Crawlspace - vent or seal?

[Manalto]

Most houses in the Mobile area sit on slabs or, in the case of older houses, on piers. (I asked my real estate agent if there were houses in the area with cellars. She told me that they were rare and any house with a cellar she had ever encountered had water problems.)

My house sits on piers and has a brick "foundation" around the perimeter with openings for ventilation. I've been told that the thinking about this configuration has changed in the last 100 years and crawlspaces under houses should be sealed as tightly as possible to take advantage of the insulating properties of trapped air. Is this correct?

[Gothichome]

James, we sealed off our crawl space and made it all part of the heated space. In doing so we covered the ventilator that provided air circulation down there. I think covering them is a bit of a mistake and am seriously thinking about either opening them up again or going with some form of mechanical fan arrangement. The issue we have discovered is the basement/crawl space get musty and it at times it permeates into the living space. I would suggest not sealing it off so tight as to stop any air flow completely so maybe some sort of passive means to keep the air moving, you should still get the same benefit with out the build up of stale musty air.

[phil]

maybe use a fan with a humidity sensor?
Ive been wondering about dehumidifiers. I dont think Ive even seen one but I have a bunch of cars and keeping the wet out can be challenging.

I think with some RV's they use silica bags or a big block of salt? I sometimes put bags of silica in there , then put them on top of the water tank to let them dry but that's obviously no help here. I can put heaters and fans in the cars but it costs quite a bit of power. I thoight maybe I could buy a plug in unit that could dry things out now and then, and I could move that from car to car occasionally.

[GibsonGM]

Honestly, the deal is....Vent. OR, Seal. In other words...100% vented, like you see with a home on pillars. Or seal it completely. That means an impermeable vapor barrier (rubber, fairly pricey) that's affixed to the foundation (brick, cement, etc.) nearly air-tight. Nothing else will do (as Gothic notes). Poly sheeting is good, and does ok, but is not really able to be made 'airtight'.

The 'musty smell' is from moisture getting in. It's gases coming up from decomp in the soil, and a little of our house decomposing from moisture too, ha ha. In a retrofit like we all have, that's pretty much a given. Who can TOTALLY seal that basement or crawlspace so that soil moisture can't get in?? Not me. So, the most easy solution to realize seems to be either to set up an exhaust fan off run off a hygrometer, to exhaust at a set humidity (being not that well sealed, make up air will come in from outside via tiny cracks). That could be vulnerable in winter, I'd think.

OR, you can run a dehumidifier down there that's piped to some exit point (dry well....) - that's my choice. It's never more than 60% humidity in my (fieldstone) basement in summer, and the thing doesn't need to run in winter, it just stays off (40% on average). The dehumidifier keeps the air 'sweet', and it circulates it to minimize pockets of 'damp' that perhaps could exist down there. It keeps the floor timbers at a nearly constant humidity - much more so than if left natural, anyway, and prevents floor buckling. I piped it into my waste line, so no worrying about a full pan. I think about it once a week or so, just to make sure the hose is attached and so on. Operating cost is quite small, couple $ per month.

That's what I got! Hope you get it figured out Oh - the OTHER thing you could do would be to make sure the underside is WAY OPEN (big air flow). But, in cold times, that'll make your floor cold...the key is to avoid a temp. difference from inside to outside, or moisture will condense on your wood, as your underside will be colder than outdoors, generally. Warm, humid air comes in, meets colder air underneath, and gives up its moisture - UGH. Rot.

[Manalto]

Honestly, the deal is....Vent. OR, Seal. In other words...100% vented, like you see with a home on pillars. Or seal it completely. That means an impermeable vapor barrier (rubber, fairly pricey) that's affixed to the foundation (brick, cement, etc.) nearly air-tight... the OTHER thing you could do would be to make sure the underside is WAY OPEN (big air flow). But, in cold times, that'll make your floor cold...the key is to avoid a temp. difference from inside to outside, or moisture will condense on your wood, as your underside will be colder than outdoors, generally. Warm, humid air comes in, meets colder air underneath, and gives up its moisture - UGH. Rot.

