Frost on Underside of Roof Sheathing

Got a call to inspect an attic, homeowner stated he hasn’t gone up there in a long time, when he was up there he noticed the underside of the roof sheathing had frost on it. He recently had a new heat pump, and Aprilaire humidifier installed (in basement). He wondered if that could be the cause.

Its been 20 degrees or less here for a couple days, house is a 3500sq. ft. colonial. He said the HVAC contractor set the Aprilaire and said this is the setting to leave it on.

I was planning on looking for vents improperly terminating in attic, poor ceiling insulation, and poor attic venting.

Anything else I should look for?
Thanks, Steve

not realy. it’s going to be one of those thing your going to look for. most likely attic venting is insufficient. too much warm humid air is hanging around in the attic and just like the side of a cold beer can, it condenced. except now the “warmer” air is hitting a surface that’s below freezing. you may want to discuss the option of a gable power vent set up at one end of the attic in the place of a gable vent.

Thanks Jay.

Seal around all penetrations that may run through into the attic. Ceiling fixtures, pot lights, plumbing vents, weather strip the attic access hatch. Sometime air can be pulled up from under the baseboards and up into the attics.

If there’s a humidifier in the basement I recommend a vapor retarder for the attic floor.

Erol Kartal

I believe you are on the right track, but I would check to see if there is a fold down stair that is not gasketed or an attic hatchway that is not insullated or any of the above mentions from other HI’s, any item that could relate to heat loss and would also check the insulation thickness, ridge venting or gable vents and hopefully supplemented by soffits vents, and check for proper vent installation and if not installed, make sure that the airway is not blocked to the eave vents.
Check to see if there is a discoloration of the wood sheathing wich would indicate a long term problem of the such and could eventually create dry-rot to the substrate.

I would definitely note that there is a concern in the ventilation, insulation and any other relations to this concern.

Hope this helps.

Marcel :slight_smile: :slight_smile: :slight_smile:

good add in Marcel, we can’t think of it all off the cuff.:mrgreen:

Checked out the attic, the homeowner had already reset the humidifer and the frost on the sheathing had disappeared. The sheathing was still damp in areas and the roofing nail points still had water droplets on them.

The attic hatch was not gasketed, the roof is 20 years old, the house is in a heavily wooded setting,there are soffit and passive vents and I think 2 can light fixtures. There were no vents improperly terminated in the attic.

They have a small child, the house was 73 degrees and felt humid. I believe it is/was a combination of not enough attic venting and the setting they had the humidifier set on.

If venting should be 1ft. per 150ft. a rough calculation indicated there are not enough vents.


The HVAC contractor was wrong to tell the homeowner to leave the humidistat alone. As outside temps drop the humidistat setting should be lowered for LESS humidity. If you look at almost any humidistat it will indicate a setting of 35% for an outdoor temp of 20 degrees or above and 15% for 20 below outside temp.

Hi. Steve;

I believe ewBarker is on the right track for his response.

Too much humidification will find it way to a dryer area and combined with the natural moisture accrued by the occupants activities, will find its’ way to the attic through vapor diffusion.

I can elaborate more on this subject, but enough said to make the point, that the attic might be an area of distraction of the real problem.

Marcel :slight_smile: :slight_smile:

I’ve seen attics so humid in the winter, and so poorly ventilated that the condensation built up on the nails so much that it was actualy sleeting in the attic. True story, happened last winter in an attic of one of out building here in good ol’ N.H. crazy man, craaaaaaaazy.

Hi. Jay,

How did you write this one up?


Marcel:) :slight_smile:

I wasn’t on an inspection. it was an attic for a 24 unit 3 floor apartment building. most bath vent for the top 8 apts. were damaged and venting into the attic, years of neglect, dumb *** maintenance techs blocking up the soffet vents. the list goes on. i informed the boss what was wrong, and what to do, from there it’s up to him.

Hi. Jay;

Since it is obvious that you learned something from this experience, I would call that and educational asset to ones experience factor and should be upholded as such.

We are always on the learning trend and never will we be satified with the blaa. in learning. Education is food for the mind and turns to energy when you use it. Isn’t this fun? Nowonder I do not dream of Blondes on the Beach at night. ha. ha.

Marcel :slight_smile: :mrgreen: :wink:

Here Here Brother.

Observed extensively frost in attic at my inspection today. What are the causes and recommendations for repair. No insulation on attic access and only R-13 fiberglass insulation throughout the attic.

Suggest all air bypasses be sealed. Insulation is not the cause.

You got air warm moist leakage .
Could be from attic door or some vent pipes .

Please complete your profile so we know where you are located. Helps us give you answers

I agree with Roy and Paul


Better yet, become a member and you may learn your way to an answer.

Are you asking what is the cause or are you asking what is the source? There is a huge difference.

The cause is dew point.

To fix, you need to know the source.