Thread: Should I vent my crawlspace?

My house is near San Francisco, California, i.e. humid, temperate climate. No freezing.

My crawlspace currently has zero vents. No insulation at cripple wall or at floor joists. Crawlspace floor is dirt, feels dry at surface, but darker color earth 3" down indicates the soil isn't moisture-free.

Like many of our neighbors, we have a lot of moisture in the house. Sometimes the carpet feels damp-ish. Tightly packed closets without air circulation are susceptible to mold. Wintertime is the worst, as those familiar with the climate would know.

I'm not sure if opening some passive vents will let moisture out or let more in. So far, my research indicates there is arguments for both sides. I don't know what to do.

I know the IRC requires vents. But my understanding is that code requirement might be very outdated? Above the crawlspace in the living areas, the exterior walls and ceiling are fully insulated.

I can't justify the cost of "encapsulating" with plastic membranes.

I'd really appreciate the chance to hear your opinion.

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Originally Posted by Geoffrey Brunn

I can't justify the cost of "encapsulating" with plastic membranes.

I guess then your just s**t out of luck.

See
http://www.crawlspaces.org for my recommended solution of proper crawlspace management and control of moisture. It works.

In general it is preferable to ventilate the crawlspace and provide a vapor barrier on grade and between the conditioned space and the unconditioned space. There are many other solutions that deviate from this but the issues are too complex for this forum and the limited information provided.

Originally Posted by Geoffrey Brunn

My house is near San Francisco, California, i.e. humid, temperate climate. No freezing.

My crawlspace currently has zero vents. No insulation at cripple wall or at floor joists. Crawlspace floor is dirt, feels dry at surface, but darker color earth 3" down indicates the soil isn't moisture-free.

Like many of our neighbors, we have a lot of moisture in the house. Sometimes the carpet feels damp-ish. Tightly packed closets without air circulation are susceptible to mold. Wintertime is the worst, as those familiar with the climate would know.

I'm not sure if opening some passive vents will let moisture out or let more in. So far, my research indicates there is arguments for both sides. I don't know what to do.

I know the IRC requires vents. But my understanding is that code requirement might be very outdated? Above the crawlspace in the living areas, the exterior walls and ceiling are fully insulated.

I can't justify the cost of "encapsulating" with plastic membranes.

I'd really appreciate the chance to hear your opinion.

If you are having that much of a moisture issue in the home I say yes, vent the crawl.3 inches down and the soil is pretty moist say you do have a lot of moisture in the crawl. As it evaporates it goes into your home from the leaky floor of the older home. If it was not going into your home the underside of your floor as in the wood in the crawl would have an excessive moisture content and then you would have mold in the crawl.

You can read any article you wish but with the problems you are having you could ONLY benefit from venting the crawl space in my humble opinion. Short of completely encapsulating the crawl and I am not a huge fan of that because most i have seen never really "encapsulate" the crawl. It still allows a good bit of moisture into the crawl and up into the home.

Mold or suspected mold in closed closets. Damp feeling carpet. A lot of fogged windows from time to time I am sure.

Are you venting the bathrooms with a ceiling vent coming on the entire time one is in there? Are you venting those vent to the exterior and not the attic?

As far as any sizable closets I strongly suggest in adding a small HVAC vent to that closet.

I hope this helps. As far as I am concerned this whole "don't vent the crawl" thing is way out of hand even in some of the more humid climates. I lived either next to or within a mile or two of the ocean for 50 years. Lots of humidity. Venting the crawls did nothing but good. Unless your home was built with an enclosed crawl properly from day one it will cost you a bundle to have it done correctly and I doubt it will ever be like one that was done while building and leaking moisture into the crawl space

Originally Posted by Geoffrey Brunn

My house is near San Francisco, California, i.e. humid, temperate climate. No freezing.

My crawlspace currently has zero vents. No insulation at cripple wall or at floor joists. Crawlspace floor is dirt, feels dry at surface, but darker color earth 3" down indicates the soil isn't moisture-free.

Like many of our neighbors, we have a lot of moisture in the house. Sometimes the carpet feels damp-ish. Tightly packed closets without air circulation are susceptible to mold. Wintertime is the worst, as those familiar with the climate would know.

