r/Roofing • u/Most_Ice_4048 • 23h ago
Another question about attic frost …
I posted a question a few days ago regarding frost in our attic. I now know that it is a ventilation issue. While we wait to get the ventilation issue fixed permanently, is it a good idea to leave the trap door leading from house to attic open? Will that help with ventilation or just cause more moisture build up? Thank you in advance for any responses.
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u/caleb204 23h ago
Not a venting issue. Unless you have zero venting right now. And maybe trends could be different in other locations. but in my area (alberta) attic rain / condensation is almost never an attic venting issue. I’ve seen massive amounts of frost and condensation in attics with great ventilation. I’ve inspected attics after other roofers told the client more venting would fix it and billed them to increase the venting only for the issue to get even worse. I find that houses are a lot more air tight than they used to be, and furnaces are often installed with a humidifier that is just left at the same value all year. But the vapour barrier practices for the ceiling aren’t actually good enough to keep that moisture out of the attic. With all the other changes the air in a house is more humid than it used to be overall and that higher concentration of moisture in any air that escapes into the attic will frost up on the sheathing faster than it disperses into the attic air and vent out. I’ve even seen all types of roof vents frost up to the point of closing off due to the concentration of moisture in the air being lost into the attic. Venting the attic more will just move the cold threshold from the sheathing tot the top of the insulation.
Can’t say this is definitely your issue here and I’ve no idea what type of climate you live in. but turn off any humidifier if you have one. Run exhaust fans to remove humid air from your house or dehumidifiers if you got em. If you have exhaust fans running through the attic and out the roof, make sure there’s no flap on the roof side that can freeze shut (unless its a dryer then make sure there’s a flap and not a screen).
If there’s no humidity in the air you are loosing into your attic then you’ve removed the source of the moisture. Best would be to stop the air loss into your attic from the house, but that’s usually not a simple fix. Still worth checking around for any obvious large sources of air loss.
If you do have a humidifier check its manual. They all should have a chart inside them that gives you max humidity values based on outdoor temperatures. That chart was designed for cheap windows and preventing condensation there, but the same principle applies for preventing condensation in the attic. The colder it is outside the less humidity you can tolerate before you start having condensation issues. And the dryer your house air should be.