I had a roof repair done this summer. A portion of new decking, underlayment and shingles.

Its been frigid here and the roof is covered with snow and I was in the attic and noticed the new nails that penetrate the decking visible from the attic are frosty. Old nails in other portion are not.

Is this something that I should be concerned about?

 

No, it's perfectly normal.

 

The frost does indicate moisture in the attic so some air sealing should be considered. What type of attic venting is being used? If a ridge vent, sometimes the new roof gets installed right over the vent and they forget to cut it back.

Have you or anyone made an effort to seal off air leaks into the attic. Bath fans vented to the outside and not the attic or soffit are a must. Attic hatch or stairs along with recessed lights and space around the chimney or plumbing vents are other major paths.

Bud

 

Theres a few roof vents and they are not obstructed. No bathfan in use. So your saying its caused by warm indoor air infiltrating the attic?

How come the old nails don't have frost on them?

I like this explanation best, but please elaborate.

 

Although a little moisture in the attic is normal, it is not good. Sometimes there are specific areas that show more moisture than others (or mold) because the source is located directly below. As for your old vs new nails it is hard to get specific, perhaps aluminum nails vs galvanized. Al conducts heat 5 times better than steel. But regardless as to why, the moisture is there.

There is a suggested amount of ventilation, 1 ft² of net free vent area for every 150 ft² of attic floor. That can be cut in half if the ceiling plane has been well air sealed. A typical ventilation arrangement that meets this standard would be continuous soffit venting combined with a full ridge vent. A few roof vents doesn't sound close.

What was the reason for replacing the portion of roof decking? Was it moisture related? What living space is below those frosty nails?

Bud

 

I'm guessing the smooth surface of the newer nails better transfers cold from outside into the attic. Older nails are probably covered in a layer of rust. It's coarse, irregular surface traps a boundary layer of air which acts as an insulator. This is coming from a bald guy who recently grew his first beard. Even a days stubble on my head is noticeably warmer and the same for my face. It's amazing the difference a days stubble can make. It's not long enough form a protective fur coat but the stubble is enough to slow down air movement at the surface.

 

Elaborating takes more time. LOL

Are the frosty nails on the shady side of the house and the non-frosty nails on the sunny side of the house? That would explain the entire thing right there. The sun melted the frost off the other nails. They might ALL be frosty if you go up there at 7AM.

It could also be the difference in ventilation in different areas of the attic.

It could be something to do with the type of metal used in the nail. Some metal alloys conduct heat better than others. If the nails are right next to one another on the same side of the roof, I would say this was the explanation. If the old nails with no frost are all dusty with rust, that might have something to do with it too.

It's just not surprising to find a little frost in an attic, especially on the nails. The moist air from the home is going to condense on a cold surface. If it isn't the nails, it will be the bottom of the roof sheathing. Light frost is not usually that big of a deal, unless it is building up and getting thick, then like Bud said, it is indicating a bigger problem. A lot of light frost forms and melts daily as the temperature in the attic changes and the dewpoint goes up and down... except on the shady side of the house where the roof deck (and nails) stay cold. When the temperature and the dewpoint cross, you're going to get condensation or frost. It's bound to happen.

 

THANKS guys for the explanation.

Budd, just to clarify there are surely soffit vents as well in my attic. The repair was not moisture related either, I had removed an inoperable attic fan and had the hole repaired. The frosty nails are right above the attic access hatch.

I don't know about the sunny side vs. shady side.... this is up towards the ridge if its any consolation. I think theres a ridge vent too, I see a little bit of frost right up there at the ridge as well.

 

Sounds like you should look ar some air sealing options for the attic hatch. For every cubic ft of air that leaks out a similar volume must be leaking in, usually in a variety of little places. Seal one and you reduce the other.

Enjoy
Bud