[Greenbuilding] Crawlspace

nick pine nick at early.com
Sat Jun 2 20:22:00 CDT 2012


Stuart Fix <sfix at renubuildings.com> wrote:

> Humidity within the crawlspace should be controlled via poly on the ground

That would seem to invite condensation from warm moist house air in a cold 
climate, without a vapor barrier below the floor and insulation below that.

> and proper ventilation (either passive screens or active fan/humidistat).

Passive screens would seem to invite condensation from warm moist outdoor 
summertime air. So would automatic foundation vents that are open at 70 F... 
http://www.amazon.com/AIR-VENT-RABL-AUTO-FOUNDATION/dp/B000IJWOV6 How about 
a smart vent, ie a fan with a differential absolute humidity control... 
http://www.smartvent.biz/ ?

If a simple humidistat turns on a fan to move outdoor air through a 
crawlspace when the crawlspace RH is too high, say higher than 60%, what 
prevents the fan from moving warm moist outdoor summertime air through the 
crawlspace, with condensation?

> Eli's comment:
> The issue I am trying to understand is how to maintain in a ventilated 
> crawlspace to be warmer than dew point.  The vapor barrier on the ground 
> only stops ground sources of humidity.  A high standard thermal barrier in 
> the floor keeps the ac cooling from cooling the bottom surface.  However, 
> the shading will keep the crawlspace cooler than outside.  Could this 
> cause condensation?

> Response...

> The only way to STOP condensation from occurring, in the situation you've 
> described, would be to heat the crawlspace, or to ventilate the space at a 
> high enough rate that it maintains the same conditions as outdoors. Both 
> are likely a waste of energy.

The second way seems especially wasteful, since maintaining exactly the same 
conditions as outdoors requires infinite fanpower :-)

Gary BIs <gary at builditsolar.com> wrote:

> I converted my crawl space to a conditioned space as discussed in this:
> http://www.builditsolar.com/Projects/Conservation/Insulation/CrawlSpace29238.pdf

Do they suggest a vapor barrier under the floor? They say "Air sealing 
between the house and crawlspace is less critical" with crawlspace wall 
insulation.

> I put two layers of poly on the dirt and sealed it around the edges and at 
> seams. I blocked all the vents and I foam can sealed all around the rim 
> joist. Then insulated the crawl space walls with polyiso foam board with 
> tape  on the seams... its been about 6 years, and its worked fine for me. 
> The crawl space feels dry.  I do live in a dry climate (SW MT)...

Amen?

Nick 





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