Attic moisture Free Press 2014

ASK THE INSPECTOR: Attic moisture plaguing Manitoba homes

By: Ari Marantz

Updated: March 29, 2014

QUESTION: We had water dripping from our second-floor bedroom ceiling the evening of the first mild day a few weeks ago. Today, when I had time to get up into the attic, I found quite a bit of snow, about an eight-foot-square area, lying on top of the fiberglass batt insulation at the north end of the attic under the gable vent. There was some under the south gable vent, but not as much.

It was impossible to just remove the snow, as it had refrozen and was partially embedded into the batts. I have removed the top layer of batts to the garage to await thawing and drying out. The remaining insulation, another layer of R12 batts and wood chips, will also have to be dried out.

In 14 years in our home, we’ve never had such water leaks before. I don’t check up in the attic regularly, and so assume there may have been some snow in previous years, but never enough to cause liquid water leaks. This year, I assume that one of the many snowfalls we had was accompanied by the necessary direction and strength wind to blow this amount of snow through the gable vent into the attic.

I know I should not block the attic vents, especially in winter, to maintain attic ventilation. Do I just need to get a better vent? The current one is a newer, plastic vent installed about seven years ago when we re-sided our home.

I’ve read suggestions online about installing a furnace filter on the inside of the vent, which is claimed to block wind-blown snow but not prevent adequate air flow. Is this a good idea, or are there other options? Thanks, Ray


ANSWER: Having dealt with the issue of attic moisture in a recent article, I was reluctant to revisit the issue so soon, but the current situation in our area warrants further attention. You are only one of hundreds of homeowners who are having issues with unusual leakage from wet attics, and I’ll to explain why this is happening.

There is a chance that the frozen moisture you have seen inside your attic is partially due to snow blowing through the gable vent, but it’s even more likely that it’s from frost that has formed from moisture within the attic itself.

With the high winds and blowing snow during this unusually bitter winter, some snow getting through the louvers on a typical gable vent is not abnormal. In fact, with these conditions some snow can often be seen below normal roof vents as well. These amounts, while looking troublesome, rarely cause much concern, other than temporarily wet insulation. Isolated wet spots like this should easily dry over the long, hot summer, as long as attic ventilation is adequate.

However, I think it’s more likely that attic moisture has condensed inside the vent and frozen, creating a thick layer that may be partially blocking the vent. Wet, blown snow may also have stuck to the screen on the inside of the vent, further complicating matters.

With the recent warmer weather, this heavy layer of frost has partially melted and fallen on top of the insulation. What you may not have seen is additional frost that formed on the underside of the roof sheathing and fell on top of the batts, contributing to the leakage.

This additional melted ice may have exceeded the normal absorptive capacity of the fiberglass batts, allowing water to seep through to the ceiling. This larger-than-normal amount of moisture is likely why you haven’t experienced this is previous years.

This issue is happening in lots of homes in our area. While excessive attic moisture and condensation is usually linked to high relative humidity in the living space and poor air sealing of the building envelope, this year there is another important variable: exceedingly long periods of -20 to -30C, which have prevented normal thawing and melting of attic frost.

A normal winter freeze/thaw cycle situation prevents the large, single-event thawing we’re seeing late this winter, which is wreaking havoc in our homes. Unfortunately, some of the blame falls squarely on the shoulders of Mother Nature, not the building or its occupants.

You have taken the right step by removing some of the wet insulation, preventing the excessive attic moisture from causing further damage. But care should be taken not to remove all the insulation until the weather is consistently above freezing, as that may cause further problems.

Unfortunately for many homeowners, particularly those with blown-in, loose-fill fiberglass, dampness may be present in different layers of the thermal protective material. This makes partial removal much more difficult. I’ve also recently seen several homes where actual water was sitting on the polyethylene air/vapour barrier below the insulation.

While it may appear that a poorly designed attic gable vent is allowing snow to blow into your attic, causing leakage when the weather warms, that may not be the whole story. A combination of environmental moisture, trapped in the attic over the excessively cold winter, and air leakage through the building envelope, are more to blame in this case. Blocking any attic vent with any type of material or filter will only cause bigger issues and will delay quick drying of the wet insulation once really warm weather arrives.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors – Manitoba ( Questions can be e-mailed to the address below. Ari can be reached at (204) 291-5358 or check out his website at