What is the ideal temp/humidity level for a crawlspace ?

[gregtwojeeps]

A have a 1959 vintage one level home over a crawlspace that houses the horizontal flow gas furnace/AC/air handler. I went and checked the crawlspace one 90 deg.f. day when the AC was working hard and found all of the uninsulated metal duct work in the crawlspace was sweating profusely. ...

I bought a temp/humidity digital unit with a remote sensor and put the sensor in the crawlspace and it read 89% R.H. which probably explained why all the duct work was dripping water from condensation whenever the AC was on. ..

So I bought a 70 pint dehumidifier and put it in the crawlspace and now have the RH at 65% with a avg. temp of 71 deg.f.. I am looking for a RH level I can maintain, that will not be too high for the wood floor joists/real oak hardwood floors in my home, yet not so low where this de-humidifier is going to run too often .... and spike my electric bill needlessly. Thanks, Greg

 

[Radix2]

What is the floor in the crawl space ? Dirt? Crawl space vents?

If you can make sure you have a good grade away from the house and put a vapor barrier on the floor, you will have the best shot at keeping the humidity down. The ideal would be to maintain the humidity similar to your comfortable Indoor spaces...tough to do.

 

[larry_g]

A have a 1959 vintage one level home over a crawlspace that houses the horizontal flow gas furnace/AC/air handler. I went and checked the crawlspace one 90 deg.f. day when the AC was working hard and found all of the uninsulated metal duct work in the crawlspace was sweating profusely. ...

I bought a temp/humidity digital unit with a remote sensor and put the sensor in the crawlspace and it read 89% R.H. which probably explained why all the duct work was dripping water from condensation whenever the AC was on. ..

So I bought a 70 pint dehumidifier and put it in the crawlspace and now have the RH at 65% with a avg. temp of 71 deg.f.. I am looking for a RH level I can maintain, that will not be too high for the wood floor joists/real oak hardwood floors in my home, yet not so low where this de-humidifier is going to run too often .... and spike my electric bill needlessly. Thanks, Greg

I think that your are spending money to run a dehumidifier and at the same time loosing $$ through the uninsulated ducting. Spend the money to insulate the ducting and the RH will not be a problem there.

lg
no neat sig line

 

[gregtwojeeps]

I think that your are spending money to run a dehumidifier and at the same time loosing $$ through the uninsulated ducting. Spend the money to insulate the ducting and the RH will not be a problem there.

lg
no neat sig line

Good point Larry,

A question though. If I insulate the heat/cool ducts I will then basically have no heat in the crawlspace where my water lines are and we do get sub-zero winter temps here. The floor joists do have 5.5 in. faced batts 16 in. o.c. between them keeping the hardwood floors warmer in the house. Do I insulate the ducts and maybe just cut an adjustable supply register in to one of the branch lines to put a little heat in the crawl space ?...

I don't think it will take much heat to keep the water pipes from freezing though, because with the crawlspace being totally under the grade the soil may give it some "insulation" factor like the old "below ground cellars" or is that just a old mans tale ?

 

[DC73]

Radix2 is on the right track. Is your crawl space vented or unvented? Do you have a vapor barrier on the floor of the space? In your humid climate, your crawl space is probably better off being unvented. In a dry climate, vented crawl spaces can work.

You do need to insulate your duct work and make sure it has NO leaks. You don't want to be drawing radon or other out-gasses from the soil into your home.

Here are two good articles to read on unvented crawl spaces:

http://www.greenbuildingadvisor.com/blogs/dept/musings/building-unvented-crawl-space

http://www.greenbuildingadvisor.com...paces-and-psychrometric-chart-are-not-friends

And, from BuildingScience.com, here is a fairly technical article on moisture control:

http://buildingscience.com/documents/digests/bsd-012-moisture-control-for-new-residential-buildings

One take away from the BuildingScience.com article is this: "Leaky return ducts located in vented crawlspaces draw significant amounts of soil gas, moisture, possibly pesticides, radon, and other pollutants into the conditioned spaces, often creating moisture problems and increasing heating loads during the heating months and cooling loads during the cooling periods as well as risking occupant health and safety. Leaky supply ducts located in vented crawlspaces lead to the uncontrolled depressurization of the conditioned space, leading to excessive infiltration of cold air during heating periods, increasing heating loads and potentially supplying sufficient interior moisture to attic and roof assemblies to create roof sheathing moisture and decay problems. During cooling periods, the same mechanism can lead to the infiltration of exterior warm moisture laden air, increasing cooling loads."


In short, insulate your ducts, seal them against leaks, insulate your pipes, and create an unvented crawl space in accordance with the article listed.


DC

 

[gregtwojeeps]

Good links to read, thanks. Looks like duct insulation will be the helper for a solution. The dehumidifier right now is keeping the CS an avg. temp of 71 with RH % of 62-68. I just don't know how much it is running to maintain these numbers though. I bet my electric bill will show me next month when it comes in.