Thermal bridging is the movement of heat across an object that is more conductive than the materials around it. The conductive material creates a path of least resistance for heat. Thermal bridging of a flat roof can be a source of energy loss in buildings, leading to higher utility bills and less efficiency.
Most Common Roof Type?
Most common roof construction is a “warm” roof construction where the principal thermal insulation layer is located above the structural decking, resulting in the structural deck and support structure being at a temperature close to that of the interior of the building. It is necessary to incorporate a vapour and/or air control layer beneath the insulation in order to prevent moisture vapour being forced into the insulation through thermal pressure from within the building. The waterproofing membranes are placed over the insulation to completely encapsulate the roof system.
What is condensation?
The temperature at which air becomes fully saturated with moisture (100% RH) is called the dew point. When warm moist air meets a cold surface it is cooled, and if its temperature drops below the dew point it will give up moisture in the form of surface condensation.
What is R-Value?
R value is a measure of a materials’ resistance to heat transfer. The higher the value, the more resistant the material is to heat transfer. Modern rigid foam roof insulation can be a very effective way to add R-value to your roof structure. Polyisocyanurate insulation (often referred to as polyiso) is extremely popular and offers an R-value of 6 per inch of thickness and has good compressive strength (resists compression). This insulation is available in a large variety of thicknesses and can be installed in multiple layers. Polyiso is also available in pre-formed tapered panels which can be used to increase roof slope or add roof slope where none exists. It is important to consider the existing R-value of your roof system and how it relates to your heating & cooling costs. This will allow you to determine if adding R-value makes sense. If your roof slope is inadequate, it is typically much more cost effective to increase the roof slope utilizing tapered insulation than by structurally altering the roof. There is also the added benefit of increasing the R-value. The infrared image below shows a standard insulation application and the thermal inefficiencies of the fasteners.
Thermal insulation in roofing systems plays a substantial role in the overall thermal performance of the building envelope. Energy code requirements for the R-value of the roofing insulation are becoming ever more stringent, requiring increased insulation thickness. Mechanical fasteners are commonly used to secure the insulation and roofing membrane to the structural roof deck.
Each metal fastener creates a thermal bridge that reduces the effectiveness of the insulation. For a single fastener, the impact would probably be negligible. A typical roof, though, may include thousands of fasteners. The effect of these myriad thermal bridges adds up. That is, the combined impact of the fasteners can substantially reduce thermal performance of the roof insulation.
Gaps between the insulation boards can also create thermal gaps and allow condensation to occur.
Your roof consultant can help determine the type of insulation and fastening method required for your new or replacement roof by consideration of the factors listed below:
- Required thermal performance of the roof.
- The build-up thickness that can be accommodated at roof level.
- Imposed weight loading to the deck structure.
- Compressive strength required (the ability of the insulation to withstand loads applied directly onto the roof system surface).
- The level and type of traffic that the roof will experience both during and after construction.
- Compatibility with other roofing components.
- Required fire resistance.
- Required acoustic performance.
- Environmental properties.
What to do?
You can increase the energy efficiency of a flat roof system by either increasing R-value or by applying the insulation in a more efficient manner. Installing double layers of insulation boards, with the seams staggered and/or tapped is one such installation technique. This eliminates heat loss at the perimeter of each insulation board. Another energy efficient installation option is called the “screw and mop” application. This process also involves (at least) two layers of insulation installed with the seams staggered. The first layer of insulation is fastened to the roof deck and the second layer is adhered to the first. This eliminates heat loss through the fastener.
If building owners are seeking optimal thermal performance for their roofing projects, it is in their best interest to insist that the consultant complete the necessary steps to gather sufficient information required to develop correct specifications for the project.