Frost in Attic

Never before have I ran across an attic with so much frost. When I opened up the attic access panel, I thought they painted the underside of the roof white! Wow… Soffit vents were covered with insulation, missing baffles at eaves and 2 roof vents.

Very nice… What was your analysis and recommendation?


I bet you have some bath vent fans exiting into the attic, or maybe an uncovered attic fan. Attics have to have ventilation in any weather; hot or cold. I hope the weather at that home does not warm up too fast, as it will be raining inside the attic. Check that flue pipe exit.

I was thinking same. Has to be some source of water vapor to make that ice.

With that much frost its going to be more than just a bath fan venting into the attic. Is there a moisture problem in the basement or lower level? Bet there’s a lot of mold growing on the roof sheathing when the frost isn’t covering it up.

I bet Alan is right. There is probably mold all over that sheathing

The humidity level inside the home was very high (45%). Thermostat at 78 degrees. All the bath and kitchen exhaust fans were vented thru the sidewalls. It was very cold here yesterday, -10 below. Forecast is warm weather, 50 degrees. Yes it will be raining soon in that attic. Flue was for an older gas water heater, no longer being used. I recommended an immediate evaluation of the attic ventilation system and once the frost is gone, review with a mold professional. (Me)!

I believe 45 percent in humidity inside a home is Normal? Definitely a moisture issue in the attic.

Look for an open plumbing stack it could be under the insulation .
Also confirm the dryer vents to the outside with no leaks in the attic .

  • 45% humidity sounds high to me in the cold season .*
  • We run around 30~% in the winter*

If you run 45% humidity around here you will have frost on every window when it goes below 0.

The frost is typically from the attic/roof ventilation (improper, obstructed, to many high vents), insulation (lack of or obstructing), penetrations (wires, cables, rodents, roof) and bath exhaust systems. (terminations, no insulation)
Also any HVAC ducting can affect the attic space conditions. Good chance there is mold. The frost/water will affect the insulation when warming. It also drips at the sidewalls and can affect the homes finishing.

I usually recommend corrections by someone qualified to do an energy assessment, building diagnostics or a ventilation, insulation specialist.

How did I do Dave A?

Oh the Nachi energy tool is kinda week IMHO.

More like ice damns and no more bottom rails or stools in a season or 2. :wink:


I am with Roy on this one. Was there condensation on the inside windows? Perhaps an on furnace humidifier running full blast. 30% is enough in the winter. Lots of kids in the home? They like long hot showers, and do not know what a vent fan is for.

The house is losing too much heat into the attic area and it is not being exhausted. Hot goes to cold and the sheathing condensates bringing the moisture content of the wood over 20%. The dry lumber standard is 9-12%. this condition is probably promoting microbial activity. I am thinking there is no venting here. That type of insulation also allows heat to move more rapidly to the attic as it does not pack densely. 1 sq ft of venting is necessary for every 150 sq ft of floor space (maximum standard). The best application we have installed on a roof like this are surface mounts on all sides of the roof, in the upper third of the roof. The second best application is a larger turbine vent. Most people don’t like the look of these but they are very effective in this application and roof style. I have seen soffit vents only, installed in these systems, but without the upper exhaust point, it is ineffective. Wonder what the lumber looks like after the frost is gone. We do about 50 attic mold clean and re-vents per year. Big part of our service. It is a common issue.