I'm not sure if opening some passive vents will let moisture out or let more in. So far, my research indicates there is arguments for both sides. I don't know what to do.

I know the IRC requires vents. But my understanding is that code requirement might be very outdated? Above the crawlspace in the living areas, the exterior walls and ceiling are fully insulated.

I can't justify the cost of "encapsulating" with plastic membranes.

I'd really appreciate the chance to hear your opinion.

Well, if you do not have a moisture barrier on the ground in the crawlspace you need to put one down. A plastic moisture barrier is cheap, especially if you do it yourself.

If your house is damp/humid you more than likely have a ventilation problem. Are the bathroom and kitchen vents vented to the outside/exterior of the home? They need to be..... If you have no vents you need them!

You might want to run your A/C more or buy a dehumidifier to help control the moisture in the air of the home in the winter months, if your heat is not doing enough to dry out the air. Usually the winter months are the dry months as the heat drys out the air inside a home, unless you have gas heat that is not vented properly...

Thanks for the great responses guys.

To answer a few items you've raised:
- In my area, San Francisco, the winter months are the wet months (think Mediterranean climate)
- there is no A/C system, standard for the area
- However, I asked my HVAC contractor about a whole home dehumidifier. He tells me that would be an extreme measure. I figure if the crawlspace vents don't do it, then I might move onto something like this
- the bath and range hood are both vented through the roof, although I am considering a stronger bath fan since the current one might be underpowered (Tim Allen, anyone??) Related but tangential issue, though.

I think I'm leaning towards Ted's response above. I really appreciate all the input from everyone. Going to do the crawlspace vents next weekend. I never knew closets might be vented, but then when I think about it, why not. I'll look into this too.

Before you run out and buy anything, read up on conditioned crawlspaces. You did not give an age or sq footage of the home. Nor the height of the crawl, but if it is dirt, it is likely a low crawl.

My climate is similar, but more to the extreme, colder and wetter. An acceptable way to cure moisture issues here is to install a heat source in the crawlspace, simply an electric baseboard heater, and insulate the outer walls of the crawlspace with foam panels.

Yes, you need to lay down a 6 mil poly vapor barrier over the dirt, that is step one. Ideally, it is sealed with Tuc Tape or mastic to the concrete foundation walls. Here, we have gone completely to concrete skim coat over the poly. That is the best way to seal out the rising damp.

Leave a gap all around the lower edges of the insulating panels. This is so that you can inspect for termite tubes.

Now set the heater thermostat to a reasonable temp, so that the dew point is never reached and your floors are always a bit warm. No need to let moist outdoor air in, like that San Fransisco fog.

Originally Posted by John Kogel

Before you run out and buy anything, read up on conditioned crawlspaces. You did not give an age or sq footage of the home. Nor the height of the crawl, but if it is dirt, it is likely a low crawl.

My climate is similar, but more to the extreme, colder and wetter. An acceptable way to cure moisture issues here is to install a heat source in the crawlspace, simply an electric baseboard heater, and insulate the outer walls of the crawlspace with foam panels.

Yes, you need to lay down a 6 mil poly vapor barrier over the dirt, that is step one. Ideally, it is sealed with Tuc Tape or mastic to the concrete foundation walls. Here, we have gone completely to concrete skim coat over the poly. That is the best way to seal out the rising damp.

Leave a gap all around the lower edges of the insulating panels. This is so that you can inspect for termite tubes.

Now set the heater thermostat to a reasonable temp, so that the dew point is never reached and your floors are always a bit warm. No need to let moist outdoor air in, like that San Fransisco fog.

Hmmm... Could this explain perhaps why the previous owner was blowing hot air into the crawlspace? Seriously, it was an open branch from the forced air furnace plenum just dumping hot air into the crawlspace. We removed it, thinking it was wasteful. But you're saying it may have been a clever but partial solution to our problem.

The house is 700 sq ft, and crawlspace is roughly 3' tall.

