Frost in attic

This is my first full winter inspecting. The attic I just inspected has a buildup of frost on the plywood and the nails pertruding through have ice on the tips. The frost is sparatic around the sheathing as some areas have no frost. Is this something that would be normal in the winter when the temps are below freezing? It indicates moisture in the attic to me, and that isn’t a good thing.

It’s a very small attic with a Hip roof and ventilation appears adequate.


hows the insulation… inadequate insulation allowing heat to leak up through will cause condensation and freezing/frost… I see this often in the areas of the scuttles/access where the heat leaks up through…

I would agree, insulation is probably not adequate, or the ventilation really is not adequate, thus allowing the frost and condensation. Your right Scott, there will be moisture up there when it melts and starts dripping. Look for older signs, like staining or look for signs of insulation that was once wet. Also, find out if there has been any recent work in this area, as this could be a new problem as a result of this work.

Here is one photo of what’s in there. There is a brand new shiny chimney flue on the roof and through the attic. Could the black be from an old or new chimney leak? Looks like certain smoke damage. And if so, how come it is only on the plywood and not the rafters?

What would be the best way to report this?


my guess would be an ongoing moisture problem… perhaps bath vent exhausting into attic?? but there could be a fungi issue going on as well… I saw something similar a few months ago and it came back positive for a type of mold… cannot tell without testing. definately more evaluation/testing needed.

All the white stuff IS frost. This was my second time in ther in two days and my first time there was no frost. The heat in the home was turned up a few degrees while we were there for an hour and 1/2, and now there is frost. So if the ventilation appears addequate, the insulation is probably the problem, right?

Should I recommend the client get an energy audit done to determine where the heat is getting into the attic from. Also recommend evaluation by an insulation company to see what the best alternative is.

Recommend chimney contractor evaluate fireplace and chimney for any leaks before operating fireplace.



Scott,… Just Note the obvious… “Signs of / history of trapped moisture content in the attic” Current frost condition, water stains, and delamination of the roof decking material. A condition of, or a lack of proper ventilation, insulation or the combination of the two. Recommend a qualified roofing contractor review further for determination and corrective repairs.

I was not referring to the white stuff (frost) as mold… rather the other darker material.

I’m sorry…I didn’t mean to assume you meant that about the frost. I was just pointing out that it was definately frost.

Thanks for your replies. I appreciate the help.

Just a comment – the rafters do indeed have some of the dark staining at their junction with the sheathing. It has a smokey look and the chimney was just replaced, there’s moisture now, possibly mold stain, “Further evaluation is called for.”

John Kogel

Exactly what we found today, along with only 4" insulation, none over hatch, and minimal ventilation!

1907 house.

The last time I saw this there was a plumbing line break in the home with a good amount of flooding. Was there any evidence of recent water damage in the home like a flooded basement?

Nope. No flooded basement.

If the black stuff is mold it indicates long term moisture. I’d guess there is no vapor barrier in the attic. If the black stuff is soot then the chimney or old chimney is suspect. Your job in either case is to report what you see and suggest proper evaluation by the proper trade. It’s intersting but I wouldn’t get too bent out of shape trying to figure it out. Report and defer.

Scott, I seen this alot last week. If the home is not venting properly the moisture will build up in the attic. With the cold temps we had last week there was frost on nails, and anywhere humidty couldnt escape. One thing to do is take a moisture mete in the attic and check the sheathing for moisture levels.
The first pic looks like past fire damage.


Read this document from Canada’s housing agency CMHC:

Here is an excerpt that may shock you:

"What To Do About a Wet Attic

There are many signs that an attic is wet. Prolonged wetness will rot out the roof sheathing. Often this is first noticed when re-shingling. If you have ceiling leaks only in the spring, it may be that ice has been forming on the sheathing all winter and it suddenly melts when a warm spell arrives. You may see water stains or evidence of mold on the sheathing, rafters, or trusses when you are inspecting the attic. You may find the insulation has been packed down or stained by water or ice. The smell of a moldy attic will enter the house under certain weather conditions, usually in summer.

The usual response is to increase attic ventilation. This is the wrong approach. In some cases, adding ventilation will actually pull more moist house air up into the attic and make the problem worse. (I have worked on houses like this)* The best way to fix a wet attic is to stop air movement, or leaks, from the house. Once this is done, the existing ventilation is usually more than enough to keep the attic dry."*

My own instructions are:
(1) clean up moisture sources such as damp basements, don’t store damp firewood indoors, don’t dry clothes indoors, etc
(2) Vent all areas/appliances that produce moisture to the exterior (kitchens, baths, clothes dryers)
(3) Fully airseal at the attic floor level. (this is an energy saver also!!)
(4) You now probably won’t have to add more attic ventilation.

A small bit of frost on nail points/shanks is not a large problem and won’t cause problems…perfection is not needed but management of the moisture. I don’t like adding attic ventilation if there is already some there, I have seen too much snow blow into attics…something that was mentioned in the last week by another poster.

I would say that ventilation has got to be problematic to some extent… possibly compounded by warm air rising up (insulation?)

I prolly would have poked that sheathing with an awl (curiosity) as it is delaminating…
moisture content would be almost irrelevant at time of inspection as it is actively wet.

Roofs/attics rarely have enough proper ventilation… It’s a tough balance but ultimately can save a structure such as this one.


PS: BTW, insulation only slows down and filters air rising to the attic!!!

What am I missing. What does insulation have to do with it? Lack of vapour barrier - yes, lack of ventilation - yes, but how does lack of insulation lead to high humidity in the attic.