Warm Roofs: What are they and should you consider one?

With the move towards more energy efficient buildings, insulation of low slope roofs (also known as flat roofs) is something we get asked about in combination with roofing membranes. This blog discusses the concept of warm roofs, inverted roofs, and cold roof concepts, and their role in providing insulation.

While many industry professionals in the Australian waterproofing community consider warm roofing as something that has traditionally been around for many years (in the form of “inverted roofs”), there are some key differences between what has traditionally been constructed in Australia and the more recent “warm roofs”.

What exactly are warm roofs?

Warm roof construction in both Europe and America has been around for quite some time, and is generally accepted as the norm in low slope roofing. However, here on local shores we are only just catching up.

Construction of a warm roof typically requires installing a vapour barrier and hard rigid thermal insulation board to the top of the roof structural decking, and the roof membrane is then applied over the insulation. The finish can then either be left exposed or overlaid with hard and soft landscaping, pavers on pedestals, or other suitable materials.

This form of thermal insulation is considered to be one of the most efficient forms of thermally insulating a roof structure. One of the most notable benefits is that thermal heat transfer in summer, or condensation in winter, are kept at the roof membrane level or in other words above roof deck. Thanks to this unique construction method, the roof cavity (the space between the inner plasterboard and underside of the roof structure) is better protected from excess heat and condensation build up during normal use. This means that the internal temperature of the space beneath the roof cavity can be more efficiently controlled, meaning less money and energy spent on cooling or heating – lower energy bills are always a positive!

So, what are “inverted roofs”?

The typical construction for an inverted roof (or sometimes referred to as an IRMA - inverted roof membrane assembly) consists of a roof substrate, typically concrete, upon which a membrane finish is installed, followed by a rigid insulation board. A screed layer or stones (to act as ballast) are then applied to keep the whole system in place and prevent wind uplift or floatation issues.

Inverted roofs were a reasonable roofing system for their time, but they have some performance and maintenance drawbacks. The long-term insulation performance is easily affected by rainwater which can percolate down to the interface between membrane and underside of insulation. Maintenance or repairs of the roof membrane are impractical to carry out because the membrane is buried under the insulation and screed/ballast, and this can be particularly problematic and costly if leaks occur. It is also a “heavy” system compared to a warm roof which does not require ballast.

Are you familiar with the “cold roof” concept?

We have discussed insulation placed at the top of the roof structure; however, the most predominant forms of roof insulation is the “cold roof” concept. This is a roof insulation method most people are familiar with. It involves insulating the ceiling cavity with a mineral wool-type insulation at the ceiling plasterboard level. A disadvantage of this method is that, at certain times of the year, the “dew point” can be reached within the ceiling space, causing condensation to form within the roof cavity, this is also known as interstitial condensation. A second disadvantage is where insulation is discontinuous across the roof cavity (e.g. across roof framing) and results in areas of poor or no insulation.

In our own work, Australian Waterproofing Consultants have seen this on-site. Sometimes the roof membrane is suspected as being at fault, but in fact the conditions were such that condensation formed within the internal cavity between the top of the mineral wool and the underside of the roof substrate. Generally, this condensation is formed via high moisture conditions within the space below the outer roof skin. Conversely during summer subsequent internal transfer of some built-up thermal heat to the roof cavity space can also occur. These issues can have a negative impact on HVAC performance for the rooms contained by such roofs.

Warm roofs in the remedial construction market

In the sphere of the remedial construction market, we are seeing a move towards improving overall building performance when the time comes to refurbish an existing low slope roof.

In these situations, it is advantageous to review all options so that the full benefit of any refurbishment can be realised. Comprehensive planning for these types of projects is critical to ensure a smooth construction program.

Using insulation to correct the slope of a roof

Where existing roofs exhibit poor falls, it is possible to use a tapered insulation to correct the slope of the roof. This is a lightweight solution that negates the need for heavy screeds or similar materials, and is perfect for older buildings where weight may be an issue. As with all such installations, there are some critical steps needed to make sure this type of insulation is designed and executed correctly, but Australian Waterproofing Consultants are experienced and skilled in assisting with the design of these solutions.

There are many advantages to the “warm roof” concept which can be realised on virtually any project where a low slope roof is being considered.

Want to enquire about “warm roofs” or our waterproofing consultancy services? If you have any questions or would like to explore our assistance with reviewing and programming of ongoing maintenance of your waterproof coatings, get in touch.