[Greenbuilding] Crawlspace

[Stuart Fix]

RE: Nick Pine's Comments:

Stuart:
Humidity within the crawlspace should be controlled via poly on the ground

Nick:
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.

---> All of my comments assume that you have an airtight, insulated floor,
with poly/VR on the warm side. If you have a leaky floor with no vapor
retarder, you're going to have moisture problems regardless of what you do
in the crawlspace. The floor assembly should be an effective barrier
against heat transfer (insulation), air leakage (air tight construction),
and vapor diffusion (poly/some other vapor retarder). Once this is
accomplished, the crawlspace should be treated like an outdoor space,
however one that is protected from ground moisture (site grading & poly on
the soil), wind & animals (skirting walls). BUT it should be designed to
allow outward drying in a manner appropriate for your climate.
___________________________

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

Nick:
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/ ?

---> again the game is to disconnect the crawlspace from the home above,
and then deal with it like an exterior space. Nothing you do will
completely prevent condensation within the crawlspace, you must ensure
that throughout a year, there is more drying than wetting. If you live in
a climate where there is more wetting potential due to humid summer air,
then you should likely be building the crawlspace without skirting walls,
or building slab-on grade (aka eliminate the cool underside of the home).
In colder climates, there is typically more wetting potential during the
winter, and drying out during the summer isn't an issue. Skirting walls
are used to stop cold wind washing of the underside of the home &
utilities running from the ground into the home.
___________________________

Nick:
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?

---> Sure, that could happen, if it's above 60%RH outside then that fan
would just run constantly. That's a rare occurrence in cold climates. This
is a climate dependent solution; if you like in a hot, humid climate, I'd
build slab-on grade, or raised up on stilts, no enclosed crawlspace. OR
hope that there's enough drying potential during cooler parts of the year
to dry up condensation caused during hot humid months.
___________________________

> 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.

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

---> Agreed. Back to Eli's original comment, it's important to ask this
question with your specific climate in mind. Yes, there could certainly be
condensation on the underside of the floor during this summer condition
where hot, humid exterior air wafts under the crawlspace and hits a
slightly cooled (from AC indoors) underside of the floor. How do you
counter this? You can't, directly, without some extreme measures like I
mentioned above (heating/excessive ventilation). The approach I'd
recommend is to determine what the balance of wetting & drying conditions
are in your climate. If this summer wetting condition dominates the annual
balance, and it's unlikely that it can dry out season to season, then you
shouldn't build with an enclosed crawlspace (go slab on grade or raised on
stilts). If the seasonal balance will allow sufficient drying, then you
follow the passive or active ventilation strategy. If you go with a
fan/humidistat, then either turn it off during the hot humid season, or
use a smart fan like Nick proposed above.

---> Backing way up. IF you have an older home that has an air-leaky, non
vapor retarded, uninsulated floor, with a crawlspace below, I'd recommend
you renovate the floor to fix these issues, or be content to repair
moisture/mold damage as it occurs. Regardless of climate, these issues
will cause condensation within the floor envelope & crawlspace during the
winter (vapor drive from the home), and/or during the summer (vapor drive
from exterior, into the crawlspace, into the envelope). Unless you live in
a very dry climate, this will be a problem over time. Not using air
conditioning in a hot humid climate will mitigate moisture problems as
well.

Cheers,

Stuart Fix, P.Eng., LEED® AP
PHI Certified Passive House Designer
MASc. Building Science
Mechanical Engineer
ReNü Building Science Inc.

52 Airport Road | Edmonton, Alberta | T5G 0W7| C. 780.554.8192
| sfix at renubuildings.com