Reuben's Home Inspection Blog

Frost in Attics

December 17th, 2013 | 6 comments

With the recent cold snap, 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

Frost in attic

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


Why Is There Frost In My Attic?

December 7th, 2010 | 16 comments

Frost in atticAt 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.

Delaminated Plywood

Delaminated Plywood 2

Delaminated Plywood 3

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.
  • 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 ;-)
  • If you have a crawl space, make sure that a proper vapor barrier is installed on the crawl space floor.
  • 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.

Reuben Saltzman, Structure Tech Home Inspections – Email – Minneapolis Home Inspections

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