I inspected a home yesterday morning that had a light coating of frost on the roof sheathing near the gable vents in the attic. It was about 15 degress F. the night before. The attic had sufficent ventilation. I have not run into this before. Can this happen when it so cold? Any experiences with light frost?
Thanks
Yes when its cold and sunny the attic has warmed up .
If no wind the Air cools of and the attic is still warm and the humidity collects ( same as on a cold beer ) then it freezes.
Tomorrow sunny again it will be gone . See it some times .
If it is a heavy frost look for air leak from the home
Roy Cooke
Because the vents are open to the outdoors, it is colder than elsewhere. The moisture of the air in the attic coming from the heated house below frosts there because it is colder, longer and there is also more concentrated air movement (thus more moisture passing by) at the vent location.
If it gets colder, longer the whole attic may frost dependant on the saturated vapor in the attic air.
That is what I thought.
Thank you again.
It can happen however I am curious, how did you qualify the attic ventilation as adequate?
Does it matter what kind it is Roy. If not have a cold one on me. ha. ha.
You are definitely right in your prognosis. Man did I spell that big word right.??
Marcel 
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Here is a picture of an attic that gets over $3000 in mold remediation. The attic has one power ventilator installed and working as well as passive roof vents. Bottom line is frost eventually turns in to moisture, and one part of the mold equation is then present. All the fungi need now is the right temperature and off it goes. Causes are bath vents not vented to outside, placement of roof vents too far below roof peaks, turbine vents covered in the winter etc. etc.
Did a house on Saturday, it was -4 degrees F and there was a light coat of frost on the sheathing and insulation. The attic was well insulated, well vented, there was no openings to the house and no signs of moisture damage or mold. I have come across this before and as someone else mentioned it is the result of some particular meteorological conditions that allowed the moisture in the outside air or attic to condense. Nothing to worry about if every thing else checks out.
I think there is a air movement concern >I wonder are you sure there are Soffit baffles . From your picture it looks to me like the Soffit is blocked . Are you sure there is air being able to get threw the soffit to the baffles.
There sure looks to me like some where moisture laden air is getting into the attic from the home .
If there is roof vents and no soffit vents then air could be drawn from the home via ever electric wire hole .
It also looks like the insulation is dark on the top and some of the rafters are darker then they should be .
Could there be a open or split pipe from the Kitchen or Bath vent under the insulation this could leak air all the time .
I would be concerned and look further
From my above post ( If it is a heavy frost look for air leak from the home)
Roy Cooke
That is why I put a vapor barrier in the ceiling of my house.
Roy I posted that there where no other problems in the attic, baffles are not necessary as the soffit was wide open, there was even 6 to 8 inch air space in the cathedral ceiling section. The plumbing vents where exposed and are not damaged. There are no pot lights, fan ducts, chimneys or other vents that pierce the envelope. The only possible opening would be the wire holes, but this is pretty common factor to most houses as 99% are not sealed. There was no discoloration of the insulation to indicate an air leak anywhere. If this where a regular occurrence then the wood in the attic would be discolored, the chip board is the same color as the day it was installed 20 years ago. Yes, there is a plastic vapor barrier. This is not a heavy frost build up and I have seen heavy frost build up, this is a light frost build up.
The last couple of days it has been -20 Celcius in this area of Ontario. Converts to -4 Fahrenheit. Looking in any attic at this temperature I have seen many insides of roof with the nails very heavily frosted. Usually the immediate area around the nail is frosted and discolored from moisture over the years, even to the point of the wood being black. I looked at one on Wednesday that had the bathroom vent blowing directly onto the plywood in the attic space. Temperture was -22. Only that area was frosted and blackened from this going on over the past thirteen years.
I have seen a couple that have had the complete roof frosted inside and on both, I discovered an area that did not have the moisture barrier in the complete ceiling areas.
If the whole inside of the roof is covered with frost, I would definetly suspect moisture is getting into the attic space from living quarters and would recomend further evaluation of this situation.
If you’ve got ice build-up in your attic, adding more attic ventilation is not the right solution. Additional roof vents allow more air to escape from the attic and that pulls more warm moist air from the house below, into the attic.
The proper solution is to have additional insulation seal off your attic bypasses and make sure you have adequate levels of insulation. Warmer air is getting into the attic through inadequate levels of insulation or there’s insulation missing in areas throughout the attic. Installing/upgrading the insulation in the attic cavity should eliminate frost from accumulating in your attic.