Thread: Return air duct in crawl space

Single family home built in 2009. The crawl space is a "conditioned" crawl space. The walls are insulated and the area is heated with forced air. However, there is a return air duct in the crawl space. The floor is an earth floor. I thought this was a no no.

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Originally Posted by mathew stouffer

Single family home built in 2009. The crawl space is a "conditioned" crawl space. The walls are insulated and the area is heated with forced air. However, there is a return air duct in the crawl space. The floor is an earth floor. I thought this was a no no.

Yep, it is a No No. You can have the supply for the positive air pressure but no return. The key with a sealed crawlspace is that the air supply should never cutoff. It should always be blowing from what I have read and been told about them.

This looks to be one of those situations where I would be more concerned about WHY?, instead of whether it is allowed or not.
I'll make some assumptions since I don't know any better. It's Utah so it's cold; it's a crawlspace and there isn't any floor insulation to the 1st floor visible in the pics; the 1st level floor was uncomfortable to walk on without slippers because it was so cold; heating the crawl was maybe some ones idea of getting the 1st level floor warmer; it was cheaper or easier than insulating the floor? If we put in supplies we must have to put it a return right, that's what they said on the DIY forum?
As much as I don't like the return with a dirt floor and open framing, the return is probably necessary. Since they are heating the space, let's also assume they are cooling it. I doubt the homeowner remembers to close the registers or trunkline in the summer. If they are pumping AC into the space, they'll need that return to keep levels from getting out of control in there.
I would advise my client on a couple things mainly depending on exactly all I saw onsite. Get rid of the supply and return; insulate the space properly to the 1st floor level; or install a good filtering system on the existing HVAC to manage contaminants getting sucked in from the crawl.
Also, no register on that boot means it can act as a nice highway for rodents. It's a crawl so it probably isn't a big issue that the flex is exposed but I would note it.
Hope that helps.

Matthew, this is from BuildingScience.com PDF.

"Crawl spaces must always have a drying mechanism. One of the most effective ways to provide a drying
mechanism to a crawl space is to condition a crawl space by heating and cooling the crawl space as if the crawl space
is included as part of the home. Air must be supplied to the crawl space from the home in order to provide this
conditioning. This air can be returned back to the home or it can be exhausted (Figure 2, Figure 3, Figure 4 and
Figure 5). Crawl spaces can also be included as part of the home (conditioning them) by connecting them to

conditioned basements (Figure 6 and Figure 7)."

Originally Posted by Scott Patterson

Yep, it is a No No. You can have the supply for the positive air pressure but no return. The key with a sealed crawlspace is that the air supply should never cutoff. It should always be blowing from what I have read and been told about them.

Scott, there is a difference between "closed" and "conditioned" crawlspaces. When the codes first came out both closed and conditioned prohibited return air to the living area. Don't know exactly when it changed.

Originally Posted by Vern Heiler

Scott, there is a difference between "closed" and "conditioned" crawlspaces. When the codes first came out both closed and conditioned prohibited return air to the living area. Don't know exactly when it changed.

I just do not know that much about them, and based on what I have seen installed neither do the builders in my area!

I don't know what meets the definition of a closed or conditioned crawlspace but an exposed earth floor doesn't sound like "closed" to me.

I guess regional and geographic differences need to be taken into consideration as to whether or not this configuration is acceptable. But I know how humid the summer weather gets here is SE Pennsylvania and putting a return air vent in that type of crawlspace may draw more humidity up from the floor through those loosely installed layers of vapor barrier. To a lesser extent, the return vent could also have a negative impact on radon levels in the home since the floor is exposed earth.

To me, those builders built that crawlspace on the cheap. Put a poured slab floor in there. If somebody goes into that crawlspace just one time for a maintenance issue or inspection, the stones beneath the vapor barrier could punch holes right through plastic from the weight of foot traffic and make the barrier no longer a barrier. Since it was easily lifted up to expose the stone floor as indicated by Mathew's pictures, it's effectiveness is questionable as well.

Its a dumb idea and I don't understand the code equation. There will always be moisture coming up from the dirt and moulds being pulled into the air stream, dumb, dumb, dumb. At a very minimum the dirt floor should be covered with heavy plastic, overlapped seams and sealed and plastic up the side walls and sealed to walls.

I should have mentioned based on my 1st post, I don't see that as a 'conditioned' space either. From the pics it looks more like an 'enclosed' space. Matthew stated it was insulated but I don't see any in the pics. Write the -rap out of Matt.

Originally Posted by Markus Keller

I should have mentioned based on my 1st post, I don't see that as a 'conditioned' space either. From the pics it looks more like an 'enclosed' space. Matthew stated it was insulated but I don't see any in the pics. Write the -rap out of Matt.

I think that is insulation at the top of pic #1.

They went cheap on the plastic. You can see that it was tapped in the same pic. right beside the large tear! Most of the closed conditioned crawls I see have a much heavier plastic that is glued at the seams. 6 mil and tapped is all that is required though. There needs to be a 3" gap in the vapor barrier at the top of the foundation walls for WDI inspection of the sill. I could not tell by the pic if this was done at this crawl.

In a well done conditioned crawlspace you can inspect it in your Sunday-go-to-meeting clothes. I have become a big fan .

Is there a gas fired furnace located in the crawl space? If so does it have a sealed combustion chamber?

A gas furnace in the crawl space with a type "B" vent could be a disaster waiting to happen.

Conditioned crawlspaces are the way to go and a 6 mil vapor barrier sealed at seams and foundation is acceptable. There is no need for insulation on the underside of the floor, since both areas are conditioned space.

Air supplied to the basement from a central air handler will need to be returned to that air handler. The volume of air that leaves the air handler must equal the volume of air the goes into it. It will either get the air from the crawlspace or from air leaks outside. Getting it from a conditioned source (like a conditioned crawlspace) is more energy efficient than getting it from leaks outside.

The vapor barrier should be sealed to stop moisture entry. The conditioned air (heated and/or cooled) will keep the crawlspace air fresh and dry.

It has worked very successfully in our coastal climate.

Here's a link for your reference:http://www.buildingscience.com/docum...nce-and-codes/

It appears there is crushed stone under the plastic. Is that a sump pump in the 1st pic.

As the picture shows the plastic is torn and it is not sealed at the perimeter. I would say that a new vapor barrier is needed, install 6 ml poly sealed with acoustical sealant.

The furnace should be direct vent type or else have adequate make up air vents from the house.

The insulation is what is commonly referred to as the dirty diaper type though I dont know if it is a problem in a crawl space.