Thread: No Crawlspace Ventilation

Most times when we see an unconditioned crawlspace on dirt around here, it needs to have ventilation. This 1940's bungalow had a slab addition put on the back around 1975. The addition blocked off the only access hatch in the back wall. There were not vents in the concrete perimeter walls. The owners were having trouble selling the home with no crawlspace access, so they cut a hatch in the closet. Then for some reason, they picked me to go check it out.
As you can see from the pics, it was dusty dry in there, no problem whatsoever.

My point is, what works in one location won't work everywhere.

A few weeks ago, I inspected a nearly new home on a nice conditioned crawlspace. The builder had cut 2 vents in the floor, one in a bedroom and one in the kitchen, to provide ventilation for the crawlspace. Maybe I'm wrong, but I told the buyer he should close the vents off and maybe put in a couple of vents to the outside. Who would want that musty air in a bedroom?

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There are different versions of "Closed Crawlspace", some are conditioned and others are not. I don't know of any that allow return air vents into the living space.

This is what the NC codes says about conditioned crawl spaces:


R4Q9,5 Space moisture vapor control.

Closed crawl spaces
shall be provided with a mechanical drying capability to control
space moisture levels The allowed methods are listed
below in Sections R4Q9 5 1 through R4Q9 5 5 At least one
method shall be provided' however combination systems shall
be allowed

R409.5.1 Dehumidifier.

A permanently installed dehumidifier
shall be provided in the crawl space. The minimum rated
capacity per day is 15 pints (7.1 L). Condensate discharge
shall be drained to daylight or interior condensate pump Permanently
installed dehumidifier shall be provided with an
electrical outlet

R4Q9,5,2 Supply air,

Supply air from the dwelling air conditioning
system shall be ducted into the crawl space at the
rate of 1 cubic foot per minute (Q 5 LIs) per 3Q square feet
(4.6 m2) of crawl space floor area. No return air duct from
the crawl space to the dwelling air conditioning system is
allowed. The crawl space supply air duct shall be fitted with
a backflow damper to prevent the entry of crawl space air
into the supply duct system when the system fan is not operating
An air relief vent to the outdoors may be installed
Crawl spaces with moisture vapor control installed in accordance
with this section are not considered plenums

R4Q9,5,3 House air,

House air shall be blown into the crawl
space with a fan at the rate of 1 cubic foot per minute (Q 5
Lis) per 50 square feet (4.6 m2) of crawl space floor area.
The fan motor shall be rated for continuous duty. No return
air duct from the crawl space to the dwelling air conditioning
system is allowed An air relief vent to the outdoors may
be installed Crawl spaces with moisture vapor control
installed in accordance with this section are not considered
plenums

R4Q9,5,4 Exhaust fan,

Crawl space air shall be exhausted
to outside with a fan at the rate of 1 cubic foot per minute
(0.5 Lis) per 50 square feet (4.6 m2) of crawl space floor
area. The fan motor shall be rated for continuous duty. There
is no requirement for makeup air

R4Q9,5,5 Conditioned space,

The crawl space shall be
designed as a heated and cooled conditioned space with
wall insulation installed per the requirements of Section
R4Q9 8 1 Intentionally returning air from the crawl space to

space-conditioning equipment that serves the dwelling shall
be allowed Foam plastic insulation located in a crawl space
plenum shall be protected against ignition by an approved

thermal barrier

Originally Posted by James Duffin

This is what the NC codes says about conditioned crawl spaces:

R4Q9,5,5 Conditioned space,

The crawl space shall be

designed as a heated and cooled conditioned space with
wall insulation installed per the requirements of Section
R4Q9 8 1 Intentionally returning air from the crawl space to

space-conditioning equipment that serves the dwelling shall
be allowed Foam plastic insulation located in a crawl space
plenum shall be protected against ignition by an approved

thermal barrier

That's new to me. Do you know when it went into effect?

It is not a sealed crawl space by any means. There is dirt floors in a good part of it.

To open vents into the home in the bedroom and kitchen or any room for that matter without the space below being completely sealed is about the most foolish thing I have seen in a long time.

You named it right when saying "who wants to smell that musty air". You were there. You know it was musty. If it is musty even if it was completely sealed why would anyone open it to the home.

Lets say that you did do that for what ever reason one could come up with. How about sending it thru the return air to at least be filtered....or anything but the way it is.

From a draft stopping standpoint, why would this be allowable?

Originally Posted by Vern Heiler

That's new to me. Do you know when it went into effect?
[/left]

Everything I C&P'ed was underlined in the code book which indicates a change in the 2009 from the 2006 code....so it is new in the 2009 code.

John,
After many discussions with diffrent contractors it seems that closing off the vents and putting down a vapor barrier on the floor is the way to close off the crawl space. Vents will alow the outside humid air to enter the crawl space and create mold issues. Check out on the internet for a company called " Basement Systems "
Hope this helps,
Dave

Originally Posted by Dave Wheeler

John,
After many discussions with diffrent contractors it seems that closing off the vents and putting down a vapor barrier on the floor is the way to close off the crawl space. Vents will alow the outside humid air to enter the crawl space and create mold issues. Check out on the internet for a company called " Basement Systems "
Hope this helps,
Dave

The problem with recommending this is since this is not what the code requires....if issues develop in the crawl space you could have a problem justifying your recommendation.

I tell all my clients that conditioned crawlspaces are the best way to go...
Massachusetts Home Inspection: Your Crawl Space

I should point out that the pics show two different homes. The old 1940's dirt crawl is uninsulated and unvented. It is, however, heated by a lot of uninsulated furnace ductwork and an uninsulated floor above. The house was built over a dry hummock (sounds better than hump ) and the only excavation done was around the perimeters. In a different location, that same configuration could have been a wet, rotten disaster. I don't call that a conditioned crawlspace, but just a crawlspace in good condition.

The last two pics are the 2005 home. A conditioned crawlspace but with vents in the floor.

Originally Posted by James Duffin

The problem with recommending this is since this is not what the code requires....if issues develop in the crawl space you could have a problem justifying your recommendation.

There is significant information available on incorporating the crawl into the conditioned space. When the codes will catch on is anyone's guess. I'm a firm believer in sealing up the crawl from the exterior and earth and, IF done correctly, I see little chance for problems. However, if the AHJ guys want to stick with their books then there certainly could be dispute. In my view, this is a good example of staying with minimalistic standards vs "best Practices." I for one go with the latter and I tell my clients this up front and in the report's attached documents.

The next to last pic shows a large crawl space approximately 3 feet high with insulated exterior perimeter beam walls. I have been in only one crawl space in the Dallas/Fort Worth area in the 15 years of inspecting homes that resembled this and I thought the same thing when I saw the floor registers in every room that appeared to be venting to the crawl space for the return air. Then I stuck my arm inside the floor hole and felt to the inside of the room and felt another insulated wall. The entrie permieter of the home was a wall cavity of insulated boards that becamed the return air ductwork. When I first the saw the insulation in the crawl space, I thought it was the exterior perimeter, but in fact was the inside section of the return air cavity and the floor register was on the inside of the cavity. What a concept and great design. I have not seen this again since and that was about 10 years ago.

Return air using the crawl space is not allowed. An in DFW, the crawl space better be ventilated to the exterior and not blocked off with those stupid metal covers. Leave those covers of in this area all year to allow the cross ventilation year round.

That is my 2 tenths of a cent, since our money has devalued and 2 cents can't by any sense any more.