Frost inside attic question....

I got a call today for a roof tear off and replacement. I go out and the guy takes me in his attic and the decking looks good except for in 1 area....it is kind of darkening. He goes on to tell me his house was built with plenty of soffit vents but no roof vents, so he had them added 10-15 years ago. I figured the darkening was from before he had good venting.

Then he tells me he was up in his attic back in the winter and the dark area had frost all over that decking..and ice hanging on nails that came thru shingles. I ask if it frosted everywhere and he said no, just one part of the back side of home.

This house is a ranch home...with a 6/12 roof and over 18" of unfaced insulation in attic. It is about a 2000 Square foot living area home and has a bunch of good, unblocked soffit vents and 8 roof vents.

What causes this frost in attic? Could it be because it is so well insulated?

Thanks!

Are the upper attic/ridge vents clear/unabstructed throughout the season?....If they're covered-up under snow/ice all winter....There's your likely culprit.

For a quick fix to see if the brick chimney mortar lines or counter flashings are to blame, get a roll of shrink wrap and shrink wrap the chimney from the roof line upwards to the top. I've done this a number of times to prove to a customer that the mortar lines are leaking and not the roof itself.

OK, if the pointing around the chimney bricks is not great, then it is possible for the water to enter the chimney at or above the flashing, run down interior channels or cracks, and then re-emerge through another set of cracks to the outside. If that moisture was present in the chimney when it was it use, then it would drive the water to the outside, and into the attic. Yep, that looks like a scenario that could work. Corrective action would be 1) remove the old flashing and repoint the bricks, then 2) install new flashing and counter-flashing.

To test whether this is really the problem you could pour water on the outside of the chimney and monitor what's happening at the chimney below the roof deck. If water starts coming in, you've got your cause. However, if the cracks are small, it may take a while for the water to work its way down. Good luck.

Thanks Guys...there is a vent that has leaked in the past and I noticed a chimney on the exterior wall of the house down near the soffit, I would almost bet the moisture source is the chimney! I noticed when inspecting the roof that the chimney flashing was nasty.

Could that explain why it is getting moisture to frost in that area above the chimney?

Thanks again...you guys are pretty sharp cookies!

That's exactly what I was thinking but could'nt figure out how to write it down (just kidding). :laugh:

Nice one Paul.

If you’re getting frost in only one part then there’s got to be a source of moisture that’s making that pattern. So the issue is to figure out what the source of moisture is. Under “normal” circumstances, a well ventilated attic should see a constant flow of air (from the soffits to the exhausts), moving by convection. When the temperature is at the freezing point, the moisture content of air is very low, and therefore, it is essentially dry air that is entering the soffits. As it is warmed by the heat loss through the insulation, it expands and moves upwards towards the top of the roof. Any source of moisture will usually be dried out by this movement of air, UNLESS the amount of moisture is sufficient to cause saturation and condensation when the warm air meets the cold decking or nails protruding through the decking.

Typical sources of moisture that I’ve seen are leaky air ducts, improperly vented bathroom vents and plumbing vents, and ceiling pot lights that are installed through the vapor barrier and are not insulated. Occasionally, the source of moisture is leakage from the outside, especially along sidewalls and improperly-installed valley flashings.