Frost in Attic


I live in Québec,Canada and I have a condo. The condo is 2 story high and I have an attic. Since 2-3 years, I have frost in my attic. The roof is brand new from last spring (2013) and they have put new ventilation like this one (

Soffit are clear with no debri and no insulation blocking them.

This year, I decided to try to fix things in attic since new roof/ventilation didnt helped with frost. I found some issues like big hole in vapor barrier and other stuff that I fixed with vapor barrier and tuck tape, I also insulated properly the attic access (it’s not stairs access).

How can I know if I did a good job ? Should I see no more frost in attic ? I can see that there’s less frost but there’s still some. Do I need to wait until the next winter to be sure ? (I did the fix like 2 weeks ago)

I did a visual inspection of the attic and I found mold at 3 places only (small spot) that I washed with soap/bleach/water



I do not like what has been done . Bleach does not kill mould.

More ventilation is frequently the wrong way to go.

A Google search will give you much information like these below .

You are addressing issues but are not solving the problem.

The problem is that elevated levels of moisture (in relation to the outdoor air) are entering the attic space from the living space.

Ventilating the attic does not stop the moisture from entering the attic where it freezes. It actually increases it.

Insulation does not stop air flow(which contains moisture).

You need to identify the source (which in most cases is not visible or readily accessible). The most effective way is to depressurize the building (with the HVAC or a blower door) and use thermal imaging on the interior of the house to find how the moisture is getting to the attic.

the thermal bypass may be very complex and not necessarily at the ceiling level.

Why does my gut tell me that there is a fan duct venting into the attic? :shock:

Hi Jeffrey,

No, there’s no fan duct venting in attic. I’m not sure but I think it’s against code here in québec. The reason why they have changed the roof and ventilation was because roof was getting old and needed to be changed and the old ventilation on roof was always blocked by snow accumulation.

The first thing I wanted to try before calling an expert to come see the problem is to try to fix problem that I could fix myself. Like I said, there was some big tear in the vapor barrier (and they were located right over the bathroom).

I can see improvement on the amount of frost already but I wanted to know if, after 2 weeks and temp still under freezing, it should be all gone or I need to wait until the next winter ?

Since my attic is not really big 40feet by 16feet, I did removed ALL the insulation one by one and checked all the vapor barrier in the attic and corrected ALL the tear that I saw (even the smallest one caused by a nail). I think that insulation in attic is R40 and 48"x24"x12". I also have insulated the around the plumbing vent.

Thanks again

Sorry for the bad english … it’s not my first language