Thinking Through a Flat Roof

The Prairie Village High Performance Home features an abundance of natural light, and a timeless yet modern design that integrates the indoors and outdoors.

Beyond Aesthetics

This home is the perfect example of high design meeting high performance. SixTwentyOne and Kali Buchanan Interior Design created a timeless design that will be an oasis for this active family. A truly eye-catching element of the Prairie Village High Performance Home is the flat roof. Our job was to make sure that the design would stand the test of time.

While flat roofs offer a sleek, modern appeal they also cause concerns with long term durability.

Combating Condesation

A major issue that can arise with flat roofs is condensation forming on the sheathing layer. The buildup of condensation can lead to mold, mildew, and decay of building materials. In order to ensure durability, we had to come up with a plan to keep the sheathing layer warm so it doesn’t run the risk of hitting the dew point and producing condensation.

Our strategy to combat condensation forming on the sheathing layer of the flat roof was a combination of air sealing, foam, open and closed cell insulation.

A general rule of thumb is that 30-35% of the R-Value should come from closed cell insulation when surrounding the roof sheathing. Vapor can move through open cell insulation but not through closed cell. Isolating the sheathing layer with foam on top and closed cell insulation on bottom will allow vapor to make it only so far into the assembly and then effectively dry out. This is important in any roof style, but especially important in a flat roof because we don’t have gravity on our side helping to drain water. While there is a drainage strategy implemented in the flat roof design, the roof will inevitably stay wet longer than a traditional roof causing a more dramatic temperature difference between the top and bottom of the sheathing layer, which can lead to condensation if not insulated properly.

While there are likely many routes to solving a problem like this, we found that air sealing our roof sheathing, using exterior foam, along with two inches of closed cell and six inches of open cell insulation was the most cost effective way to mitigate the risk of condensation on the flat roof for this project.