Here are some pics from todays inspection. It isn’t mold, at least it doesn’t appear to be. Any suggestions?

Hi. Gary;

The last time I observed something like this it was caused by numerous frost build-up on the underside of the roof sheathing.

The frost and thaw cycles due to the temperature of the roof will cause moisture saturation and then it dries up leaving dark water marks on the wood plywood similar to a wood window sill that gets water on it occassionally. Rust on the nails in the roof sheathing is also an indicator of previous moisture.

Ventilation seems or appears to be the problem, and heat loss is occurring at the juncture of the exterior wall and roof.

Since there are no proper vents visible, it is posible that some heat loss is occurring in that area.

The blown in fiberglass insulation is also depicting dicoloration due to moisture dropings and excessive heat in the attic space during the summer.

I am sure you will have other opinions, but hope this helps.

Marcel :slight_smile:

Blackening on plywood is indicative of improper ventilation. There aren’t any soffit vents installed. It needs ridge and sofit vents.

I’m seeing some delaminated sheathing also.

Moisture and blackening underneath the roof sheathing is firstly indicative of upward leakage of warm house air (possibly a major heat loss) that contains moisture from house activities and other sources. The warm, moist air comes in contact with the cool/cold sheathing and if its dewpoint (condensation temperature) is reached, the water condenses out of the air to wet the sheathing. In colder temps, this becomes frost. As you can see in the following references, the first approach is not to add ventilation but to cure the moisture problems. By then the little bit of ventilation in the attic may be enough to remove any small remaining moisture that gets to the attic or you can then add a bit more venting.

Look at the attic air leakage points in the first two of the following references. These are only a few of the attic bypasses than you will actually find in houses…trust me, I ran an insulation/airsealing company from 1980-1990 ; still do a few houses (10-15 per year now as a sideline; Was drilling a house until 7 PM last evening; no advertising used or needed…just turned down a $9,000 large house retrofit!!). I now try to get the homeowner to do the airsealing under my guidance since the labour/materials breakdown is around 85/15 - 90/10 and airsealing labour is straightforward and simple …it’s to know where and how to best airseal the bypasses.


I did an inspection and it was exactly like that.
Found an open 4 inch plumbing vent under the insulation.

… Cookie

Good links Brian.

Thanks, makes for good reference.

Marcel :slight_smile:

Good points. Thanks guys.

Obviously a moisture issue with plausible sources already pointed out by Marcel, David, Brian and Russel. So if it’s a moisture issue, why isn’t it a mold issue? If those dark areas aren’t mold colonies, what are they?
This looks like an intermittant moisture problem with mold colonies which only get a chance to grow when conditions are right for creating condensation on the underside of the sheathing.
Find where moist air is leaking up from the living space and the mold fungi will go back to sleep.

The black areas appear to be staining only, although I have never taken a sample for analysis. It appears to be common when there is an intermittent moisture issue in cold weather; the blotchy circular colonies of mould need continued wetting with the right conditions.

Mould needs 5 conditions to grow:

  1. a food source of which cellulose in wood, drywall paper is
  2. spores (seeds)
  3. cool to warm temps
  4. moisture in terms of wet/damp materials or constant high RH over 70%
  5. generally, dark conditions

Remove any 1 of these conditions and mould will not grow. Sheathing may get wet/damp in the coldest parts of the winter but the temps are not conducive to mould growth. If the sheathing dries before temps get constantly in the range to allow growth, mould will not grow as the moisture condition is removed. I believe that this is the situation that causes the staining- moisture but no growth.