In our
haste to build houses in the past twenty years, we have been
careless at times with some very important details. Attic
ventilation is one of them. A home that I inspected this weekend is
a really good example. Upon entering the attic space I found a
lovely even layer of cellulose insulation about 10 inches thick. It
had been well installed. Often I see it in waves or looking like a
mountain range. Cellulose is a good insulator and has become
popular in the past ten years or so because it is cheaper than
glass fibre. Its main use is attics where there is more room and
where its tendency to pack down won't be a problem as it would be
in walls.
Poor
ventilation in an attic space means that the air in the attic will
change very slowly. Let's say that we have a prolonged warm spell,
like we have had a couple of times this winter. The air in the
attic space becomes warm and more humid. If the temperature
suddenly drops, just as it has, the humidity in the moisture
condenses against the roof deck, especially on roofing nails or
gang nails on trusses. It then freezes. If you were to enter the
attic area at this point you would see a layer of frost on the
underside of the roof deck or you might just see it on the nails.
If you have seen this in your attic area, you don't have enough
ventilation. An ideal situation would be to have soffit vents with
either gable vents, roof vents, or a ridge vent. Soffit vents being
50% of the area of the total ventilation and, oh yes, more is
better!
The house I
inspected had another complication. I could see a truss which
appeared to be wet on top. Then my client noticed a black stain
near some of the baffles on the underside of the roof deck. Upon
testing with a moisture meter I found the truss was wet and got
wetter as I moved down, toward the baffles. My immediate thought
was a leak in the roof, but since water doesn't flow uphill, that
was impossible. The black stain turned out to be mildew and, again,
it was wet, Over 19% humidity in wood means it is actively rotting.
So the truss was rotting and mildew was forming on the underside of
the roof deck. As we puzzled over this rather odd localized
moisture problem, I crawled around the rest of the attic, finding
that, although it was humid, it wasn't actually wet. Crawling isn't
really that accurate for how I have to get around attic spaces. To
avoid disturbing the insulation, I sort of walk on my side, my feet
pushing on one web of the truss with my shoulder on another. Moving
from one truss to another in this position is very tiring.
The best
explanation I could come up with for my client was that since the
bathroom was in the same general area as the signs of moisture in
the attic, and the prevailing wind hit the house on that side, the
moisture came from the bathroom fan. These fans are most commonly
vented through the soffit. It should then be fitted with a proper
boot to make sure this warm and very moist air goes outside.
Sometimes this pipe is left to discharge inside the soffit as I
believe it is here. In this case, the wind is pushing the warm
moist air up into the attic, making this poorly ventilated attic
even wetter.
I am
constantly amazed at how much damage a little detail can cause. For
the very small investment of hiring a home inspector and another
small investment correcting the situation, my client will save
himself thousands of dollars of repair work later on, when the roof
would have collapsed.
Mike Lancop is a certified home
inspector and member of the American Society of Home Inspectors,
the Canadian Association of Home Inspectors, and Independent Home
Inspectors of North America, and owner of Buyer's Edge Home Inspection
Services, High River,
Alberta.
IHINALegal
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