Latest Discussions : HVAC


01:39PM | 03/02/03
Member Since: 03/01/03
2 lifetime posts
Having received a massive heating bill this month we are trying to figure the best and most economical way to alleviate this headache from the 1947 home we purchased last summer. Living in Ohio, winters are pretty cold and we need some alternatives. One route we are considering is to insulate the crawl space that is under about half of our house but we are not sure how to do this. I know we need to put a 6 mil layer of plastic on the ground but how do we do the rest? Would sheets of styrofoam type insulation on the walls of the crawl space be effective? The floor above is not really cold so I didn't think floor insulation was the best route. Also, what about the vents? There are two in the roughly 700 square foot area. Should these stay open? Help!

Jay J

04:12PM | 03/06/03
Member Since: 10/26/00
782 lifetime posts
Hi TAlthouse,

I don't have enough info to be too specific, but based on what you've said, I can get you started.

If the crawl's floor is dirt, yes, cover it w/6 mil plastic. be sure you DON'T go up the walls w/it. And put some bricks, or gravel, or something on top of the plastic all around the perimeter.

If your floor isn't cold, then the heat is doing 1 of 2 things. 1) Is it leaking from ABOVE into the crawl below? 2) Are you actually heating the crawlspace, accidentally or intentionally??!

If it's #1, you DO need to insulate. Use Kraft-side R-39 insulation, and install w/the Kraft side facing the WARM part of the floor. THen use Joist Hangers or chicken wire or some type of 'webbing' (home-made or not) to 'hold' the insulation up and to keep it from falling down. If it's #2, then fix the leaks in the ductwork or STOP heating the crawl. THEN, if the floor is cold, follow the idea in #1's answer.

Now, if the walls leak air, either patch them up appropriately or insulate. However, if water is leaking through those cracks, you need to FIX the leaks from the OUTSIDE first, then patch them up on the inside. Otherwise, you'll be revisiting this problem again and again. Assuming all the walls are not leaking, then you can insulate if you want. Hopefully, your patching of the cracks/leaks will suffice.

RE: Vents - It depends on what's done in your part of the country. I live in SE PA and we vent our crawls, year round. General rule: 1 vent per 150 sq ft of space.

My best to ya and hope this helps.

Jay J -Moderator

PS: God Bless America!


01:51AM | 03/23/03
Member Since: 12/23/02
18 lifetime posts
Yes, foam on the walls would be very effective in a cold climate. At least 2 inches of the blue styrofoam is best. You'll have to patch any holes in the wall first. If insulating the walls, do NOT use vents. When the walls are insulated, the crawlspace is to be treated just like a basement- as inside of the building envelope. Venting it only lets in cold air in winter (defeating the purpose of insulation), and moist air in summer, causing rot.
Put a vapour barrier on the floor , and lap it up the walls at least six inches, securing it with mastic. Overlap and secure all seams with aluminum tape or mastic (not regular duct tape).
If you decide to insulate the floor, you must vent the crawlspace, as it then becomes ouside the building envelope.


04:19AM | 09/14/07
Member Since: 09/13/07
2 lifetime posts
In my 1980 house I've insulated my 2 crawl space ceilings with proper insulation to save heat and try to stop musty smell - although crawls appear nice and dry. The musty smell filters up through cold air and ductwork although I can't think or see anything else to tape, seal or insulate. I've insulated crawl walls with the blue insulation; the floors previously had plastic with gravel ontop - I put additional polypropelene made for crawls down on top of that, taped, ran up outside walls 6" etc. Also as I said I have taped and insulated duct work... BUT --- I still have the musty odor problem. We do have original house crawl vents, covered with screen, and louvered so they slide open or closed. There are two vents in each side of our 2 crawls, but only one vent on each side is operable. Each crawl space is about 400 sqft (between 2 crawls there is a block wall about 3/4 up, but there is circulation space between top brick and wood floor above). So -- from previous threads...I assume I should vent crawls year-round because I have a good vapor barrier on floor and 6" up on wall(even though crawls are insulated)? Also - I guess I don't have enough VENTS? What about current vent types(screened/louvered-approx 6x8" each-are they ok? Thanks! We are going crazy with trying to get rid of the odor. We don't just want to mask it.


05:26AM | 09/14/07
Member Since: 04/25/05
1915 lifetime posts
You did not indicate your climate or location.

But in large parts of the US you have 3 or more months of the year where the outside air has a dewpoint of 60 or above. If that air is allowed into a crawlspace that is cool (both naturally from ground contact and from AC running in the house) the RH in that space will be very high and maybe even condenstation.

You don't want air inside the crawlspace. Seal it off.

If you use fiberglass batts under the floor check and see if they or the joist sub-floor are damp.

Do a google on Conditioned Crawlspace and Sealed Crawlspace.

Start at


07:58AM | 09/14/07
Member Since: 09/13/07
2 lifetime posts
The house I was talking about is in Youngstown OH area. The vents are 16x8" as well --- typ-o on last post. Also this is a partial basement. Crawl spaces are in back half of house. Dining and kitchen have regular basement. We keep inside doors on the crawl space acceses (they were there when we moved in) - probably to keep odor down, i'd guess. Thanks for any help!

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