Most people with sloping roofs have insulation below their roof deck. This approach is beneficial in many ways;
- There is lowered condensation risk as the dew point is within the insulation layer
- It is possible to build the roof system from above
- The membrane is installed on a cover board or polyiso which is a stable surface
- For an effective vapor retarder, a well installed system comprising of 2 layers of polysio together with staggering joins can be used
There are other methods used in different region and also depending on the type of building. In west coast, the low slope roofing is installed over OSB or plywood. For a wood deck roof system, the ceiling might be absent. Light commercial building can use this method and low slope buildings located in inner city.
It is challenging to install fiberglass without air getting into the deck. The following features can contribute to condensation problems;
- A cold roof deck– If the roof deck does not have insulation, it picks the exterior temperature. During very cold seasons and winter, the temperature in the deck can go below the interior air hence ‘cold decks.’
- Construction materials – Wooden material absorb moisture and when the moisture content gets to 30%, the material rots. Even if the air spaces are well ventilated, they cannot dry the wood once it absorbs a lot of water.
- Insulation type – Fiberglass is porous and if there are leaks on its edges, air gets to the system through the leaks.
Have cold decks always been an issue?
Initially, membranes were dark and they became hot during summer. Cooling membranes were then adopted.
Can you use cool roofs below the deck insulation?
Yes. Use the following leads for success;
- Add insulation on top of the roof deck. The thermal control layer is placed outside the structure. Polysio flat roof insulation is placed above the plywood deck.
- Using fully adhering membrane attachment
- Paying attention to penetrations
- In some roof replacement projects, the penetrations deteriorate with time.
- Inspect whether the exhaust and stack are operating properly
- Flash and seal off any space between the deck and the roof
What about vapor retarders?
- Vapor does not always stop moisture from flowing into the system. Use a membrane on the upper side and a vapor barrier below.
- In cases where the vapor barrier is important, it should have vapor permeability. Consult a roofing expert before adding a vapor retarder.