Rats! When your crawl space has critters

Ask the Expert Home Improvement

Q. We have a crawl space under the front of our house that, to our horror, had become a rat condo, unbeknownst to us until the odor of urine began to permeate the living space above. (We had not opened the crawl space door in about 25 years. The space has two heating pipes wrapped in asbestos and a lot of leftover construction debris, so it was not a place we ever wanted to enter.) The crawl space is about 10 feet wide and 25 to 30 feet long with a dirt floor, a concrete wall on one side, and fieldstone on the other. It runs along the front of the house under an area we converted from a three-season porch to a living space. (The rest of the house has a full basement with a concrete floor and fieldstone walls.) We hired an abatement company to remove the asbestos, decontaminate, and toss the debris, including Fiberglas insulation soaked with rat urine and droppings. Immediately afterward, the unpleasant odor of damp moldy earth began permeating the house. Because of the general yuck factor of all of this and because we want to discourage any new rat tenants, we would like to encapsulate this crawl space. We watched a few videos online that only raised questions on how best to do it: what materials should we use, is there a need for ventilation, will the encapsulating material serve as insulation to prevent cold air from seeping into the living room above, etc. Any thoughts or advice would be appreciated.



A. It sounds as if the soil is full of urine, too. I’d cover that floor with either a polyethylene liner or a thin layer of concrete, which is known as a “rat cellar floor.’’

Tips for encapsulating a crawl space:

If the crawl space is enclosed, it usually has wall vents, which are designed to allow outside air in to circulate. Over time, these vented crawl spaces can develop moisture problems. Venting and insulating a crawl space is not the solution. The best approach? Encapsulating and insulating the space through careful design and construction that mitigates moisture buildup. This practice has gained favor among today’s energy-efficient builders, and conditioning the air involves several careful steps we will discuss.

Moisture in crawl spaces

In the summer, warm and moist outside air enters the crawl space and condenses. Over time, mold grows and the wood framing rots, which then attracts carpenter ants or termites. Over the years, building codes have required crawl spaces to be vented to prevent the buildup of moisture, mold, and rot. But does ventilation really provide the moisture-control desired? Lately building science experts have touted better ways to build crawl spaces.

Advantages of eliminating the vents

If properly designed and constructed, an unvented, sealed, and HVAC-conditioned crawl space can prevent mold, mildew, and pest problems; improve the house’s energy performance; and prevent rot in the structure.

A sealed, conditioned crawl space can be a great location for mechanical equipment and ductwork. HVAC equipment takes up valuable space, and the system’s efficiency can be increased by eliminating temperature swings.

What’s the best crawl space floor?

The best covering is concrete or a polyethylene liner with sealed edges covered by a concrete slab. A polyethylene covering is one of the most effective methods for controlling moisture. The membrane provides a vapor retarder and air barrier, and if covered with a concrete pour, it is durable and long-lasting.

There are other commercially available products that can act as a standalone floor covering and are more durable than 6 mil plastic. These liners are typically 12 to 20 millimeters thick and are marketed as reinforced crawl space liners. The brand Tuff-Scrim has a white 12-mil extrusion laminated reinforced film. This product has antimicrobial additives that prevent the growth of bacteria, fungi, mold, and mildew.

Manufacturers like Tuff-Scrim, GuardianLiner, and others offer heavy-duty sheeting that is significantly stronger and more tear resistant than the standard 6 mil-polyethylene liner. Twenty-mil liners are thick and strong enough to allow for medium to heavy storage, traffic, and coverage with stone.

A standalone concrete slab will also suffice as a membrane unless the ground is very wet.

Insulating a crawl space

The goal is to build a sealed, conditioned crawl space, and the most important reason for sealing it to avoid condensation when it’s humid.

Most building codes permit the construction of unvented crawl spaces. In the 2006 International Residential Code, requirements for unvented crawl spaces can be found in Section R408.3. If an unvented crawl space has a dirt floor, code requires the exposed earth to be covered with a continuous vapor retarder with taped seams. The code also lists two options for conditioning unvented crawl spaces:

1. Installing continuously operated mechanical exhaust ventilation at a rate equal to 1 cubic foot per minute for each 50 square feet of crawl space floor area.

2. Installing a forced-air register delivering 1 cubic foot per minute of supply air from the furnace or air handler for each 50 square feet of crawl space area.

Encapsulate with spray foam

Many crawl space floor joists are insulated with Fiberglas batt, which tends to absorb moisture and can fall out if not properly installed. Insulating the perimeter walls is one way to avoid this. Another option is to fill the entire floor-joist system (the sills, rim joists, and joists themselves) with closed-cell spray foam insulation.

Tips for creating an unvented crawl space

1. Keep the crawl space dry by correcting any grading problems on the exterior so that the yard slopes away from the foundation.

2. Remove rocks and debris from the crawl space floor and rake the dirt smooth to prevent your barrier from tearing.

3. Install your poly-vapor barrier on the floor, extending up the walls to within 3 inches of the top and leaving a 3-inch-wide termite inspection strip at the top.

4. Attach the top of the vapor barrier to the wall with horizontal battens secured with masonry fasteners.

5. Seal the seams of the vapor barrier material with a compatible tape or mastic; many builders use duct mastic embedded in Fiberglas mesh tape.

6. Consider installing a 2- or 3-inch concrete slab to protect the vapor barrier.

7. Insulate the interior of the walls and rim joists with rigid foam (R-5 to R-20 value, depending on the climate), Thermax (a glass-fiber reinforced polyisocyanurate foam), or spray polyurethane closed-cell foam.

8. Install an exhaust fan or a HVAC register to meet code requirements for conditioning the crawl space.

9. Try to make the crawl space floor higher than the exterior grade.

10. If the crawl space is subject to water entry, be sure to slope the floor to a sump equipped with a drain or a pump.

11. Install gutters and downspouts, or create generous eave overhangs.

12. Avoid having bathroom or dryer vents or condensate drains in crawl spaces.

13. If possible, open dividing walls between the full basement and the crawl space and run a dehumidifier.


The best way to deal a crawl space is to encapsulate it and install a dehumidifier or a supply-air vent from the HVAC system.

Crawl spaces are difficult places to work and even more difficult to seal off. I recommend hiring a reputable company that specializes in this type of work.

Rob Robillard is a general contractor, carpenter, editor of, and principal of a carpentry and renovation business. Send your questions to or tweet them to @robertrobillard.