Why Is There Frost In My Attic?

At the last four home inspections that I've done, every home had frost problems in the attic. Have you checked for frost in your attic yet this year?  If you live in Minnesota or a similarly cool climate, now is a good time to check your attic for frost.  

Temperatures in Minnesota have been in the teens lately, which is plenty cool enough for frost to develop in attics.  Once we get a week of sub-zero temperatures, frost will really start to accumulate in attics.  The colder it is outside, the more frost will accumulate in an attic.   If you already have frost in your attic, I can assure you that it's only going to get worse - winter is still two weeks away.  

Please excuse me while I digress for a moment.  Who the heck decides when the seasons change?  Yes, I know what the winter solstice is, but what a silly indicator of winter.  Winter in Minnesota should officially start on December 1st or after the first snow fall, whichever comes first.  Half the leaves were still on the tree in my back yard when we got our first snow this year, which happened on November 13th.   That wasn't a light dusting either.  When the ground gets covered with snow, fall is over in my book, no matter what the calendar says.

Frost doesn't do much damage to roofs while it remains frost, but when it melts, it makes a big mess. When the frost melts, there is often enough water to saturate the insulation in the attic and leave stains all over the ceilings.   This is the short-term problem with frost in attics.  The long term problem is that this continual saturation of the roof decking can cause the plywood on the roof decking to delaminate; when this happens, it loses a lot of it's strength and nails are much more prone to pulling out.  The fix for delaminated roof decking is to replace it.

The photos below all show roofs with delaminated plywood; this is caused by frost in the attic.  The last picture is especially nasty.  Click on any of the photos for a larger version.

The way to prevent frost from accumulating in an attic is to prevent warm, moisture-laden air from getting there in the first place.  There are two basics ways of doing this.

Seal attic bypasses  

Attic bypasses are passageways for warm air to leak in to the attic.  A few common places to find these gaps are around furnace vents, plumbing vents, electrical boxes, and electrical wires coming in to the attic.  Any bath fans, kitchen fans, or dryers venting in to the attic space will absolutely wreak havoc.  Even small gaps in any of these vents can bring a lot of moisture in to the attic.  For more tips on locating attic air leaks and sealing them, download this handout.

Lower the humidity in your home

 I find frost problems in almost every single attic where someone uses a whole house humidifier.  That's why I don't like whole house humidifiers.  It's nearly impossible to seal every little bypass to an attic, but when interior humidity levels are kept fairly low, sealing most attic bypasses is good enough.  Here are a few ways to lower humidity levels in your home:

• Turn off your whole house humidifier (duh)
• If you have one, use your kitchen exhaust fan when you're cooking.  Gas ovens add a considerable amount of moisture to the air.
• If you have a crawl space, make sure that a proper vapor barrier is installed on the crawl space floor.
• Turn on your bathroom exhaust fan during showers and leave them on for a half hour after every shower.  If you don't have a bathroom exhaust fan, get one.  While the building code allows an openable window as a substitute for a fan, I don't ;-)
• Install an HRV or a continuous exhaust fan.  Either one of these will dramatically lower humidity levels in a home.

Will adding more ventilation to an attic prevent frost?  No, this won't do squat.  I've been in tons of attics that were completely covered in frost, yet had fantastic ventilation.  To fix the frost in your freezing attic, focus on forbidding the moisture from getting there in the first place.