Flat Roof Insulation Basics

View from a flat roof of a home under construction.

Insulating a flat roof presents a challenge even for experienced roofers and home builders. There are several different methods for insulating roofs and walls alike, but with a flat roof you have to worry about water getting in. The chances of water seepage with a slanted roof are less simply because water is designed to run off and does not pool up anywhere. With a flat roof, however, this becomes a major concern. Different types of roofing insulation exist including blanket insulation, blown insulation, and rigid insulation. The type that is right for your flat roof insulation job depends upon the state of the roof, expert opinion on the best type, and your budget.

Roof Types

The various types of roofs found on homes, sheds, garages, and other buildings fall into one of the following categories: gable roofs, hip roofs, intersecting roofs, butterfly roofs, mansard roofs, gambrel roofs, shed roofs, and flat roofs. All but the flat roof type have one or more grades built into them, allowing water to run off. Flat roofs, on the other hand, if not properly designed, can cause water to pool up. This has the potential not only of leaking into the conditioned area but of ruining the insulation that keeps the room at a comfortable temperature.

Insulation Basics

Insulation comes in essentially three types as we mentioned before: blanket, blown, and rigid. Materials used to make insulation in these three types include mineral wool, fiberglass, cellulose fiber, vermiculite, Styrofoam, polystyrene, and polyurethane. The material that works best depends largely upon preference in a given situation. Like other roofs, insulating a flat one requires ample ventilation, a vapor barrier, and sealed bypasses.

Proper Ventilation in Flat Roofs

Ventilation is necessary to allow moisture accumulated inside to escape. It also lets hot air escape in the summer. If moisture has nowhere to go, it will gather around joists and eventually undermine the insulation. Flat roofs do not have eaves, so there are no soffits—holes on the underside to allow air flow. Therefore, ventilation will have to come from side vents installed in the exterior walls or from roof vents. Roof vents on flat roofs need to be sufficiently sealed and positioned off the roof so water cannot seep in through them.

Vapor Barriers

A vapor barrier is a layer of protection between the insulation and the climate conditioned area. Vents allow moisture to escape, and vapor barriers prevent any moisture that remains from rotting the drywall or drop ceiling. Vapor barriers are usually made from polyethylene film, asphalt-laced building paper, or paper-backed aluminum. With flat roof insulation, a vapor barrier should sit between the joists and ceiling panels and the insulation on top.

Sealing Bypasses

Bypasses are anywhere that there is a break in the insulation. This could be a hole, crack, or other kind of unprotected space that the insulation does not cover such as around a light fixture. Not only can warm air leak out and cool air in, but a bypass can create pockets of moisture that lead to trouble later on. As part of insulating your flat roof, be sure to contain and seal all bypasses.

What Insulation to Use

Of the three basic types, blown-in insulation is the easiest to install, but it requires a skilled professional. Blanket insulation or rigid Styrofoam will be the least expensive, but depending on the accessibility, it may be difficult to install.

Ultimately, how you insulate your flat roof will boil down to how much you are willing to spend and the unique conditions your roof presents. As long as you cover your bases and follow the basic premises of insulating, whatever material you end up using will serve its purpose.