Ask the Carpenter: Bored of the flies, reader battles porch pests

Ask the Expert
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Q. We have a problem that is frustrating, unsanitary, and embarrassing. We have a large enclosed porch with sliding-glass windows, and have guests for dinner on the porch once or twice a week when the weather is nice. The porch was built over a dirt foundation and has painted cedar clapboard on the outside. Since we installed the windows several years ago, we have had an infestation of flies, beginning as soon as the weather turns nice. They are smaller than common houseflies.

I have sprayed insecticide, but they return after a day or two. I prefer not to use chemicals where we eat and have resorted to vacuuming them up several times a day. It is not uncommon to get 50 at a time. They are usually on the windows and screens, and are much more prevalent on the east end of the porch.

We have the outer perimeter of the house treated annually for ants. I called the pest company and was told that the issue is that the porch was built on dirt. The technician said he could spray, but the problem would reoccur until we covered the dirt with a plastic overlay and vented the area under the porch.

Have you ever heard of an issue like this? How do I cover the dirt floor or vent the space without cutting into the siding? Is there a vent designed for this purpose? Would taking those two steps eliminate the problem?


A. I’d love to be a fly on the wall during those dinner conversations. Just kidding. This is a tough one, so I’ll break it down into smaller steps.

First, I’d identify the species of fly and research it. I’m a big fan of the quote from Sun Tzu’s “The Art of War:’’ “If you know the enemy and know yourself, you need not fear the result of a hundred battles.’’ (Here’s hoping you don’t go a hundred rounds with these flies.) I recommend capturing a few in a Mason jar and bringing them to your exterminator.

It’s important to determine why they are under your porch. Is there a dead animal under there, or do they simply live in the dirt?

Once you are armed with that information, update the crawl space. Was your porch a deck that you converted? Your e-mail was missing critical information such as whether the area under the porch is open to the outside. If it’s closed off, is there access from the basement? Many older crawl spaces have dirt floors, but your best bet is a poured-concrete surface.

I’m not a fan of enclosed crawl spaces with wall vents. The vents are designed to allow outside air in to circulate. Over time, these vented crawl spaces can develop moisture problems as humid summer air enters and condenses. Over time, mold grows and the wood framing rots, which attracts carpenter ants or termites.

The best way to deal with crawl space air is to encapsulate it (seal it off from the outside) 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. If you want to seal off and encapsulate one, I recommend hiring a reputable company that specializes in this type of work.

OK, lets discuss the plastic overlay. If the porch is open to the outside, like a raised deck, simply cover the ground under it with 6-mil plastic and gravel. If an unvented crawl space has a dirt floor, my local building code requires exposed earth to be covered with a continuous vapor retarder with taped seams.

If this area is an enclosed crawl space, the best floor covering is a 6 mil-plus polyethylene liner with sealed edges covered with a concrete slab. A polyethylene covering is one of the most effective methods for controlling moisture and would also keep the flies out. The membrane provides both a vapor retarder and air barrier, and if covered with a concrete pour, it is very durable.

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 mil and are marketed as reinforced crawl space liners. Tuff-Scrim has a white 12-mil antimicrobial extrusion-laminated reinforced film. This product has antimicrobial additives that prevent the growth of bacteria, fungi, mold, and mildew. Twenty-mil liners are thick and strong enough for medium to heavy storage, heavy foot traffic, and coverage with stone.

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

Steps to create an unvented crawl space

1. Keep the crawl space dry by correcting grading problems on the exterior so that the land 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 a 20-mil polyethylene covering over the floor, extending up the crawl space walls to within 3 inches of the top. Leave a 3-inch-wide termite-inspection strip at the top of the wall.

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 fiberglass mesh tape.

6. Consider pouring a 2- to 3-inch-thick concrete slab to protect the vapor barrier and make the crawl space floor higher than the exterior grade.

7. Insulate the interior of the walls and rim joists with R-5 to R-20 rigid foam, polyisocyanurate foam, or spray polyurethane foam.

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

9. 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.

10. Do not vent baths or dryers into crawl spaces.

11. Do not install condensate drains in crawl spaces.

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. Subscribe to our free real estate newsletter at