A layer of rigid insulation covers cold concrete and makes a warm and inviting new room

When the owners of this home in Bridgewater, Connecticut called in contractor Andy Engel to convert a portion of their basement into a rec room, they weren’t sure how far he would need to go to make it comfortable and healthy. Fortunately for them, there was no evidence of water problems, so Andy was able to do all of his moisture, air, and, heat control details within the existing basement. He drew from his experience as a remodeler and a former editor at Fine Homebuilding Magazine, and from Joe Lstiburek’s Builder’s Guides, to choose a durable and economical system—essentially a conventionally framed and finished room separated from the concrete walls and floor by a continuous, well-sealed layer of expanded polystyrene.

Design approach: Make existing space more useful

The project’s driving concepts were simplicity and versatility. The family was starting to outgrow the modest-sized home and they just needed some more adaptable living space. A modest budget meant that using what they already had made the most sense.

Because the basement was relatively dry, Andy was comfortable omitting drainage and waterproofing from the project. Instead, he made sure that all of the materials he used would let water vapor pass through. The foundation is in direct contact with the ground so it will absorb water; often the best thing is to let the small amount that makes it through keep on going. A dehumidifier that automatically switches on when the air is too damp will keep that moisture in check.

Rigid foam separates old from new

Andy picked EPS because it’s the most vapor permeable of common rigid foam insulations. It also happens to be one of the most affordable. The foam does several things. First, it forms a capillary break between the old concrete and the new wood. It also does a decent job of stopping heat loss, both as an air and thermal barrier. From a structural standpoint, it has plenty of compressive strength to support a typical floor or wall.

Andy covered the entire slab with 1-inch EPS boards, sealing the seams with spray-foam and tape as he went. Two staggered layers of ½-inch plywood subfloor — the first one screwed right to the concrete floor — topped off the EPS. He then glued a 2-inch thick layer of EPS to the foundation walls with the same spray-foam he used to seal the gaps. The most time-consuming step was cutting small pieces to fit around pipes and framing against the band joist. A conventional stud wall, just inside the foam, finished off the assembly.

Finish as usual

The final surface is nothing more than standard drywall. Latex paint and glue-down cork flooring top off the list of vapor permeable layers. Just by picking the right materials and putting the time into installing them properly, the homeowners ended up with an attractive, practical addition, that should be durable, healthy, and comfortable for years to come.

Affordable compromises

After weighing their options, the homeowners found a comfortable middle ground between a DIY project and a full out design/build remodel. They hired Andy to build a clean and simple space that was pretty much defined by his building science know-how and the condition of the existing foundation. They were able to cut some costs by taking care of several of the finish details—like the oak stairs—themselves.