What is the Purpose of Installing Rigid Board Insulation on a Roof?

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What is the purpose of roof insulation? The purpose of roof insulation is to provide a substrate for the roof application and to provide thermal resistance.

What vital purposes does roof insulation serve?

  • Roof insulation can reduce the amount of energy needed to cool and warm a building.
  • Reduce condensation.
  • Reduce temperature can stabilize variations in the deck therefore reducing thermal expansion and contraction.
  • Provide fire resistance.
  • Provide slope when using tapered insulation.

Why would the roofing system potentially need to be upgraded due to using insulation?

  • Insulation resists the thermal heat transfer into the building therefore increasing the roof membrane temperature in the warm months and reducing the roof membrane temperature in the cooler months. This can accelerate the aging process of the roofing system.
  • The level of thermal expansion and contraction may increase due to the thermal resistance of the roof insulation.
  • Although the potential for interior condensation is reduced with roof insulation. Having roof insulation stuffed between the decking and the roof membrane can increase the potential for condensation to take place within the roofing system. In colder climates the use of roof insulation may make it necessary to also use air retarder and/or vapor retarder. Condensation is an extremely important consideration when designing a roofing system.

What are some desirable properties of roof insulation?

  • Compatible with bitumen and other roof adhesives. It should be able to withstand coming in contact with solvents, hot bitumen etc. without degrading.
  • Component Compatible – Should be compatible with other roof components.
  • Impact Resistant – Should be impact resistant and capable of resisting impact damage during and after roof installation.
  • Fire Resistance – It should comply with building codes and insurance companies.
  • It should be moisture resistant. It should be able to resist the damage that moisture vapor could cause.
  • Thermal Resistant – Roof insulation should have a low thermal conductivity (k-value) so the highest possible thermal resistance (R-value) can be reached in the thinnest possible piece of material.
  • Stable R-Value – The R-value should remain constant.
  • Attachment capability – Its surface should provide accommodation for secure roofing attachment. It also needs to be resistant to moisture degradation as to not compromise the roof attachment.
  • Dimensional Stability – The dimensions would need to be stable under varying temperatures.
  • Compressive Strength – It should be strong enough to resist damage to due weight and impact during installation and normal roof traffic.

It would be nice if all roof insulation possessed these properties but that isn’t realistic. Therefore, it is important to consider what properties are the most important and what type of insulation would be the best fit for a specific project.

What are the most common types of rigid roof insulation?

  • Cellular Glass
  • Expanded Polystyrene (EPS)
  • Extruded Polystyrene (XPS)
  • Glass-faced Gypsum
  • Fiber-Reinforced Gypsum
  • Stone Wool
  • Perlite
  • Polyisocyanurate
  • High-density Polyisocyanurate
  • Wood Fiberboard
  • Cement Board
  • Composite Board
  • Tapered Insulation

How many layers of rigid board insulation should be used in low-slope membrane roofs?

2 Layers or more of rigid insulation board should be used in low-slope membrane roofs. The second layer should be offset from the joints of the first layer to reduce thermal losses and membrane stress.

What is Thermal Bridging? A path for heat to conduct. For example, a metallic fastner could act as a thermal bridge conducting heat through the insulation board. To learn more about thermal bridging click here Thermal Bridging in Roofs and Framing – AE Building Systems

What does Btu stand for? British thermal unit. A Btu is the amount of energy required to increase the temperature of 1 pound of water 1 degree.

What is the primary function of insulation? To provide thermal resistance.

What is Thermal Conductivity (k)? Thermal conductivity is the time rate of steady-state heat flow through a unit area of a homogeneous material induced by a unit temperature gradient in a direction perpendicular to the unit area. It can also be defined as the number of Btu that pass through a 1-inch thickness of a 1-square-foot sample of material in one hour with a temperature difference between the two surfaces of 1 degree Fahrenheit. The formula is (Btu*inch/h*ft^2*F). NOTE 1: thermal conductivity (k) value applies to 1 inch thickness of a specific material.

What is Thermal Conductance (C)? Thermal Conductance is the time rate of steady-state heat flow through a unit area of a material or construction induced by a unit temperature difference between the body surfaces. It is the Btu that pass through a specified thickness of a 1-square-foot sample of material in one hour with a temperature difference between the two surfaces of 1 degree Fahrenheit. (Btu/h*ft^2*F)

What is Thermal Resistance (R)? The quantity determined by the temperature difference at steady state between two defined surfaces of a material or construction that induces a unit heat flow rate through a unit area. (h*ft^2*F)

What is Thermal Transmittance (U or U-factor)? The heat transmission in unit time through unit area of a material or construction and the boundary air films induced by unit temperature difference between the environments on each side. (Btu/h*ft^2*F)

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