John, sometimes the relative humidity outside here can be 90%, making the dew point just a few degrees lower than ambient temperature. Let me know if the following logic makes sense:

If the crawlspace has no plastic membrane and no vents, then the dirt floor will act as a constant source of moisture, keeping ambient humidity at--just guessing--50%. If the space is heated to, say, 70degrees Fahrenheit, then the dew point would be roughly 20degrees below ambient, maybe at 50deg. Any uninsulated sill plate or stud would be at or near outside/earth temperature at maybe 45 degrees. So they'll have condensation at their surface and will rot, right? So leaving ambient temp lower than room temp would be a solution to that problem. OR, remove moisture via plastic membranes, right?

Let's assume there is no significant source of moisture from the living spaces. There are definitely no roof leaks. Thus, the moisture inside the house must be coming from the dirt below.

Here's something that's just dawning on me: why not buy a dehumidifier and just leave it in the crawlspace?? (Run the drain hose to somewhere appropriate, obviously). Sure, it'll run and use some electricity, but if I am selling the house in 2 years, then I'd never recoup financially on any significant capital improvements.

I've recommended dedicated crawlspace dehumidifiers on a few occasions. Most crawlspaces around here will take on more humidity if you vent them due to the humid summers we get. I typically recommend sealing the vents and putting a vapor barrier on the floor if it is exposed earth or stone. Crawls are more the exception than the rule here but not uncommon either. Full basements are the norm here in SE Pennsylvania.

Originally Posted by Geoffrey Brunn

Hmmm... Could this explain perhaps why the previous owner was blowing hot air into the crawlspace? Seriously, it was an open branch from the forced air furnace plenum just dumping hot air into the crawlspace. We removed it, thinking it was wasteful. But you're saying it may have been a clever but partial solution to our problem.

The house is 700 sq ft, and crawlspace is roughly 3' tall.

John, sometimes the relative humidity outside here can be 90%, making the dew point just a few degrees lower than ambient temperature. Let me know if the following logic makes sense:

If the crawlspace has no plastic membrane and no vents, then the dirt floor will act as a constant source of moisture, keeping ambient humidity at--just guessing--50%. If the space is heated to, say, 70degrees Fahrenheit, then the dew point would be roughly 20degrees below ambient, maybe at 50deg. Any uninsulated sill plate or stud would be at or near outside/earth temperature at maybe 45 degrees. So they'll have condensation at their surface and will rot, right? So leaving ambient temp lower than room temp would be a solution to that problem. OR, remove moisture via plastic membranes, right?

Let's assume there is no significant source of moisture from the living spaces. There are definitely no roof leaks. Thus, the moisture inside the house must be coming from the dirt below.

Here's something that's just dawning on me: why not buy a dehumidifier and just leave it in the crawlspace?? (Run the drain hose to somewhere appropriate, obviously). Sure, it'll run and use some electricity, but if I am selling the house in 2 years, then I'd never recoup financially on any significant capital improvements.

I believe it was mentioned already but blowing air into a crawl that is not completely sealed or encapsulated will dry the soil by evaporation and that pressurized air blows up into the home from the leaky floor pushing all the moisture into the home.

You mention fog and humidity. That is for only part of the day or should i say early morning in most cases then it is quite pleasant outside all year.

I canot repeat enough to not blow air into the crawl because it is going to pressurize and blow right back into the home. If as you say you have three feet in the crawl then you can get down there and maybe and I say maybe seal the floor off with plastic taped at all the joints and then to the wall. Taping to the walls no matter what the material is about an impossible task so you will have to think mechanically fastening the plastic to the walls and then seal it with a mastic/caulking. That will help tremendously. As far as insulating the stem walls? It is California and the outside temp is almost never a concern since the ground temperature will always be higher than outside temperatures in the winter.

Anyways, way to much to figure as to what you should do. An experienced man from that area would be better as he will know the conditions in your area and inspect the crawl and moisture level to see what is best.

All is this is a guesstimate from any of us as we have never sen the home or know what conditions there are in the area and in particular your home. A lot of the info I gave you should ring true but maybe not all of it.

70 degrees F is too much. You don't need to heat the crawlspace, just take the chill off.
If you already have duct pipes running thru there, the heat loss from the ductwork is often all you need.

Vapor barrier is number one. Insulate the outer walls.