BCT 313 / FALL 2016

Hampshire College came to Wright Builders looking to increase their workshop space at their grounds and maintenance facility. Originally the shop was to be attached to the existing structure as shown in the plans. Permitting complications made it necessary to add fire protection, greatly increasing the cost of the project. This led to the decision to build the shop as a stand alone structure, negating the need for sprinklers. The building has no plumbing and heating is limited to one small electric unit. Keeping the shop as bare bones as possible allowed for it to be zoned as a “shed”. It will purely serve the practical purposes of a workshop.

This photo shows electrical conduit and junction boxes in the center of the floor. These 220 volt outlets will service the shops large machinery. A four inch concrete slab will be poured around the electrical boxes. Vertical rebar is spaced at regular intervals to hold the wire mesh. It will be removed as the slab is poured. The wire mesh will give the concrete structural integrity.

This picture clearly shows the under layers of the concrete slab, as well as how they connect with the wall. Foam was sprayed along the corner to create a continuous thermal barrier with the rigid foam boards. The gravel that is covered up, serves as a capillary break. The plastic sheet is a vapor retarder that allows moisture to flow out of the building, while blocking moisture from entering through the earth.

Rigid foam board was laid over the gravel sub floor and covered with a plastic vapor retarder. At the edges the plastic is taped to the insulation. Wire mesh is laid over these two layers to serve as the base for the concrete slab that will be poured. The edge shown in this picture connects with soil because it is the garage door bay. The lumber supported by rebar is a form for the concrete slab and a barrier to the backfilled soil.


Capillary movement is water traveling through a porous material via surface tension, adhesion and cohesion. This surprisingly strong force can make water travel in the opposite direction of gravity, which is why basements can pull moisture from the earth when not properly sealed. Capillary breaks can simply be crushed stone or gravel with pore sizes greater than ¼ inch. ¾ inch stone gives this desired pore size. Capillary movement requires solid surfaces for water to cling to, therefore large gaps stop movement. A properly sealed foundation will feature a rubber membrane between the footing and foundation wall as shown below. This stops capillary movement inside the footings from entering the foundation and the buildings interior.

thermal barrier

The second layer shown in the diagram above is rigid foam board. According to NESEA member Mark Rosenbaum, soil is “on average 40-50 times more conductive than air”. It also has a much greater heat capacity than air, which means it can store lots of energy and stay cold for long periods of time. This means insulating under cement slabs, especially when on grade, is vital to creating an efficient building. According to Fine Homebuilding, under slab XPS or EPS insulation should have a minimum rating of R-10. It is important to make sure the subsurface gravel is level and smooth to avoid puncturing and cracking the insulation.

vapor barrier

The third layer in the diagram is the vapor barrier. These sheets, typically made of polyethylene, prevent water vapor from entering the building. What sets vapor retarders apart from water barriers is their one way permeance. Condensation forms on the interior of all buildings. In old leaky structures, this is not an issue as vapor eventually dries out due to air movement. In new super-insulated, air-tight buildings, condensation can lead to mold, rot and occupant health problems. Vapor retarders allow interior moisture to leave the building through the envelope. This is especially key for keeping wall assemblies dry. The photos below show different types of vapor retarders used for concrete slabs, walls and roofs.

The blue membrane below is comparable to the one used by Wright Builders at the shop building. The light blue membrane two images above stops capillary action between the footing and foundation walls.

Since the last site visit, windows have been installed in the rough openings. Aside from painting or polyurethaning, the exterior trim is finished. It covers up any taping, caulking, insulation and flashing details the builders did to waterproof and air seal the window. Metal flashing has been fastened to the top of the trim to divert water from the wood. These double hung Pella windows open horizontally and are likely double paned. The interior side of the window and wall remains rough framed. Drywall and insulation will be installed around the glazing in the coming weeks.

Created By
Gabrielle Bernier

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