Ventilation Attic Access, PNW house

Published on February 15th, 2017 | by Henri

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Attic ventilation changes with climate

Q. My wife and I just bought a home in the Seattle area. During our inspection the inspector noted that there was some mild mold growth in the attic and in his opinion, if the ventilation was increased he said the mold would die naturally. The home inspection was done in October 2016.

It is now January 2017 and the last 5-6 weeks have been unseasonably cold for the Pacific Northwest. The average high is 47 and average low is 36. The last few weeks we have had highs in the low to mid 30’s and dipping into the mid teens at night. I have began to notice that there is condensation on a couple of exterior facing walls. We have cathedral type ceilings in the upstairs and only a small accessible side crawl attic without access to the attic above the upstairs.

I popped my head inside the attic, as the closet that has the attic access was showing lots of condensation and paint bubbling and chipping. Inside I could see ice on the sheathing above the soffits and also above me where the cathedral ceiling proceeds up into the attic over the rooms. There is mold on only some of the insulation, and maybe 30-40% of the roof sheathing.

We do not have a furnace, as this house was built in 1970 and a lot of older homes in the PNW use some form of wall heaters. We also do not have any exhaust fans in the house. We open the window in the bathroom when we shower and open the window in the laundry room when we do laundry.

I’m sure the house will dry out in the summer, but what do I do about in the winter and moisture problems? Do I call a roofer and have more roof vents installed? I also think whomever installed the attic insulation inadvertently blocked off some of the soffits and maybe also blocked off slanted ceiling with insulation. My fear is that air flow is so poor up there that we will continue to get mold. I know these roofs are poorly designed to begin with, but what should I do or who should I call to help me solve this problem?

Thanks very much for your help. Right now I have the attic access door off to help air circulate and a fan blowing in the room.

A.  What you describe is a case of warm, moist air from the conditioned space convecting into the attic through a number of cracks and crevices in a house not built to today’s tighter standards. As a result, you are experiencing the serious moisture problem familiar to those living in colder climates.

The cold weather you are experiencing now may turn out to be a more frequent event in the coming years.

The first thing you need to do is to put the attic access panel back down, and if it isn’t weatherstripped and insulated, consider doing both.

The sudden change from the usually more normal Pacific Northwest winter climate is causing a considerable increase in the relative humidity (RH) of the colder attic air, resulting in major condensation issues.

Having this panel off is introducing huge amounts of warm conditioned air into the cold attic, adding to the problem your inspector pointed out last fall.

Don’t bother calling a roofer to increase the roof’s ventilation; it’s not the solution. Increasing ventilation is not going to solve the problem, as it can’t, and will not, overcome the amount of convected warm, moist air from the conditioned space. You need to find and seal the convective paths, and there may be many considering the type of construction done in the early ‘70s. This is best done with a blow-door test and infrared thermography. The combination of both will identify the sources of convection, which then can be dealt with. Consider contacting an energy auditor who will do the testing.

Houses in your area, built before the energy crisis of the early ‘70s, have not been constructed to withstand the much lower temperatures you are now experiencing. They were built to match the climate.

I am not clear as to what you mean about condensation on a couple of exterior facing walls. Are you seeing the condensation on the accessible attic walls or on two actual house walls in the living space? That’s an important distinction.

If you are referring to the attic walls, it is not surprising: high levels of RH are causing the condensation on cold walls.
But if you are referring to two exterior walls of the living space itself, is the condensation on the exterior siding or the interior surfaces of the walls? This would be of greater concern, as it may indicate inadequate or missing insulation in the walls.

The cathedral ceiling is another issue. Is there adequate ventilation above the insulation? Is there a sufficiently tight vapor retarder in the ceiling? These may not have been important considerations in a more clement climate, but they may become more important if colder weather is becoming the norm. Excessive moisture build-up in the cathedral ceiling may condense and run down the walls, which is why it is important to find and seal any convective paths breaching the integrity of cathedral ceiling’s integrity.

Although summer may dry thinks up, any moisture trapped in sealed cavities may not dry up.

Opening windows after showering is helpful, but fans are still an option worth considering.

Some of the other problems you mention are difficult to assess from a distance; they need to be checked on site. An engineer well-versed in moisture problems may be the best person to call. Best of luck dealing with these issues.

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