I'm looking a building in an area that is always hot and very humid. One can close the building up and run A/C and dehumidifiers, but this isn't very green - plus electricity is expensive there. The more efficient and typical way is to leave things open and maximize breeze and shade. One might also try to take advantage of thermal mass (it's a little cooler at night), but my experience has been that the cooler walls cause condensation or mildew. Ie. cool, high mass walls and hot moist air aren't a good combination in the interior. On the other hand, things like wood and wallboard can't be used either - they can't take the moisture and so homes are all concrete and steel and sometimes some foam. So what is a low mass, moisture proof interior wall surface for a concrete wall? Perhaps foam and then a 1/4" coat of synthetic stucco? MgO? Some type of plastic wall paper that could be applied directly to foam? How about floors - tile over concrete is common, but it has the same high mass cooling/condensation problem.

Posted By jonr on 17 Jul 2010 10:59 PM
I'm looking a building in an area that is always hot and very humid. One can close the building up and run A/C and dehumidifiers, but this isn't very green - plus electricity is expensive there. The more efficient and typical way is to leave things open and maximize breeze and shade. One might also try to take advantage of thermal mass (it's a little cooler at night), but my experience has been that the cooler walls cause condensation or mildew. Ie. cool, high mass walls and hot moist air aren't a good combination in the interior. On the other hand, things like wood and wallboard can't be used either - they can't take the moisture and so homes are all concrete and steel and sometimes some foam. So what is a low mass, moisture proof interior wall surface for a concrete wall? Perhaps foam and then a 1/4" coat of synthetic stucco? MgO? Some type of plastic wall paper that could be applied directly to foam? How about floors - tile over concrete is common, but it has the same high mass cooling/condensation problem.

The only way you can avoid mildew at high ventilation rates is to keep the interior well above the outdoor door dew point, which isn't very comfortable in truly hot & humid areas. 

For instance, in central FL today the dew points were running ~78F, and to get interior down to the 65% RH level where the mold potential falls off would require running the interior of the house at ~90F(!).  If, by a combination of high mass and good solar gain control the interior were able to stay as cool as 75F there would be a thin coating of condensation on all interior surfaces deposited by the ventilation air.  Anything below the dew point of the ventilation air becomes a condensing surface, independently of it's mass or thermal mass.  Low mass stuff like vinyl or foil wallpapers with insulation between it and the thermal mass of the wall may rise quickly to the dew point, but it'll still have a film of water on it, making any organic contamination on it mold-food, in perfect mold-banquet conditions.  Porous materials like masonry or much stone & tile can wick in a bunch of the condensation via capillary action though- going fully waterproof isn't always the right solution.

Mold & rot on the building materials might be controlled by selecting less  moisture-susceptible materials- mold doesn't much grow on most plastics ceramics or metals until/unless they get sufficient surface contamination. Stone & tile are good- about anything you'd uses in a shower or tub-surround is fine.  But keeping the mold off cabinets furniture, etc. is also important for keeping indoor air quality at non-fungal-infection levels for the occupants, not to mention the comfort level of clammy damp surfaces everywhere, and sweat lingers on skin & clothing  rather than evaporating.

At low ventilation rates it doesn't take a heluva lot of electricity for small compressor based dehumidification to keep latent loads under control when the sensible loads are otherwise handled by a combination of ventilation rates, thermal mass, insulation, and solar gain management.  The duty-cycle of the dehumidifier then becomes primarily a function of the ventilation rate & outdoor absolute humidity.  Building it tight, controlling ventilation rates, and using mechanical dehumidification  is still a good strategy, even at 50 cents/kwh diesel generator island-style. With low ventilation rates and some hygric buffering one can even use off-peak or PV solar to power the dehumidifier for a more favorable benefit/rate.

Nighttime ventilation schemes don't work very well in humid environments. Even if the sensible cooling load is manageable with that type of strategy, the latent load is typically several times that of the sensible load, and pulling in 80-99%+ relative humidity air that is 5-10F cooler air than the interior may cool then interior, but raises the total humidity burden, increasing mold & rot hazards on food/furniture/humans, etc.

Thanks. Even if the building is sometimes sealed, I like to be able to open it up when the weather is tolerable. At least a low mass or insulated, non porous surface doesn't collect moisture for very long - as you say, it quickly warms to above the dew point and then it starts drying. On the other hand, thick concrete stays cool for a long time, collects lots of moisture, then stores some of that moisture in the pores where it will evaporate later.

Not clear to me is the effect of intermittent low humidity - ie, does getting down to 60% RH for a few hours every day make higher levels at other times OK in terms of mildew? I suspect that it what keeps the problem tolerable for the locals who don't use AC or dehumidifiers and have stucco over concrete walls.

For a floor, about the best I can think of is concrete slab, foam insulation,1/4" hardibacker, then vinyl flooring. It won't absorb moisture and it's not very high mass (on the interior side).

Posted By jonr on 20 Jul 2010 07:50 PM
Thanks. Even if the building is sometimes sealed, I like to be able to open it up when the weather is tolerable. At least a low mass or insulated, non porous surface doesn't collect moisture for very long - as you say, it quickly warms to above the dew point and then it starts drying. On the other hand, thick concrete stays cool for a long time, collects lots of moisture, then stores some of that moisture in the pores where it will evaporate later.

Not clear to me is the effect of intermittent low humidity - ie, does getting down to 60% RH for a few hours every day make higher levels at other times OK in terms of mildew? I suspect that it what keeps the problem tolerable for the locals who don't use AC or dehumidifiers and have stucco over concrete walls.

For a floor, about the best I can think of is concrete slab, foam insulation,1/4" hardibacker, then vinyl flooring. It won't absorb moisture and it's not very high mass (on the interior side).

Actually it warms up quickly to the dew point and stays wet.  It can't keep accumulating more & more water on it's surface once it's at the dew point, but it doesn't dry until it's temp is above the dew point of the surrounding air.  The low-mass waterproof approach will APPEAR wet sooner & longer than high mass-porous stuff, even if the latter accumulates more water.

With biological processes like mildew it's average humidity x time.  If the AVERAGE relative humidity is below 65% the mildew potential is low, even with intermittent bouts of very high humidity.  Taking a shower in a 70F bathroom with low ventilation you get 100% humidity for 5-10 minutes at a time, but if you vent the room or mechnanically dehumidify to 60% RH shortly thereafter you don't end up with mold growing on the walls.

Vinyl flooring might be awfully slick when at the dew point with a film of water on it.

> but it doesn't dry until it's temp is above the dew point of the surrounding air.

Which, with the incoming air above the dew point and nothing cooling it, happens. Wet things DO dry, and thin coatings of water on non-absorbent, ~air temp surfaces dry reasonably quickly.

I haven't seen mildew on low mass, non-absorbent, open to the air surfaces even when the humidity is always high; but I often see it on concrete and in bathrooms with absorbent surfaces.

Some period (perhaps daily) of low humidity will cause mildew to go dormant and it can't come back quickly.