Attic Frost

cougar01 · 01-21-2009, 08:58 PM

I have a standard gable roof without an overhang on a 40 year old colonial. I installed DCI Products SmartVents at the eaves and have a continuous ridge vent. I tore out all of the old mineral wool insulation, air sealed all top plates and penetrations that I could reach, and installed up to R-49 fiberglass insulation being careful to detail everything and without any gaps. Attic is unfinished.

Within the last week, we've had several snowfalls, and there is now about a foot of snow on the roof (which entirely covers the ridge vent). I have absolutely no ice dams anywhere or any icicles hanging. Snow pack has remained on the roof where I see some neighbors melting off and ice hanging from their gutters.

Temps have been about mid-high teens during the day and zero or sub-zero at night. Outside relative humidity steady at about 75-80%.

I am getting frost forming on my west side gable wall sheathing. No other parts of the attic has frost, and the roof sheathing is dry. Double checked, and the top plate along this wall is sealed with spray foam and/or silicone caulk. Wall sheathing is sections of OSB and celotex. The sections with OSB is where the frost is forming. Had the house re-sided with insulated siding and Tyvek. OSB was used to replace damaged sections of the celotex. There is an attic pull-down stairs near this wall. However, the stairs have the styrofoam attic hatch cover over them, and where the stairs meet the framing, I installed foam weatherstrip. I had the wife close the stairs with me in the attic, and with the attic dark, I can see absolutely no light sneaking around the frame, so I can assume that the weatherstrip is doing a failry decent job.

Was wondering why frost is forming only in this area (west facing wall), and can it be caused by the high outside RH coupled with the frigid temps----plus the ridge vent being covered under a foot of snow. Would this affect the ventiation in the attic? Attic relative humidity right now is about 70% which I know is high, but with the ridge blocked, how does the air flow? Plus, it seems to equal the outside RH level.

Was considering placing pink foam board against the OSB and spray foam along edges to studs to make a good air seal against the OSB. Ideas? Suggestions?

THANKS!

Maintenance 6 · 01-22-2009, 03:50 PM

You didn't mention anything about a vapor retarder under the insulation. Is there one and if so, is it continuous? Insulation height isn't blocking the smart vent, I hope. Vapor is probably passing through the celotex, but the glue in the OSB is acting as a vapor retarder. Have you had any issues before the snowfall blocked the venting?

cougar01 · 01-23-2009, 10:09 AM

Thanks for the reply.

I have faced R-19 between the joists, then I put unfaced R-30 perpendicular on top of it. I also installed the "chutes" to prevent the insulation from blocking the SmartVent.

I originally only had 2 large gable vents that we blocked off once the SmartVent and RidgeVent were installed. With the gable vents only, I remember seeing frost on the underside of the roof sheathing all the way across the attic, but with the new ventilation, that problem was eliminated. Now I only get frost build up on the new pieces of OSB on the west facing gable wall.

Wasn't up in the attic prior to RidgeVent being blocked by snow, so can't asnwer that question at this time.

Maintenance 6 · 01-28-2009, 08:00 AM

Make sure your bathroom exhaust is venting to the outside and not just dumping into the attic space. Also make sure everyone is using the exhaust fan. Make sure any gas fired appliances are properly vented to the outside. Basically, make sure that anything going on in the house that produces a lot of water vapor has a vent path, rather than load the interior with excess moisture.

Gary_602z · 01-28-2009, 08:35 AM

Also never-ever go up in the attic and let your wife close you in there!

Especially if she has a padlock in her hand!

Gary

creamaster · 01-28-2009, 09:33 AM

"Now I only get frost build up on the new pieces of OSB on the west facing gable wall."

Just throwing out a guess here but was the osb put up recently? If so it may still contain moisture from the manufacturing process and maybe that is why it is the only portion that is frosting up.