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Old 04-02-2010, 10:11 PM   #1
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Room in attic insulation air gap

I have a garage with room in attic trusses. they are 2x8 top chords. there is a portion of the truss which is part of the roof and part of the ceiling in the attic. For this portion of the truss, I intend to insulate with rigid insulation. My plan is to use a 3/4" air gap between the top of the insulation and the bottom side of the 1/2" osb roof sheathing. This will leave 6.5inches of room for insulation giving me an R32.5. I've been searching the web on what minimum dimension this should be and I've found answers between 1/2" and 2". This is a biiig disparity. Sooo, I could go with less insulation to have a bigger air gap. Are there any "pros" that can recommend what the air gap should be? Can anyone point me to any online pages indicating what the minimum gap should be?
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Old 04-04-2010, 04:39 PM   #2
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The recommended minimum air gap is 2" and perhaps more importantly you will need to provide ventilation to the cavity (eaves/ridge vents).

The reason for the ventilation gap is to combat condensation which will occur from time to time, failure to provide adequate ventilation could lead to the onset of timber decay, which would cost you dearly at a later date.

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Old 04-04-2010, 05:06 PM   #3
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"Roofing professionals sometimes specify 2 x 4 wood members as furring strips to create an air
space. Vented nail-base insulation systems also are used, and these products typically provide a
1- to 2-inch air space. In many cases, the air space depth of these preformed insulation products
is not adequate.
The U.S. Army Corps of Engineers' Cold Regions Research Engineering Laboratory (CRREL)
researched ventilation and cathedral ceilings. CRREL determined a roof assembly's R-value,
slope and airway length (ridge to eave distance) affect the sizing of air space depths and intake
As a result of that research, CRREL developed simplified guidelines for sizing ventilation
spaces based on those three variables.
CRREL developed graphs for designers to determine appropriate sizing by referencing the airway
height or inlet (intake) area. Keep in mind, the research's main purpose was to reduce ice-dam
formation on cathedral ceilings. These guidelines may indicate a need for ventilation spaces that
are too large to be practically achieved in new or reroofing projects. These graphs can be found
in the Moisture Control section of The NRCA Roofing and Waterproofing Manual, Fifth Edition."
From: page #11,

I had the Army Report results once but can't find it. Hope this helps.

Be safe, Gary
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