I was told of an add-on for my crawl space encapsulation: insulation under the 20 mil white casing. Is that preferred?

No. Insulation does not go under the 20-mil floor vapor barrier in a crawl space encapsulation. The point of encapsulation is to seal the dirt floor off from the house, and burying insulation beneath that barrier traps moisture against it, which is the one thing you are trying to stop.

What your installer pitched may have meant insulation on the foundation walls, which is a real and sometimes useful add-on. That sits in a different spot and does a different job. Here is what each piece does.

A 603 crawlspace encapsulation is a stack of parts. Dimpled drainage matting goes down first. The 20-mil floor vapor barrier lays on top of that. A 12-mil vapor barrier covers the walls. We spray-foam-seal the seams. Then a dehumidifier and a sump keep the sealed space dry. Every layer points the same direction. Keep ground moisture out, keep the air dry.

The floor barrier sits flat on the dirt. Nothing soft belongs under it. Soil holds water. The reason water pools at your foundation in the first place is disturbed backfill, the looser fill that got put back when the house was built. It drains slowly and holds water. Bury fiberglass or rigid foam down there and it sits in that damp, then it sags, molds, or just stops working. So the answer on “under the barrier” is a firm no.

Now the part that may be what your installer actually meant.

In a sealed crawl space, the right place for insulation is on the foundation walls, on the inside, over the vapor barrier. The U.S. Department of Energy’s Building America program treats a sealed crawl space as part of the conditioned house, so the walls are where it pays off. The DOE also notes that up to about a third of a home’s heat loss can come from an uninsulated foundation or crawl. In New Hampshire winters, that matters. A cold crawl floor over your living space pulls heat all season. Wall insulation in a sealed crawl can cut that down. So if that was the add-on, it can be worth it. Ask which surface they mean.

What it is not is insulation between the floor joists left exposed to a vented, dirt crawl. That is the old way. It gets damp, it falls, and the space stays cold and wet. Once you encapsulate, you seal the space and insulate the walls instead. One approach or the other. Not both halves of two.

You may not need wall insulation at all. If your crawl is shallow, already mostly sealed, or your heat loss is coming from somewhere else, adding it can be money you don’t need to spend yet. We would rather look first. A free inspection tells us whether the encapsulation alone solves your problem or whether the walls earn the extra cost.

On cost, a full crawlspace encapsulation runs $3,000 to $25,000. What moves that number is square footage, how tall the crawl is, how much debris and old material we haul out, and whether you add a dehumidifier or a sump. Wall insulation, when it makes sense, is part of that scope we price after we see it. We give you a free estimate and a quote within 24 hours, so you see the line items before you decide.

Two more things worth knowing. The encapsulation carries a 25-year warranty, and the wall vapor barrier carries its own 25-year warranty. That coverage is conditional. It can void if another contractor or a homeowner cuts into the work later, so keep the sealed system sealed.

And if your crawl is dirt or fieldstone, ask about radon while we are down there. A sealed crawl is the right time to add sub-membrane depressurization, which pulls radon gas out from under the barrier before it reaches your living space. We install those in-house. The EPA action level is 4.0 pCi/L, and EPA suggests considering a fix between 2 and 4. One trip, two problems handled.

Want a clear answer for your own crawl space? Call us at 603-610-1770 or book your free inspection. We will tell you what your crawl needs and what it doesn’t.

Learn more about our crawl space services, our radon mitigation, and the 603 guarantee.

Related questions

Does a crawl space encapsulation include a dehumidifier? Yes. A 603 encapsulation includes a dehumidifier plus a sump, the 20-mil floor barrier, the 12-mil wall barrier, dimpled drainage matting, and foam-sealed seams. The dehumidifier holds the sealed space dry year-round. Pricing runs $3,000 to $25,000 depending on size and condition.

Is crawl space encapsulation the same as the Forever Dry System? No. The Forever Dry System is our basement waterproofing setup, full-perimeter interior drainage with a sump, wall vapor barrier, and dehumidifier. Crawl spaces get encapsulation instead, which is built for a dirt or low floor. Different space, different system. See basement waterproofing for the basement version.

Should I insulate the crawl space walls or the floor joists? The walls, once the crawl is sealed. DOE Building America treats a sealed crawl as part of the conditioned house, so wall insulation is where it pays off. Joist insulation over a vented dirt crawl tends to get damp and fall. We will tell you on a free inspection whether walls are worth it for your home.

Does sealing my crawl space fix radon? Sealing helps, but on its own it does not guarantee a safe radon level. For a dirt or fieldstone crawl we add sub-membrane depressurization, which vents radon from under the barrier. We install these in-house. EPA’s action level is 4.0 pCi/L. See radon mitigation.