Currently i am removing all the sheetrock from the upstairs due to someone doing a really bad DIY job. The rafters are 2x6 and stuffed with kraft faced fiberglass. There is no soffit vents or roof vents in this home. (shingles all look good no signs of damage from hot roof deck). Anyhow i would like to add 1.5 inches of rigid foam insulation to the interior of the exterior walls and roof rafters while leaving the fiberglass insulation in place then going over it with drywall. I am looking for some insight as to any potential problems this may create. I would rather not have to remove the fiberglass and try to install soffit vents if i dont have to. Thanks for your input and time.

First, the purpose & function of roof deck venting is to purge moisture, not to cool it off and save the shingles. The roofing is highly impermeable to water vapor, and under temperature and humidity conditions where the roof deck dwells for long periods below the dew point of interior air (say, wintertime in a cold climate) moisture will accumulate to mold or rot levels in the wood unless it is allowed some drying path. The severity of these is issues is clearly climate dependent. Got a zip code?

The affect of roof deck venting on shingle temp while measurable, is less consequential than the color of the shingles or the side of the house. Vented or unvented, the primary cooling path for the shingles it direct radiation back at the sky, followed by convection of the exterior air that is in direct contact with the shingles. The shingles are thermally isolated from the interior/attic or vent-channel air by about R1 of wood, making the thermal effects of that presence or absence of ventilation air only a secondary effect.

Is there an air gap between the fiberglass and roof deck?

Are there any signs of mold or moisture staining on the roof deck where the fiberglass has been installed?

Posted By Dana1 on 21 Oct 2013 03:33 PM
First, the purpose & function of roof deck venting is to purge moisture, not to cool it off and save the shingles. The roofing is highly impermeable to water vapor, and under temperature and humidity conditions where the roof deck dwells for long periods below the dew point of interior air (say, wintertime in a cold climate) moisture will accumulate to mold or rot levels in the wood unless it is allowed some drying path. The severity of these is issues is clearly climate dependent. Got a zip code?

The affect of roof deck venting on shingle temp while measurable, is less consequential than the color of the shingles or the side of the house. Vented or unvented, the primary cooling path for the shingles it direct radiation back at the sky, followed by convection of the exterior air that is in direct contact with the shingles. The shingles are thermally isolated from the interior/attic or vent-channel air by about R1 of wood, making the thermal effects of that presence or absence of ventilation air only a secondary effect.

Is there an air gap between the fiberglass and roof deck?

Are there any signs of mold or moisture staining on the roof deck where the fiberglass has been installed?

My zip code is 67575. I removed a few batts and there is no signs of moisture, condensation, or mold anywhere. The batts are directly against the roof deck and there ia no vapor barrior of any sort except the craft paper on the batts. The roof decking is not plywood it is 1x boards with about 2 inches of space between each one covered with roofing felt then the shingles. Shingles are a dark brown in color if that is any help. Thanks

Putting the batts in direct contact with the roof decking is a code violation. The kraft facers are a "smart" vapor retarder that becomes more vapor-open when wet, more vapor-tight when dry, which probably helped, but no doubt about it, closing it in with more R on the interior side would make it more susceptible to moisture in a 67575 is in US climate zone 4 climate.  If instead you put R15 or higher on the EXTERIOR (between the structural roof deck and the shingles) you would then be able to safely leave the existing batts in place. See:

http://publicecodes.cyberregs.com/i...sec006.htm

The reason that it's in issue is that when you  add rigid foam-R to the interior the average seasonal temp of the roof deck will drop, making it retain on more moisture than it does currently.  The north facing pitches are by far the most susceptible, so if the part you peeled back was on some other side, see if you can't find the shadiest north-facing corner to take a peek, since that's where the problems would be.

If you still want to add interior side foam, the safest thing would be to remove the existing batts and replace them with unfaced R15 rock wool (or unfaced R15 HD fiberglass), so that you would have a code-legal air gap for the deck to dry into, and put 2-4" of unfaced 1.5lb density (aka "Type-II") EPS on the underside of the rafters, carefully  air-sealing it at the seams with fiber-reinforced duct mastic. (EPS shrinks with age, and will open up gaps if you don't use something that glues it agressively.)   At 4" it will have a vapor retardency of less than 1 perm, but over 0.5 perms, which allows the assembly to dry toward the interior when necessary, but limits the rate at which moisture from the conditioned space accumulates in the cold roof decking in winter.  At 2" it'll be about 1.5 perms, which would still be good enough. If you only have room for 1", use XPS (~1.2 perms @ 1", but far more envirnonmentally damaging than EPS.)

An alternative which doesn't meet the letter of code, but still works is to pull the batts, apply 1" of closed cell spray polyurethane against the roof deck, re-install the batts (compressing them is OK), then put 2-4" of EPS.  An inch of  spray polyurethane is between 1-1.5 perms, and doesn't wick moisture during condensation events, and is by-itself sufficiently protective in your climate, and 2" would be even better (but at an extra buck a square foot you may want to stay at 1".)  Take the time to read and understand this before proceeding.