Crawl Space Encapsulation in Londonderry, NH

Encapsulation seals a crawl space off from the ground. Liner on the dirt floor, barrier up the walls, the whole thing tied together and sealed, and a way to control the moisture that is already in there. In Londonderry it usually runs $3,000 to $25,000, quoted in writing after we look at it.

How do you know your crawl space needs it?

You usually smell it before you see it. That damp, closed-up smell in the room above the crawl is air coming up out of the crawl, because that is what air does in a house. It moves up.

Other things we get called for:

Floors above the crawl that feel cold or springy
Condensation on ductwork or pipes down there
Rot at the bottom of framing, or fungus growing on joists
Mice, red squirrels, anything that found a way in and stayed
A dirt floor that is damp to the touch even in a dry stretch

An unhealthy crawl space will rot your house from the inside out. It is not the kind of thing that holds steady and waits for you.

What actually goes in

A real encapsulation is not a sheet of plastic thrown on the dirt. Ours is a liner across the full floor, barrier run up the foundation walls, everything lapped and sealed so there is no open seam, and moisture control sized for the space. We spec a heavier liner on the floor than on the walls, because the floor is what gets walked on when somebody has to get down there to run a wire in ten years.

If there is standing water, that gets solved first. Sealing a wet crawl just traps the water on the inside of a nice new liner.

What it costs in Londonderry

$3,000 to $25,000, and that is a wide range because crawl spaces are wildly different from each other. What moves your number:

Square footage. The single biggest driver.
Headroom and access. A crawl you can sit up in is a different job from one somebody has to army-crawl through with materials.
What is down there now. Old insulation falling out of the joists, debris, a previous half-attempt at a vapor barrier. It all has to come out first.
Whether there is water to deal with. Drainage or a pump adds to it.
Rot. If framing is gone, that is structural work and it happens before the liner.

We give you a written number after somebody has been in the crawl. Not before.

The warranty

Crawl space encapsulation carries a 25 year warranty on the liner, the wall barriers and the workmanship. Dehumidification carries 5 years per the manufacturer terms.

One thing worth knowing: if somebody else later fastens into a sealed wall and puts holes through the barrier, that damage is not covered. Tell whoever works down there next that there is a system on those walls.

Questions we get

Should I just add vents instead?
Venting a crawl space in New England means inviting in humid summer air that then condenses on cool surfaces. Sealing is the approach we use.

Do I need a dehumidifier too?
Usually yes for a full encapsulation, because sealing controls where moisture comes from but does not remove what is already in the air. We size it to the space.

Can I do part of it now?
Yes, and people do. Tell us the whole plan so we do the first part in a way that does not have to be undone.

How long does it take?
Most crawls are a day or two once we start. Rot repair or drainage adds time.

What you are seeing, and what it usually means

What you notice	What it usually points to	What we normally do
Musty smell in the rooms above	Crawl air moving up into the house	Seal the crawl, then control the moisture left in it
Cold or springy floors	Insulation failed, or framing has softened	Check the framing first, insulate after the seal
Condensation on ducts and pipes	Humid air meeting cold surfaces	Dehumidification sized to the space
Damp dirt floor in a dry spell	Ground moisture coming up continuously	Full floor liner, lapped and sealed to the walls
Standing water after rain	Water entering, not just vapour	Drainage or a pump before anything gets sealed
Fungus or soft spots on joists	Rot, which is structural	Repair the framing first. Sealing over rot hides it

How the job runs

Somebody comes out and gets into the crawl. Not a look through the hatch with a torch, actually in there, because you cannot see the far corners from the opening and the far corners are usually where the trouble is.

Then you get a written scope. What is coming out, what is going in, what it costs. If there is rot or standing water, that is its own line and its own sequence, because sealing over either one just hides it under a new liner.

On the day, the old material comes out first. Debris, fallen insulation, whatever the last owner left. The liner goes down across the full floor, runs up the walls, and every seam gets lapped and sealed. Then the moisture control goes in and gets set.

Most crawls are a day or two. Rot repair or drainage adds to that, and we will tell you which one you are before we start rather than after.

What is actually under Londonderry

Londonderry sits across four USGS quadrangles, all New Hampshire: Derry, Manchester South, Nashua North and Windham. Published surficial maps exist for all four. Bedrock is a thinner record, with a 1:24,000 map published only for Windham.

Four bedrock units underlie the town, including the Berwick Formation of the Merrimack Group and the Massabesic Gneiss Complex. The USGS notes that the well-bedded Berwick is exposed in roadcuts near the interstate in Londonderry, which is to say you can see it on the drive.

Drainage, by the numbers

Of the town’s mapped soils, about 56 percent are well drained and 11 percent excessively drained. But 12 percent are very poorly drained and another 3 percent poorly drained, so roughly one acre in seven is ground that holds water. The town’s own plan reports 52 percent of its soils in Hydrologic Soil Group B and about 22 percent in Group C.

The restrictive layers are the detail that matters for a foundation. Hollis soils record lithic bedrock at a representative depth of about 38 to 41 centimetres, Chatfield at about 76, with rock outcrop at zero. Where compact till is the restriction, water moving down stops there and moves sideways instead.

Shallow bedrock and dense till do the same thing to water. They send it somewhere else, and somewhere else is often the nearest foundation.

Where the town is dry, and where it is not

Over 93 percent of Londonderry is FEMA Zone X, which is minimal flood risk. The higher-risk ground is specific: upstream of the Little Cohas Brook Reservoir, upstream of Kendall Pond, near the Beaver Brook headwaters. Wetlands cover about 2,501 acres, roughly 7 percent.

The town is 72 percent bedrock aquifer and 24 percent stratified drift. Impervious cover is 10.3 percent, just over the 10 percent mark at which stream quality is generally considered impacted.

Being in Zone X does not mean a basement stays dry. Most of the wet basements we see have nothing to do with a river and everything to do with water in the soil beside the wall. That is a different problem with a different fix, and it is what the inspection is for.

Aquifers, wetlands and the shape of the town

Londonderry has four major stratified drift aquifers, and the town’s plan places them primarily near the municipal borders. The bedrock is recorded by the town as 82 percent metamorphic and 18 percent plutonic igneous, with the plutonic ground concentrated in the northwest corner.

Wetlands run to roughly 2,501 acres, about 7 percent, and about 60 percent of that is freshwater forested and shrub rather than open marsh. Forested wetland is easy to miss from a driveway and it is still wetland.

The town is drained east by Beaver, Indian and Black Brooks, and west by Little Cohas Brook and its neighbours, all within the Lower Merrimack River Basin. Three of its watersheds already carry impervious cover above the 10 percent mark.

What our customers say

Hired 603 to encapsulate my crawl space and install some support jacks to help with my sagging floor. They were honest, professional, and hardworking. Great company through and through.

Eric Morales, Google review, 5 stars

That is one of more than 240 Google reviews. We would rather you read them than take our word for anything on this page.

Related pages for Londonderry

Book a free inspection in Londonderry

Somebody comes out, looks at the actual space, and tells you straight what it needs and what it does not. You get a written number for a defined scope. No two hour pitch, because we would rather be at the next house.

If you are looking for a team that isn’t going to treat you like another number, we would be happy to give you an estimate free of charge. We don’t cut corners. We stand behind our work. We care.

Call 603-610-1770 or book your free inspection. We cover Londonderry and the surrounding towns from our shop in East Kingston, New Hampshire.

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