Chris: There are also good apps for smartphones that do all the psychrometric calculations, too. I mostly use the Munters PsychroApp.

I remember when I read the paper (“Unplanned Airflows and Moisture Problems”) where Lstiburek and Terry Brennan first mentioned accidental dehumidification and thinking how brilliant that was. I put the link to the paper at the bottom of my article on that topic:

Accidental Dehumidification – A Preventable Mess

Bill: How far are you willing to go? That duct in the wall is probably an uninsulated supply duct. The drywall in the closet gets cold enough to accumulate moisture and grow mold in summer. One solution would be to open the wall and insulate the duct. You may need to expand the space for the duct by stealing space from the closet.

Barring that, you try to get some air flow into the closet. A small muffin fan might be enough if you have a decent undercut on the door. Ideally you’d place the fan high and on the back side of the closet, but putting it above he door could work, too.

You could also put a small dehumidifier in the closet.

A dew point calculator such as http://www.dpcalc.org/ is super-handy for figuring out target indoor dew points. For example, if the inside of my window frames are at 7℃, I’d need to maintain an indoor dew point of 4℃ or less to avoid mould. In a continental winter that might be no big deal, but on the coast it’s quite a challenge!

At present the house tends to grow mould on the aluminium window frames and on curtains in poorly-ventilated bedrooms, and it’s very hard to heat the house without the indoor dew point becoming undesirably high in terms of condensation. The frames are not thermally broken and are often only a couple of K above the OAT. And the curtains can be their own worst enemy, as their insulative property causes it to be colder on the window side in winter, leading to increased RH, as they’re exposed to the same dew point conditions as the rest of the indoor air.

I always got a chuckle out of Joe Lstiburek’s jocular euphemism for condensation: “accidental dehumidification”!

Great article. Just sent it to my son to explain why his big bean bag type chair on our cold floor ( concrete slab ) and up against his wall will have moisture under it in the winter.

We have a fiberglass Therma Tru with foam that’s performed pretty well. Just checked its temperature. Room temp 67-68F, wall surface 65-66F, door surface 63-64F, except right at the edge I get around 59F. It’s 22F outside.

Thanks, Alex! Glad you liked it.

Two great questions, Roy. The Passive House community has good doors because they require it. Unfortunately, I think they’re still pretty expensive.