Hot Topics: Moisture on the Underside of the Roof Deck

snowy roof in winter

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If you've noticed the underside of your roof showing signs of moisture, you need to address it immediately. As snow and rain fall, the moisture should not be penetrating through to the inside of your attic or crawl space. Take a look at what these DIYers say about why it could be happening, and make sure to get it fixed before more serious damage occurs.

Original Post: Moisture on Underside of Roof Deck

Geochurchi Member

Why could there be moisture on the underside of a roof deck? The attic had blown-in insulation installed two years ago. It has soffit vents and a ridge vent; it has no gable fans; all exhausts are vented outside; fiber-type covers keep soffit vents clear of insulation; and the attic stairs access point is insulated. Could it be that more soffit venting is required?

Highlights from the Thread

Bud9051 Member

We can review how much venting is recommended, but number one is air leakage into the attic. Before you added the new insulation, did you undertake an aggressive air sealing approach?

Next, would be the humidity in the house and where those exhaust fans are vented. If vented into a soffit, that isn't far enough outside. Have you measured the RH (relative humidity) inside the house?

Do you run a humidifier?

Now, as for the ventilation, 1 ft² of NFA (net free area) for every 150 ft² of attic floor. That number is then shared half high and half low. It can be every 300 ft² instead if 150 ft² if there is a vapor barrier and the air sealing job was excellent.

Vents have NFA ratings but if unavailable use 50% of the opening.

These vent guidelines are not rock solid requirements, but a good starting point.

XSleeper Member

Why does a car have frost on the windshield in the morning? Is there anything wrong with the car's windshield? No. Same thing is happening in your attic... it's not surprising.

Geochurchi Member

The house was insulated by a state agency supplied contractor. Everything was sealed according to that standard, all recessed fixtures were covered with boxes before this insulation was blown in, and all vents are through the roof. I haven't checked RH-percentage.

Bud9051 Member

As the attic cools in the evening and the inside to outside temperature difference goes down, so does the air flow. That leaves whatever moisture was in the air in the attic. When the roof continues to cool, that moisture may condense out and form the frost you see. This is X's example in more words Here on we enjoy providing a place where home improvement novic.

Whether this is a long term problem or whether that moisture can come and go as it does without doing damage, I can't say. I don't usually associate NH with a humid climate.

Also, give us the age of the house. That will tell us what measures were part of the construction.

Geochurchi Member

The house was built in approximately 1969. It's a split-entry with pretty standard construction. It may very well be a temporary situation. It's at my sons house, and he called yesterday when he saw the moisture.

Bud9051 Member

Builders in 1969 were not inclined to spend extra money on air sealing and related design changes, and a split lever can create some hidden air paths where walls and floors do not always align. Finding them can be difficult. As for a state program, often they have targets for air sealing. If they seal too much, they then have to add fresh air in some manner. The old target most often referenced is 0.35 air changes per hour and much of that goes directly to the attic. So, even if they did their job well, that is still a lot of inside air moving into the attic. Note, that is 1/3 of all of the air inside your home every hour.

We are still at the "too much moisture" or "not enough ventilation" point. Let us know what you find for inside humidity and some outside pictures of the house would help.

Also, double check to be sure the ridge was opened up properly when that vent was installed. All too often the underlayment remains covering the vent. We have a thread going here about that very topic.

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