I've recently moved to a new home. This home had an addition that was built on top of a Crawlspace that is vented to the outside. Needless to say, the morning after the first cold night made me realize how much this affects the temperature in the main family room.

Before we move in an inspection also revealed a small mold problem on the joists in the crawl space. This was taken care of. While they were removing the mold they went ahead and added plastic to the floor and insulation to the walls. However, it's still vented to the outside and the plastic/insulation is doing next to nothing as there is no tape and there's gaps all over.

Also, there are some areas in the room above where I can feel air coming through.

So first thing I want to do is to seal any gaps between the crawl space and room above with caulk and expanding spray foam( not insulating the ceiling ).

Then comes the hard part. After doing some reading, it seems sealing the crawl space and doing some proper insulating/vapor barrier is the way to go.

How do I seal the vents? Next steps and order of importance is greatly appreciated. I'm fairly handy and have access to a family with plenty of experience, but I want to tackle as much if this as I can.

Here are some pics:

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2 Answers 2


It is a good idea to seal any apertures between the crawlspace and the main house. If you have to rely on caulk and spray foam you're probably in a situation where the house has some weird structural problems - that is, you should not have giant gaps between the crawlspace and the floor above in any normal construction regimen.

It is not a good idea to seal off the crawlspace, ventilation is not only a good idea but it's required by code in locations where crawlspaces are an accepted norm (note that in places where winters are exceptionally cold the norm is finished and unfinished basements, not crawlspaces). The fact that you have had mold in the crawlspace is actually an indication that you might need MORE ventilation not less.

A proper insulation/vapor barrier installation at the level of the floor joists should solve any draft problems. Also, looking at your photos I would recommend that you insulate your ductwork/boots/wyes per local code.

So to reiterate, please do insulate and install a vapor barrier! Do not seal off the ventilation for the crawlspace - indeed look into your local code to see if you have enough ventilation.

As a side note - any issues of moisture (and therefore mold) in the crawlspace need to be addressed. Possible sources of moisture include but are not limited to: leaky plumbing supply or waste lines, inadequate drainage system at the perimeter of the house (undersized gutters/downspouts, poor grading, no storm drain, etc.), and a possible high water table.

Good luck staying warm this winter!

  • Thanks for the response! OK that makes a lot of sense. I've been reading some conflicting things, some newer reasearch saying a crawl space should never vent, etc,etc... but what you said makes sense to me. Regarding the mold...the previous owners installed a deck with a virtually sealed off space under it over top one of the vents. I believe this is causing stagnation of the air in the crawl space.
    – bf1618
    Commented Dec 16, 2012 at 2:06
  • Can you please site the "code" that you are referring to?
    – Tester101
    Commented Dec 17, 2012 at 13:23
  • 1
    Citing code is fun. California Building Code: 1203.3 Under-floor ventilation. The space between the bottom of the floor joists and the earth under any building except spaces occupied by basements or cellars shall be provided with ventilation openings through foundation walls or exterior walls. Such openings shall be placed so as to provide cross ventilation of the under-floor space. 1203.3.1 Openings for under-floor ventilation. The minimum net area of ventilation openings shall not be less than 1 square foot for each 150 square feet (0.67 m2 for each 100 m2) of crawl-space area.
    – Paul
    Commented Dec 17, 2012 at 13:59
  • There's a lot more information here: publicecodes.cyberregs.com/st/ca/st/b200v10/…
    – Paul
    Commented Dec 17, 2012 at 14:02

You don't seal the vents, ventilation of the crawlspace is needed to prevent moisture buildup and rot.

I had to take on the very same project. There are three objectives.

  1. Under house air can have mold in it, you shouldn't be breathing it, block all air exchange between the living space and the crawl space.
  2. Under house moisture levels must be brought down so your house under-structure doesn't rot. Reduce the amount of damp soil in contact with the air, allow no pooled water, maintain ventilation to remove the rest.
  3. Reduce heat exchange into the crawl space. Insulate the floor, it completes the side benefit of sealing the floor seams and from personal experience, makes quite a difference.

Our '70s stick-built house is built with 1 1/8" T&G plywood subfloor. Normal construction now is to lay a heavy bead of Liquid Nails or similar subfloor adhesive in the groove and drive the sheets together, something they didn't do back then. I stripped the shag rug carpets out, caulked every seam shut, laid down 3/8" plywood and refloored the whole house. This eliminated all the drafts from the seams.

The crawl space was built on two levels. The next step was to improve the drainage so there was never any standing water under the house. We dug in a drain to the ditch and then laid lay down two layers of 6 mil black poly on the upper level and cemented over the lower level with about 2" patted into place which converted it to a useful storage area. A tremendous evaporative surface for moisture was eliminated and the dampness of the air much reduced.

We then insulated between the floor joists with fiberglass bat. 1/4" black expanded plastic mesh stapled in place on the floor joists holds it in.

One added thing to watch for is anywhere air leaks. If each seam in the floor had a 1/8" gap over a span of 28ft, you have 42 square inches of air slot there. Caulk kills a world of sins.

Following that thinking, the place was built with these decorative beams in the ceiling. Close inspection showed there was a 1/4-1/2" slot on each side of the three beams. All the heat from the stove headed for the ceiling and through the slots, you could see the air trails in the dust under the insulation. If you figure average slot width of 3/8" for a 14ft span, this was a total of 378 square inches.

Holy stinky stuff, that's like having a large hole cut in your ceiling... (18"x18" = 324 square inches).

The next winter was rather oppressive the first time we started using our wood stove. We can actually overheat the place in the wintertime now.

  • Thanks for the response as well. I have yet to inspect under the carpet or from underneath, but I plan to seal everything up, and I will hopefully soon, insulate the floor and put a vapor barrier on the ground and walls in the crawl space.
    – bf1618
    Commented Dec 16, 2012 at 2:08

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