I'm renovating a Virginia home from the mid-1800's. The house is balloon frame construction, sitting on top of maybe four layers of brick, sitting directly on top of the dirt.

In the past, the crawlspace was not sealed and there was a lot of air movement under the house due to gaps in the foundation, which I have closed off with bricks and mortar. Half of crawlspace is inaccessible (no room to move under the floor joists), and the other half has a shallow access trench for the plumbing.

Presently, I have managed to poly and seal the half of the house with the access trench. My question is what to do about the other half of the house? There is no way to access it. There is a dirt floor, and no ventilation at all. By sealing up the accessible area, I have cut off all ventilation to the inaccessible area.

This house has never had air conditioning or insulation, but it does now. By installing these two things, I have significantly changed the environment of the house. I cannot assume the crawlspace will behave as it did over the past 100 years, during which it stayed dry and free of mold. Should I vent the dirt portion of the crawlspace by cutting vents into the foundation, or do I leave the dirt portion closed up?

Physical access to the dirt portion is not an option either way.

  • Floor is not insulated. Closed the crawl to keep critters and drafts out. I'd be comfortable adding vents if that's the solution.
    – Mattzees
    Jun 2, 2016 at 23:57

1 Answer 1


You have an interesting problem, with a lot of compounding issues. If you only have 4 courses of brick on top of dirt, that's not much of a foundation either way. I personally would be greatly concerned about movement, but the fact that its been that way for a couple of hundred years is kind rendering it null and void.

I wonder too what you mean by "poly and seal"? Do you mean you have applied vapour barrier to the underside of the joists and sealed it? If so, this will have catastrophic results. Moisture from inside the house will permeate the floor and when it hits the cold (October to March) side of the joist (the bottom) it will condense and pool in the VB. This leads to rot of the joists and mold. If you have insulated as well, it will generally make it worse, unless you have used spray foam (which solves the problem entirely), but I doubt that as you said you have limited access. You could use an air barrier (Tyvek™ or TYPAR®) instead of a vapour barrier, but if you have no insulation, you aren't really achieving anything.

By adding the AC and insulation, you will be significantly altering the thermal and moisture dynamics inside the house and underneath in the crawlspace. In my experience, Virginia is quite humid, but doesn't get that cold, correct? If that the case, frost may not be an issue as far as the foundation goes (heaving and ice buckling), so if it was my place or a customers place, I would probably approach it this way:

1) Dig out access trenches below to give you access to the entire area. A three foot trench every eight feet (on center) usually works pretty well.

2) Get a spray foam company to come in and do an R40 Polyiso foam spray throughout.

3) Back fill all of the access trenches to make sure a soil subsidence doesn't compromise your "foundation".

4) Install venting openings or louvers all the way around.

This way the insulation encapsulates all of the existing structure and insulates it and seals it from moisture ingress from below. The foam will make sure the wood is kept above the dew point in the crawlspace to make sure you don't get water logging of the joists. The venting will ensure the Polyiso cures alright and then keeps the crawlspace vented permanently.

  • What I meant by "poly and seal" is we put down a sheet of 6mil poly on the ground and ran it up to the bottom plate of the house and sealed it with icynene spray foam (which is what the entire house was insulated with). That describes half the crawlspace. The inaccessible half is an unvented dirt crawlspace right next to the newly-sealed crawlspace. What makes it inaccessible is a structural beam that runs the length of the house with no way through except under it, which is very sketchy.
    – Mattzees
    Jun 3, 2016 at 0:07
  • okay - now i get it. so your joists have all been encapsulated, correct? then you don't really need anything further. just make sure the "foundation" is well vented. you need not worry about the inaccessible area, it will just breathe naturally. the foam encapsulation will protect all of the floor system members from moisture from the sub-building area, including moisture coming from the crawlspace soils. i generally do not use isocyanuric foams, but this kind of application is exactly where they are the best possible solution. Jun 3, 2016 at 3:14
  • The inaccessible crawlspace has had nothing done to it at all. It has a dirt floor, maybe 10 inches of space between the ground and the floor joists. It is unvented currently. Any airflow it used to have has been cut off by 1) sealing the accessible part of the crawlspace, and 2) spraying foam into the stud bays in the house above. To be clear, it is a completely unvented crawlspace with a dirt floor. Some people are in favor of vents. Others say no vents. I'm confused.
    – Mattzees
    Jun 3, 2016 at 13:15
  • The joists in the inaccessible area have not been encapsulated.
    – Mattzees
    Jun 3, 2016 at 13:20
  • then it goes back to the original suggestion. you have to trench and get in under there with foam Jun 4, 2016 at 4:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.