Flat Roof Insulation

Most homes are built with peaked roofs to provide for adequate runoff of rainwater and prevent accumulation. But not all homes are built with peaked roofs. There have been a number of homes built at one time or another with flat roofs, especially in areas of the country which see little rain. Some architectural styles lend themselves to flat roofs, rather than peaked ones. Even homes with peaked roofs may contain a flat portion, if the home was designed to include a second-story sun deck or the garage was made with a flat roof.

The main reason why flat roofing isn’t as popular as peaked roofs is the higher risk of leaks. Not only doesn’t a flat roof shed rainwater as well as a peaked roof does, but it doesn’t shed snow load either. In times of severe snowfall, especially a very wet snow, it can become necessary to climb up on a flat roof and shovel off the snow, as the weight of the snow can cause damage to the roof.

Another problem associated with flat roofs is insulating them. Due to the difference in construction, different insulation techniques need to be used. A peaked roof isn’t actually insulated, but instead the floor of the attic (which is also the ceiling of the floor below) is. But with a flat roof, there is no attic area to speak of. In a very real sense, insulating the roof is also insulating the ceiling below.

Even so, it’s not a good idea to treat insulating a flat roof as insulating the floor of an attic. While that has been done, there are some serious problems with it, especially problems with moisture. It is much better to insulate a flat roof with a flat roof insulating system, which has been designed specifically to deal with the problems associated with flat roofs.

Why Bother Insulating Your Flat Roof?

While some people try to avoid insulating flat roofs, due to high costs, there is still ample reason for insulating them. Twenty-five percent of a home’s heating can escape through the roof, especially if it isn’t insulated. This raises your energy costs, without providing any benefits. Since heat rises, roofs actually need more insulation to prevent heat from escaping, than walls do.

The lack of adequate insulation also makes it hard to maintain a comfortable temperature in your home. The room directly under the uninsulated roof will be hotter or colder than the rest of the home, depending on outside conditions.

Rotting of roofs can occur wherever there is inadequate ventilation. Many older homes, which have flat roofs, have problems with hidden roof rot, increasing the chances of leaks, as well as the chance of someone falling through your roof, if it is also used as a deck. This is especially bad in homes where OSB was used for roof sheathing, rather than plywood, as OSB is more susceptible to damage from moisture.

Insulation Mistakes to Avoid

Fiberglass batts should never be used for insulating a flat roof. Unlike using them in walls or in an attic, there is a strong possibility of condensation forming and making these batts wet. That moisture will compress the insulation, destroying its effectiveness and possibly even causing it to fall out away from the roof, increasing the chance of more condensation forming in the future.

The old way of insulating a flat roof is referred to as “cold roof” insulation. This requires cutting insulation to fit into the spaces between the roof supports. A space is left between the insulation and the roof itself, which makes it possible for condensation to form in that space, attacking the roof sheathing and support beams. Using cold roof insulation techniques will result in rotted roof sheathing and beams, requiring major repairs in just a few short years.

Some roofing contractors will recommend a combined roofing system, where a thinner layer of insulation is applied on the top of the roof, with additional insulation below it. But this is no different than cold roof insulation and will result in the same problems.

Another option is to insulate the ceiling, somewhat like is done with a pitched roof, rather than applying the insulation to the roof itself. This is a more workable solution, if proper ventilation is applied. However, it requires dropping the ceiling enough to provide space for the insulation, which can cause ceiling height issues. It also makes it difficult to install any lighting or hardware down the road.

On the other hand, spray foam insulation works, because it adheres directly to the underside of the roof sheathing, covering it and the support beams. This eliminates the problems normally associated with cold roof insulation, as there is no space between the insulation and the roof for condensation to form However, you are limited to the amount of insulation you can apply to the underside of the roof in this manner, so there is no way you will end up with sufficient insulation value to meet building code.

insulation, flat roof, workers
Insulation on flat roof, Phi’ls Main Roofing

Warm Roof Insulation

Warm roof insulation is an insulation system which is either applied as part of the roof, during new construction, or over existing flat roofs which have no insulation. In the case of applying the insulation over an existing roof, care must be taken to ensure that the existing roof structure and sheathing are not damaged. If they are, then it may be necessary to replace the roof or at least the roof sheathing as part of the process.

This sort of insulation system uses polyurethane foam, a closed-cell, rigid foam product. While not intended to be used as decking, the foam is strong enough to support the necessary snow load, as well as walking on it during construction, as long as it is coated with an additional layer of sheathing.

A typical installation consists of a multi-layer approach, placed above the joists or beams. Working from the bottom up, it includes:

  • A layer of plywood or OSB
  • Vapor barrier 
  • Flat roof insulation 
  • Another layer of plywood or OSB
  • Weatherproof membrane roofing 

When retrofitting this onto an existing roof, the roofing that is already installed provides the bottom layer of plywood or OSB and the vapor barrier. There is no need to remove already existing weatherproof roof membranes, rubberized roof coating or other roofing. In the case of new construction, the vapor control barrier is there to protect the sheathing from condensation.

In any case, the weatherproof membrane that is applied over the insulation is important to protect the insulation and the roof from water damage. These membranes are usually part of a roofing system, designed with other matching components which help to seal the membrane to the roof, especially around the edges. It is inadvisable to take shortcuts in the installation or to substitute other materials, especially in the areas of edging and adhesives. Using the wrong material could cause premature failure of the roofing system or leaks.

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