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nealtw 02-22-2011 11:40 PM

Insulating crawl space ???
I keep reading about insulating the walls and sealing up the crawl space, I have a few questions.
1 After closing up crawl space, are you heating it?
2 If your not heating it, it is still a cold zone, no?
3 Condensation will occur when warm meets cold, at the floor yes?
4 When a house is sheeted with plywood or osb there a gaps left for airflow and under windows holes are drilled so trapped moisture can get out behind the siding.
5 Many crawl spaces have some walls that are wood frame and were never ment to be insulated, so the vent holes were never drilled. If you end up with any amount of heat in the crawlspace you will have moisture build up against the sheeting and this is a big problem isn't it?
6 When you have any kind of plumbing leak water spill in the house, it will find its way to the crawlspace, are you going to leave it there untill spring.

WindowsonWashington 06-03-2013 03:41 AM


1. Yes. You should supply heat to the space once it is insulated.
2. No.
3. Potentially, however, ground temps are pretty stable and once the stem walls are insulated, pretty high.
4. Not sure about this one.
5. If you do not use the proper insulation and air barrier, yes.
6. Never had to deal with it but I would imagine that cleaning it up and fixing the leak are advisable.

nealtw 06-03-2013 08:09 AM


Originally Posted by WindowsonWashington (Post 87692)

1. Yes. You should supply heat to the space once it is insulated.
2. No.
3. Potentially, however, ground temps are pretty stable and once the stem walls are insulated, pretty high.
4. Not sure about this one.
5. If you do not use the proper insulation and air barrier, yes.
6. Never had to deal with it but I would imagine that cleaning it up and fixing the leak are advisable.

If you devert hot air from the furnace to the crawl space, are you taking return air from there too. Are you not adding moisture from the house to the crawls space. Once you do have a leak in the roof or plumbing and water gets in the crawlspace, you have to have a way to clean it up, as it often isn't descouvered soon enough, won't you be bringing back into the house.
Perimeter drains often fail in older house and can result in inces of water in a closed area, do you think the poly on the floor will hold that back.

WindowsonWashington 06-03-2013 09:53 AM

If you have a leak, you will get water in the crawl space and there isn't much you can do about that besides be aware of the leak.

Most crawls have enough communication with the home that they do not require return air.

The only way that you would condition the space is if you are making sure that the moisture is not getting into the system (i.e. proper foam on the stem walls, proper vapor retarder on the floor and sealed to the stem wall, etc).

Sealed poly on the floor that is sealed to the stem wall will hold a bunch of water down.

Again, if something fails, that is a failure and cannot really be accounted for except that there are failures in the home regardless of what system is in place.

nealtw 06-03-2013 01:09 PM

Systems in the houses have failed for hundreds of years, for the most part the venting of a crawl space has worked well. I newer tight houses ever gap and hole is filled for air infiltration and firestopping. I can't immagine the bennifet over risk here. And what is the bennifet.

GBR 06-03-2013 07:39 PM

Here in the States, air exchange is required for a closed crawl, 5 ways to do that, pp6: "The bottom line is that crawl space ventilation is not required by the model codes if:
• a ground cover is provided
• the perimeter walls are insulated
• the crawl space is conditioned"--- from;


nealtw 06-03-2013 08:30 PM

Thanks Gary; So by conditioned , you mean air movement, not likey hot water and or electric heat?

WindowsonWashington 06-04-2013 05:49 AM

Roof fail as some point but that does not mean I am going to not install one for that reason.

Vented crawlspaces are fine and in certain applications, make sense. The issue is that most often times, they are not insulated properly and are very difficult to do so with the additional difficulty of insulating around supply lines, duct work, electrical, framing, etc.

Most times, it is easier to just move the insulation to the outer wall and treat it as conditioned.

In a perfect world, you should have some return air section in there. That being said, most crawls are well connected to the home and what small amount of air you need to supply will easily circulate back to the main returns.

Regardless of conditioned or not, the floor should be sealed to block the migration of fumes, moisture, and humidity.

Benefit to a conditioned crawl is that is more effectively keeps the floors warm, makes for more conditioned storage space, dryers and more healthy air, better smell, less bugs, etc.

The biggest and most notable is that the floors above are usually much more livable.

nealtw 06-04-2013 09:43 AM

I've dealt with two new house and one wartime house that were insulated and sealed like this with problems.
House one, 3 story with boiler and hotwater in the crawl space with was heat tubes pinned to underside of floor with insulation below that. Built on a mountain side the front of the foundation was something like 16ft high. Some water had found its way under the foam onto the concrete floor at that wall. We found a plumbing leak at the first floor kitchen sink that only show up when they rented the sweet. The smell in the crawl space was something else, we vented this as this was as there was no other way to dry this and keep it dry.

House two had pooly installed windows, in removing rotten framing under window, the water had reached the crawl space and rotted the rim joist and sill plate, the smell and moisture in the crawlspace was left for homeowner to deal with. No conditioning in crawl space.

House three was an old house that had been repaired with new perimeter drain and waterproofing. Poly and concrete on the floor and foam on the walls. Three heat ducts and a return.
The homeowners were getting sick and had an air quility test down, they found the problem to be air comming from the crawl space and we went in to open the old vents, they had the heat closed off and insulated the floor.

When you get a little further north if you don't heat the basement the pipe they will freeze, so expecting heat to migrate down is questionable at best.
A dry well vented crawspace already is a good storage area.
"If" a floor is sealed between the two floors which is code for firestopping and if the crawlspace is sealed as suggested and if water gets in there, and we know it will. This area will become high humidity and promote rot and mold growth. If you take that to the extreme you could have mold using all the oxigen in the space like a shop rusting.

I would like to see peoples comments after having this done like 5 , 10 , and 15 years later.

GBR 06-04-2013 03:31 PM

The 'conditioning" part is with air exchange with house above. Not simply supply air, as the 5 ways per code all included exhaust also(previous link). One should spread the vapor barrier (6-12mil.) to ease crawling, then air seal the crawlspace floor above, all the wiring, plumbing holes, chases, flues, ducts, etc. with appropriate fire/draft stopping materials to slow the "stack effect" from air rising to the attic through interior/exterior wall cavities (feeding convective looping in low density insulation and degrading R-value due to air movement);
Then go into attic and do the same, to stop the pressure differences that bring crawlspace air up as well as conditioned house air (balanced passive/active ventilation system).

Just as you mentioned piping/venting in the crawl, each house is individual, not to "blanket" cover all by saying; "conditioned crawl is better", that may not work for some;

Energy cost loss rather than savings from changing- Flagstaff;

The vents role in the South;


Check out some of the links from "Technical Reports" at end;

Interesting read;

BSC---- hot dry air;

Open-cell rather than closed-cell SPF in flood areas;

Then there is always the danger of radon or termites when "conditioning" your crawl which many forget to include when giving advice;

Each region/circumstances are different.

PS. I had another link on dangers of a heated crawl in the Great North- melts the surrounding perma-frost to damage foundation when it settles, lost it...

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