With the cold snap last week, I've seen a rash of frost covered attics. I also received an email this week that inspired me to blog about this:
A couple of years ago my husband and I contacted you about frost accumulating in our attic. My husband filled in all the attic bypasses as you suggested and also put in an outside attic vent. This year with our extremely cold weather, frost is accumulating again. Do you have any other suggestions? - Wendy Wils
I've blogged about frost in attics before, but it's been a few years and it's time to re-visit this topic. To start, frost shows up in the attic when moisture-laden air from the house gets into the attic. That's about it, pretty simple. When the moisture gets into the attic, it condenses on the roof sheathing in the form of frost. The frost itself doesn't do any damage, but once it melts things get wet, which is when the damage occurs. Melting frost can lead to deteriorated roof sheathing, mold on the roof sheathing, wet insulation, and water stains on the ceilings. All bad stuff; you definitely don't want frost is your attic.
Frost comes from air leaks
Frost gets into the attic from air leaks, or attic bypasses. I've blogged about attic air leaks many times, and I've shared photos of attic air leaks; check out my post on moldy attics for some good examples of attic bypasses. Of course, any type of exhaust fan needs to be exhausted directly to the exterior, and never into the attic. Even if the exhaust fan is aimed at a roof vent, this isn't good enough. A lot of moist air will still find it's way back into the attic.
The best way to prevent frost from accumulating in an attic is to seal off attic air leaks. Click here for an excellent guide to attic air sealing. While seemingly small air leaks may not seem to be important, these can add up to a lot of frost accumulation in the attic. It's important to seal all attic air leaks; not just the big ones. Once every little air leak has been perfectly sealed, the attic will be frost free. The only problem with doing all of this air sealing is that the air leaks are located underneath the attic insulation, and it can be very difficult to find every air leak without completely removing the attic insulation. For this reason, it's nice to start with the easier stuff first.
More indoor humidity = more frost in the attic
The more humid a house is, the more frost you'll find in the attic. The houses with the worst frost problems always have a whole-house humidifier running, which is why I'm not a fan of humidifiers. They destroy houses. If you have a frost problem in your attic, be sure to take care of all the easy, obvious stuff before crawling around in your attic. For the love of love, turn your humidifier off.
Replace the standard switches on your bathroom exhaust fans with timers that will run the fans for an hour at a time. Here's an example of a timer switch that can be used for motors and doesn't require a neutral wire. Once those timers are installed, train everyone in the house to run the bathroom fan for 30 - 60 minutes after every shower or bath; this is how long it takes to get indoor humidity levels back to normal. Just running a fan while taking a shower won't do much.
If you don't have exhaust fans installed in bathrooms that are used for showers or bathing, fix that. I don't care what the building code says, you need a fan in these bathrooms.
If you have a kitchen exhaust fan, use it while cooking. Ovens generate a lot of moisture.
Consider installing an HRV if you don't have one. HRVs replace damp indoor air with dry outdoor air, and recapture a fair amount of heat at the same time. This will certainly lower humidity levels in the home. If you already have an HRV, make sure it's properly installed, properly maintained, and operating.
If you have too many plants (or weeds) in your home, get rid of 'em. I can't say how many is too many, I just know it when I see it.
If you have a damp basement or a crawl space with no vapor barrier, fix it. These are both major contributors to indoor humidity and attic problems.
House pressure affects frost
With all other factors being equal, the air in your house sees your house as a very wide chimney, because warm air rises. The trend is to have air leaving the house at the top, and entering the house at the bottom. The taller the house, the greater this effect. Split level homes with more than one attic space will always have the worst attic problems at the uppermost attic.
When a home has a combustion air duct connected to the return plenum, the house gets pressurized when the furnace runs, which increases the effects of attic air leaks. Combustion air ducts should not be connected to return plenums; they should just be dropped down into the room.
Unbalanced HVAC ductwork can also cause pressure problems. If there are too many return openings in the ductwork in the basement, the basement will be under negative pressure while the upper levels are under positive pressure. Sealing up all of the holes and gaps in your furnace ductwork can actually help to decrease the severity of attic air leaks. One simple test to find out if your basements "sucks" is to position a door to the basement about 1" away from being closed, then turn the furnace fan on. If the door closes by itself, it's an obvious sign that the ductwork is not properly balanced.
Will adding insulation help? No way.
Let's think that one through. If an attic doesn't have enough insulation, it will be warm. Adding insulation will make the attic colder. The colder it is in the attic, the greater the potential for frost accumulation. Adding insulation to an attic will only make things worse when it comes to frost. Insulation should only be added after air sealing has been performed. If it's not in the budget to do both, then just have the air sealing done. This is much more important.
What about more roof vents?
Bah-humbug. Focus on all the other stuff listed above first. Proper ventilation in the attic may reduce frost accumulation, but if done wrong, simply adding more roof vents might actually make for more frost. I'll have more on roof vents next week.
Author: Reuben Saltzman, Structure Tech Home Inspections