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Insulating a crawl space: walls and/or ceiling?

Posted by thomas1975 (My Page) on
Mon, Mar 4, 13 at 12:07

I've read through many posts on this but wanted to post with my specific information.

I'm in the northeast
Just moved into the home
Home is 40 years old with a 20 year old addition that created a crawl space that's about 40' x 5 or 6'.
Crawl space is cinderblocks and (knock-on-wood) has so far been very dry.
Crawl space has 3 vents to the outside which are as closed as they can be but let in a lot of cold air.
Between the crawl space and original basement are the 2 original casement windows (which we keep closed) and a full size, well-sealed, door.
The main basement area has a powerful Sante Fe dehumidifier.
Above the crawl space is our kitchen which gets freezing the winter.

I've been told various things by various people (insulation companies, basement companies, contractors...) and wanted to get some opinions here on the following:

1. Should I permanently close off the 3 vents in the crawl space?
2. Should I insulate the walls of the crawl space?
3. Should I insulate the ceiling of the crawl space?

I've been told basically the opposite on all of these questions: yes close the vents they are not necessary, no don't close them as you need the airflow. insulate the walls of the crawl space since if you insulate just the ceiling it will drop the temperature of the crawl space and could freeze my pipes, insulate just the ceiling as that will warm up the kitchen and won't affect the crawl space, etc...

Thanks for your help!

Follow-Up Postings:

RE: Insulating a crawl space: walls and/or ceiling?

In regions with extended periods of humid hot conditions, such as the northeast US, closed insulated unvented crawlspaces work best in keeping out mould-inducing humidity from the crawlspace and the living space above.

So air seal and insulate the walls of your crawlspace but ONLY with foamboard or sprayfoam.

Here is a link that might be useful: The Scary Crawlspace

RE: Insulating a crawl space: walls and/or ceiling?

Nice article worthy, thanks for that...
I love this part... "Don’t vent dryers or other appliances into the crawl" Who would do that? Every load of washing machine water direct into your crawlspace. Sheesh.

For closed conditioned crawl spaces the recommendations were...

•Insulate the perimeter walls to the level suggested for above-grade walls in your climate.

•Exterior perimeter wall insulation is a good option to consider, especially in new construction. It eliminates the problems of moisture management associated with interior wall insulation and provides the possible benefits of additional thermal mass within the structure. The downside is you have to protect the insulation from termites and the above- grade portion of the insulation from deterioration caused by weather exposure.

•When insulating a crawl from the interior, you should protect rim joists from condensation with foam insulation.

•Use water-resistant insulating materials, such as rigid-foam board or spray foam properly protected to meet fire ratings. Avoid using air-permeable insulation, such as fiberglass batts or spray-on cellulose.

•Power venting a sealed crawl space to the exterior can reduce the movement of air from the crawl space into the house. This can improve IAQ, but at the added cost of operating and maintaining the exhaust fan.

I just might do that. i.e. insulate the walls and rim joists cuz our floors over the crawl space zone of the house are really cold in the winter. The issue there is "properly protected". Per the fire ratings, XPS sheets are to be covered w drywall or finished. Obviously a crawlspace you're not going to finish in drywall like you would a full height basement. So they don't really address what to do there -- cant leave it exposed but you're not gonna frame and finish...

RE: Insulating a crawl space: walls and/or ceiling?

If the crawlspace is only used to access utilities, an ignition barrier--as opposed to a thermal barrier--may be used, which may be somewhat less costly to install. Intumescent coatings can be used. Thermax, a polyisocyanurate sheathing with a metal foil covering, is often used in commercial settings.

Get the approval of the local building authority to be sure. In case of damage claims, insurance companies may use non-code compliant installations to deny coverage.

RE: Insulating a crawl space: walls and/or ceiling?

I live in the capitol of hot and humid and closing off the foundation during hot and humid is not what we do here as the air stagnats and it rains under the house. We generally use the simple vents that close themselves when temperature gets below 50 degrees they are a stock item at the home depots around here.

I made the mistake of closing in one of my original homes I built here with an insulated foundation with closed foundation flood vents and failed to put a vapor barrier on the soil i.e. visquinn so it turned into a sauna after awhile.

Try covering the earth with visquin first its inexpensive and may solve your problem.

Good luck,


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