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Reuben Saltzman

Why Is There Frost In My Attic?

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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No responses to “Why Is There Frost In My Attic?”

  1. mark
    December 23, 2012, 3:57 pm

    I live in Illinois. I have a very slight build up of frost at the eaves in my attic approximately 4-5 ft up. slight condensation on the nails also in some areas. My eave overhang is almost 2 ft. By late morning early afternoon the frost and condensation on the nails is totally gone. I dont see any mildew anywhere, the plywood and framing still looks fine. Soffit vents dont appear to be blocked with insulation and I have adquate venting at the ridge. The only logical explanation I can come up with is that the cold air intake at the eaves is bonding with the slighly warmed air in the attic causing the frost right at that point in the attic. As the day goes on and the sun rises combined with airflow at the eaves it disappears and evaporates. Anyone have any comments

  2. Reuben Saltzman
    December 24, 2012, 5:42 am

    Hi Mark,

    I’m sure the frost is the result of attic air leaks / attic bypasses.

  3. Richard Martin
    January 6, 2013, 3:17 pm

    Hi, I went in my attic today and saw frost everywhere. We had a big snow storm 2-3 week ago and when I climbed in the attic, I saw that the two vent on top of roof were blocked by snow. I barely see lights coming in. That’s the first time I get frost in attic cause I check that each year.

    I guess if I never had any problem with frost and didn’t made any change or reno in house, it must be that (vent blocked) ? Now that I have frost, can I run into major problem when it gonna melt ? Is there something I can do NOW to prevent damage from frost melting in spring ? Tomorrow, i’m gonna go on the roof to make sure that the 2 vent’s are not blocked by snow.

    Thanks for any advice


  4. Reuben Saltzman
    January 6, 2013, 8:42 pm

    Hi Richard,

    Sorry, but I don’t have any great tips to give you on dealing with the frost. Making sure the vents aren’t blocked would be a good idea, and you should also go around the exterior of your home to make sure your soffit vents aren’t blocked over with dirt and debris. Try reducing your indoor humidity levels to help prevent it from getting worse, and hopefully a lot of the frost will end up evaporating before it melts.

    – Reuben

  5. patrick
    February 11, 2013, 10:00 pm


    I was just in my attic and noticed frost everywhere and what looked to be black or dark colored mold on the botton side of the sheeting. My house is 13 years old now but I also noticed brown colored water dripping and or in icecicle form dripping from my soffit. Could this be the nasty stuff from the attic running down the boards and coming thru the soffit or is it possibly melted frost from the attic mixing with dust and dirt trapped in the soffit as we do live on a dusty road. Any advice is greatly appreciated!


  6. Reuben Saltzman
    February 12, 2013, 4:36 am

    Hi Patrick, it’s the latter. Dust and dirt, as well as tannins in the wood that are coming out.

  7. patrick
    February 12, 2013, 2:00 pm

    Thank you!

  8. Anthony
    December 7, 2013, 2:35 pm

    Last year noticed frost and mold on underside of roof. Insulation was also less then adequate and vent duct work was not insulated. This summer I removed mold and added kilz mildew and mold resistant paint (water resistant paint) to underside of roof. I also installed schutes, insulated duct work for vent and added adequate insulation (R38). Live in MN my question “is my roof going to be okay…” (really don’t want to intall new roof) thanks

  9. Reuben Saltzman
    December 7, 2013, 2:41 pm

    @Anthony – the stuff you did will probably make things worse. I recommend re-reading this post.

  10. Frost in Attics | Structure Tech Home Inspections
    December 17, 2013, 4:44 am

    […] blogged about frost in attics before, but it’s been a few years and it’s time to re-visit this topic.  To start, […]

  11. Buffy
    January 6, 2014, 4:45 pm

    We have frost in our attic as well as frozen outlets along an outside wall. Frost is coming through the drywall at the ceiling and baseboards.

    Who would you call to fix something like this??

  12. Reuben Saltzman
    January 7, 2014, 4:40 am

    @Buffy – a good insulation contractor.

  13. Andrew
    January 6, 2014, 8:15 pm

    We redid our siding, windows and roof this summer. The windows were replaced with triple pain, the siding with a styrofoam backing (giving it an R4 value) and the roof deck was redone as well. With the frigid cold temperatures this last week I noticed frost starting at the ceiling line of our back bedroom. Frost in the attic I know is common, but coming into the ceiling of our bedroom. Is this something to be worried about?

  14. Reuben Saltzman
    January 7, 2014, 4:42 am

    If you have frost at your ceiling, you have a problem with the insulation in your attic. I wouldn’t ‘worry’ about it; what you see is what you get. I’d just fix it.

  15. Joey
    January 16, 2014, 6:49 pm

    Hi there. We recently did some renos to our house, which included all new windows and doors, replaced our wood burning fireplace with a natural gas one, as well as replaced our old cast iron boiler system with a new energy efficient one. We live in Canada, and we noticed early this winter that our house had a lot of humidity in it. We then had about 6 weeks of extremely cold weather. We tried to run the bathrooms and kitchen exhaust fans often to help get rid of the humidity. Our humidity is now at normal levels. We just had a quick rise in temperature (at melting point) and we noticed 2 wet spots on our bedroom ceiling. When we checked out the attic, we found a large amount of frost in various locations. All we have for ventilation is perforated soffits and a power vent, that only runs in summer when it’s hot.. Could the cause of our problems be our house is too airtight now, and our roof doesn’t have enough ventilation? Any thoughts or suggestions? Without any duct work is it possible to get any kind of air exchanger for better air circulation?

  16. Reuben Saltzman
    January 16, 2014, 8:26 pm

    @Joey – you really need to work on sealing the attic bypasses to eliminate the frost issues in your attic. Lowering the indoor moisture levels will help, but that shouldn’t be your main focus. Here’s another more recent post I wrote on frost in attics: .

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