Crawl space and underfloor insulation

Is there a crawl space under your house?

If you go down a step or two from the front door to the yard, and there are small screened vents along the bottom of the outside walls, your house most likely sits on a raised foundation with a crawl space underneath. That's typical of many older Bay Area houses, and plenty of later ones too.

Houses built on a concrete slab, including Eichlers with heating pipes in the slab and many post-war tract homes, have no crawl space. This page isn't for them.

Signs your floor needs attention

• Cold floors in winter, especially in rooms over the crawl space rather than over a garage or slab.
• Insulation hanging down between the joists, or lying on the ground. Batts installed without proper support sag and fall over time.
• A musty smell in the house that's stronger near floor registers or in closets.
• Standing water or damp soil under the house after winter storms.
• Rodent signs: droppings on the ground or the vapor barrier, insulation pulled into nests, chewed ducts.
• Ducts running under the floor with torn wrap or disconnected joints. Those are worth fixing at the same visit: see ductwork.

On the fog side of the Bay, the air under a house stays cool and damp through much of the year, so cold floors aren't only a January problem.

What goes in

Insulation between the floor joists. Usually batts sized to the joist depth, pushed up snug against the subfloor and held there with supports such as wire hangers or netting, so they can't sag. Insulation that drops even an inch leaves an air gap that undoes much of its value.

A vapor barrier over the soil. Polyethylene sheeting laid across the ground, with seams overlapped and taped and the edges run a short way up the foundation walls. ENERGY STAR's guidance for crawl spaces is a continuous, sealed ground cover of vapor-resistant material. It keeps ground moisture out of the crawl space air and out of the framing and insulation above.

The rim joist. The band of framing around the edge, on top of the foundation, is awkward to batt. Closed-cell spray foam seals and insulates it in one pass.

Gaps in the subfloor. Plumbing and wiring holes that go up into the house get sealed, so crawl space air isn't pulled into the rooms above.

Batts, blown-in or foam under the floor?

Bay Area contractors offer all three under floors, and each fits a different crawl space:

• Batts are the standard. They're the least expensive per square foot, easy to inspect later, and easy to replace in sections if a plumber or a rat disturbs them. Their weak spot is support: without hangers or netting they sag.
• Blown-in fiberglass held behind netting stapled across the joists fills around pipes, wires and bracing more completely than batts, which helps in crawl spaces crowded with plumbing. It costs a bit more, and it's harder to work around later.
• Closed-cell spray foam sticks to the underside of the floor, seals air leaks as it goes and doesn't sag or fall. It costs the most by a wide margin, so it's usually saved for the rim joist and for tight spots where nothing else fits.

The inspection is where this gets decided. Clearance, how crowded the joist bays are and what's living down there all point toward one of the three, and the quote says which and why.

Moisture comes first

Insulation under a wet house is worse than none: it holds water against the wood. Bay Area winters bring heavy storms, and many crawl spaces have low spots where water collects, gutters that drain against the foundation, or a slope that runs toward the house. If the inspection finds standing water or wet soil, that has to be solved before insulation goes in. Drainage and grading are other trades' work, and we'll tell you plainly if that's the first step.

How we do it

1. Inspection. We measure the clearance, check for moisture and standing water, look at the condition of any existing insulation, note rodent activity, and look at ducts, plumbing and wiring under the floor.
2. Clear out. Fallen and soiled insulation and debris come out first, with any droppings dealt with before anything is moved.
3. Seal. Plumbing and wiring holes in the subfloor are sealed.
4. Vapor barrier. Sheeting goes down over the soil, seams overlapped and taped.
5. Insulate. Batts go up between the joists and are supported so they stay tight to the subfloor.
6. Close up. The access door is fitted and closed, and foundation vent screens are checked. Torn screens are how rats get in: see rodent proofing.

Safety under the house

A crawl space is a confined, dusty space, and it's where Norway rats tend to live. UC's pest notes describe them as burrowers that dig along foundations and stay low in a house. If there are droppings under the floor, the California Department of Public Health's guidance applies to anyone cleaning up: air the space out, soak droppings with disinfectant or a 10% bleach solution, let it sit, and never sweep or vacuum them.

Older houses can hide other things under the floor too. If we find suspect material, such as old pipe or duct wrap that could contain asbestos, or vermiculite, we stop and refer you to a qualified asbestos consultant rather than disturbing it, which is what the EPA advises for vermiculite.