Tips from the Architect: Winning the Battle over Mold and Mildew and Waterproofing your Old Basement

May 11, 2017

Many homes in the Boston metro area are over a hundred and have what is often referred to as "rubble stone foundation". Rubble stone foundations are most commonly found on buildings built before 1915, and they are frequently a concern for owners as over time the mortar between the stones in the foundation deteriorates, allowing water to enter the basement. This problem occurs because no exterior waterproofing was ever installed below ground.

One of our architects owns a 2-family house in Somerville, built in 1906. It has a rubble stone foundation dressed with concrete block on the exposed portion of the exterior typical of vintage homes of the era. To help our clients and community learn from his process, he prepared a case study outlining the steps he took to waterproof his own basement. We hope you find it helpful!

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"Our basement was a mold and mildew nightmare and half of what we stored down there was destroyed because of it. The rubble stone leaked like a sieve and the "concrete" slab was so porous that you could pour a glass of water on the floor and it would disappear. It was likely mostly a sand mix with very little cement.

The first thing we did was clean everything out of the basement. We had asbestos, loose knob and tube wires, abandoned heating pipes, old oil tanks - you name it. All of this was removed. At this point it is a good idea to redo any under slab waste pipes you may have. We then replaced the main line but missed a smaller one (it was probably crushed during construction) that later had to be dug up - what a mess.

The next step is to put down a 6 mil or better poly vapor barrier over the existing slab.  We used *HUSKY Model CFYG1514-140Y 14 ft. x 140 ft. 15 mil Yellow Guard Vapor Barrier. We ran this up about a foot along the perimeter rubble stone foundation. Then, we poured a 2 to 3 inch thick slab on top of that. In the finished half of the basement, we put in radiant tubing over 2 inches of extruded rigid insulation before the pour, and that part of the basement sits an inch or 2 above the mechanical side of the basement where all the plumbing is. On the mechanical side, we sloped the slab toward the front of the house into a small area where I have no slab. In this area we a made a gravel pit to help drain any potential water problems. Just this past year, we had a pipe burst in the tenant's bathroom - all the water stayed on the mechanical side with no flooding.

Next, we installed metal studs around the entire perimeter of the basement about an inch off the rubble stone wall (varies with the irregularity of the stone). Between these studs, and right on the rubble stone, we sprayed closed cell insulation. As this insulation quickly expands to many times its initial coverage, it fills all the nooks and crannies in the rubble stone foundation and locks into the foundation. This produces a watertight seal with the foundation. It is sprayed over the poly vapor barrier to form a contiguous seal between the slab and the foundation. The C-channel shape of the metal studs helps to lock the spray foam into place. You so spray any more than 2/3's of the cavity depth of the stud. This void space allows you to run wires in the walls through the cut-outs in the studs for electrical outlets.

Next you need to cover the closed cell insulation, as it is highly flammable. Rather than use a paper based drywall system (cellulose plus moisture provides the perfect breeding ground for mold and mildew), we chose to use a paperless drywall system. I choose 5/8" thick DensArmor Plus. DensArmor Panels have fiberglass mats that eliminate a potential source for mold growth and may reduce remediation and scheduling delays associated with paper-faced drywall. They can also be finished either by taping and plastering or by skim coating the entire surface in 1-2 coats for a smooth plaster finish - much like blue board. We chose not to plaster the mechanical half of the basement and opted for a 2-coat finish in the finished areas.

That brings us to finishes. Continuing our battle against mold and mildew, we chose not to use any wood trim or doors. Instead, the doors are steel with steel jambs, and the new basement windows are a fiberglass replacement. The stair treads and stringers going to the bulkhead are ipe. All the baseboards, window and door trims are composite (mostly PVC). Finally, all the painting was done with a mold and mildew proof paint. I recommend Rust-Oleum® Zinsser PERMA-WHITE Mold & Mildew-Proof™* Interior Paint.

After seven years, there is not a spec of rot, mold or mildew - just a well insulated and dry basement."