Attic moisture and ventilation

This page exists because a good share of the calls that sound like leaks in Boone County are not leaks. They are condensation, and the difference matters because the repair is completely different and a new roof will not fix it.

What is actually happening

A house produces a surprising amount of water vapour. Showers, cooking, laundry, breathing, houseplants and an unsealed crawl space all add moisture to the indoor air. That air is warm, so it rises, and it finds every gap into the attic: the hatch, recessed light housings, the holes cut for plumbing stacks and wiring, the top plates of interior walls.

In a mid Missouri January the underside of the roof deck sits below freezing. Warm moist air reaches it and the moisture comes out of the air onto the cold surface. On a hard cold morning you can go into the attic and find frost on the point of every roofing nail, which is exactly what it looks like. When the weather turns, the frost melts, it drips, and it lands on the insulation and the ceiling below. The homeowner sees a damp patch and reasonably concludes the roof is leaking.

Summer runs the same process in reverse on air conditioned houses, though winter is where the damage happens.

How to tell it apart from a leak

Timing. A leak follows rain. Condensation follows cold, and especially a thaw after a cold spell.

Pattern. A leak has a track. Water enters at one point and runs, so staining traces a path from a penetration or a valley. Condensation is even. It appears across a broad area of decking, often worst on the north slope and near the ridge, with nothing at all resembling an entry point.

Nails. Frost or beads on nail points in winter is conclusive. Roofs do not leak through hundreds of nails simultaneously.

What it does if it is left

Decking that is repeatedly damp delaminates and loses its grip on the nails, which shows up as shingles that will not stay fastened. Insulation that has been wet does less, so the house costs more to heat and the attic gets colder, which makes the condensation worse. And mould grows on the underside of the sheathing, which becomes a finding on a home inspection when you sell.

The fix, in the right order

Seal the air paths from the house into the attic. This is the part that actually stops the moisture, and it is often the cheapest.

Vent the moisture producers outside. Bathroom and kitchen fans that terminate in the attic are a recurring finding, and they are pumping warm wet air directly at the deck.

Get the ventilation balanced. Intake at the soffits and exhaust at the ridge, in proportion, with the deck properly slotted where a ridge vent sits. Blocked soffits are extremely common after insulation has been topped up without baffles.

The right moment for all of this is while the roof is off, because the deck is visible and the ridge is open. It is a small addition to a reroof and a separate project afterwards. Worth raising specifically when you get a quote.

One independent mid Missouri roofing company receives requests from this site. The replacement page covers the wider job, and leaks and flashing covers the case where it really is water coming in from outside.