent baffles designed to hold this air-space open. They are installed in the rafter bays, where ceiling joists meet the roof rafters (see Easy2DIY for an illustration).

However, the lack of eave vents in your attic may be a problem. Vents at the eaves allow air to circulate in the attic. Even with baffles installed to create an air-space, you also need air flow to help moisture dissipate.

Recommendations

From the sound of it, the roof has already been framed, and that limits options.

Here are four approaches I recommend you investigate for insulating your attic.

  1. Apply a 2-inch layer of spray-foam insulation (R-10 to R-12) directly to the interior surface of the roof sheathing. This will keep the surface warm and help minimize condensation. You can then apply batt insulation around the foam without worrying too much about the air-space. Generally, spray foams have to be applied by a professional. For this application, I recommend a closed-cell foam. There are several water-blown, soy-based insulation products on the market, including Emega 1-lb. and 2-lb. foams and BioBased's 1701 foam. Prices (including installation) run about $0.85/board-ft. for 1-lb.-density foams and $1.75/board-ft. for 2-lb.-density foams. (A board-foot is one square foot in area and one inch thick, so for a 2-inch layer you must double these prices.)
  2. Apply a continuous vapor barrier (such as CertainTeed's MemBrain) to the underside of the insulation and make sure that it is well-sealed. (The barrier should face down, sealing off the warmer living area below, leaving the insulation exposed on the attic side.) This will help ensure that little or no moisture can get from the inside of the conditioned space to the roof sheathing.
  3. Install eave or soffit vents (typically an accessory to whatever siding or exterior cladding system you are using), retrofitting the existing framing as needed, and install air-space baffles to properly vent the roof. You'll be glad you did in the long run.
  4. If you are planning to replace your roof, you might consider adding exterior insulation at the same time. Apply two inches of rigid foam insulation board to the top of the roof sheathing. Roofing membrane or 30-lb. felt (for air-barrier and water-shedding plane) then goes on top of this exterior insulation before installing roofing materials such as shingles or standing-seam metal. Care should be taken to properly seal the edges of the air-barrier and flashing against any leaks. (The addition of exterior insulation may significantly complicate the installation of roofing material. Consult your roofing contractor or the roofing-material manufacturer to ensure that the proper installation procedures are observed.) Two inches of rigid insulation will generally cost about $1.50 to $2.00 per square foot installed (depending on contractor and trade discounts), but keep in mind that the project may raise the labor costs of roofing material installation.

Whatever you decide, for the mid-Atlantic region you'll want a total R-value of R-38.

 

For more information:

To learn more about insulation, moisture control, and ventilation, consult the DOE's Energy Efficiency and Renewable Energy website.

Tagged In: insulation, spray foam, vapor barrier

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