I neglected to mention in my OP that the rear section of the house, the part that was added on in the 1950s, is completely open at ground level. Because of the wide roof overhang, it remains dry underneath the house. Air circulation is optimal. The neighborhood cats, unfortunately, are attracted to this spot. (This can't continue.) Since hermetically sealing the crawl space (and controlling its humidity) is not a practical solution, installing some kind of mesh or discreet barrier to wildlife seems to be the right plan of action.

As you say, the floors are uncomfortably cold during those rare times when Gulf-Coast temperatures drop into the 30s or 20s. (Laying the VCT kitchen floor last winter was an exercise in juggling heat guns and space heaters in order to get the materials to behave themselves.) The air circulation under the house that moderates the temperature on hot days allows for toe-chilling floors on the coldest days of winter.

This winter I plan to install my cast iron radiators and wonder if the pipes, which will run directly under the floors, if left uninsulated, can serve to alleviate some of that chill. I worry that the heat loss may mean tepid radiators and an ineffective system. I suspect that this kind of radiant heat under a wood floor will simply be a waste of energy unless a significant amount of insulation is installed beneath it, which brings us back full circle to the impractical task of creating a hermetically-sealed space.

I guess good radiators, slippers and rugs are the answer.

[matchbookhouse]

I had an extremely damp crawlspace, since the water table was less than a foot below the surface of the yard and I was 3 blocks from the river. I also had a French drain and 2 sump pumps to remove water, but the rising damp was really affecting the old joists. When I had the crawlspace sealed, they installed access doors with rubber seals and closed off the vents entirely. Then they put down special heavy liner throughout the crawl that also went up the walls and attached to the side walls below the sill plate and up all the support piers as well. I had a commercial dehumidifier installed that ran automatically and kept the humidity around 45%. Of course, it naturally dropped lower in the winter. It was an expensive but very worthwhile investment for my particular house and situation. Since my crawlspace was only 2 feet high, the process also made it very clean and bright, which the plumbers and electricians really liked.

[Gothichome]

Slippers and a good sweater would be the period thing to do James.
We sealed with expanding foam so our basement is too tight. Although in the winter when the furnace is on it does stay warm down there so the relative humidity is not an issue. Gothichome is two thirds dirt crawl space, the last third being unsealed cement floor. So it is all water permeable to some extent. And Gibson is correct the mustiness is coming from the dirt both in the crawl space and basement. Mitigating this mustiness is not practical but getting it out of the basement and fresh out side air in is.

[Manalto]

Slippers and a good sweater would be the period thing to do James.

Sweater? I think not, Ron. With my new (1927) cast iron radiators, the house will be toasty warm. I may even have to crack a window from time to time.

I'm surprised, because of a number of factors including Mobile's high annual rainfall, stray cats, termite damage, the house being closed up for extended periods of time, etc., that there is no musty smell. And it isn't just my nose-blind self that thinks so. I've asked visitors who wouldn't be shy about speaking up and, for some reason, the house smells good. From the conversation here I've determined that what I need is merely to exclude wildlife with a barrier that continues to permit airflow.

[GibsonGM]

Yes, that's what I'd do, Manalto. We are much much farther north. How low does it go in winter for you? Gothic is in Ontario, I am in Maine. Winter can give me -30 degrees F. No open crawlspaces for us up here!! We tend to install a membrane and seal them.

But you may be able to absorb the cost of having your heat pipes down there in winter. I wouldn't bother leave them uninsulated 'for comfort'...then they will lose too much heat. Maybe insulate the pipes well, and accept the fact that the floors may be cool; wear slippers The last resort you have is to insulate the floor, but any moisture getting in there will rot the floor pretty quickly - it's not advisable. I'd sooner close in and seal the crawlspace.

[Manalto]

Yes, that's what I'd do, Manalto. We are much much farther north. How low does it go in winter for you? Gothic is in Ontario, I am in Maine. Winter can give me -30 degrees F. No open crawlspaces for us up here!!

It will regularly go down into the 30s at night during the winter, 20s is unusual and, rarely, the high teens, so winters are mild and often quite pleasant. Summers are another story, with high temperatures and humidity. I've adapted somewhat but avoid working outdoors in the sun.