Flat roofs
Thermal efficiency
Most existing buildings that are ready for a roof retrofit have insulation levels below current energy standards. Although increasing the thermal performance to current standards may not be necessary when conducting roof renovations, it can be critical for energy savings. Depending on the retrofit strategy, this may be achieved by adding a few additional inches of insulation over the existing roof, or it may require a full re-roof (tear-off). From a thermal efficiency standpoint, different roofing systems will encounter other thermal effects depending on the application, type of insulation, temperature exposures, number of fasteners, and material properties. Insulation boards used in roofing applications may be fastened into place using screws with varying densities and lengths, or adhered. Roof systems are subject to higher and larger temperature fluctuations, meaning their membranes and insulation materials are subject to expansion. For example, foam plastics are at more significant risk to thermal deficiencies caused by gaps and holes caused by thermal expansion and contraction, along with their in-situ climate-dependent thermal performance.
Durability
In some cases, insulations like polyisocyanurate will be susceptible to lower thermal performance at cold temperatures, affecting overall performance, especially in cold climates. To reduce the effects of expansion and contraction, double layers with offset seams are necessary. A design solution to limit the impact of climate-dependent thermal performance and thermal expansion and contraction includes using a dimensionally stable insulation board (such as stone wool) as the second insulation layer or as the system coverboard.
Constructability
In the case of recovery or full re-roof, consider the existing roof’s parapet heights to ensure there is enough space to add the desired thickness of insulation. Installing crickets to modify drainage is a simple solution, mainly when the parapets’ roof edge is low and does not allow for increased depths. Most importantly, the roof’s structural capacity requires engineering evaluation to ensure it can handle the weight of the new or added layers and confirm the existing deck is compliant with structural requirements, determined by a pull-through test.
For roof remediation systems incorporating additional insulation, choices will vary depending on the new membrane type. Adding stone wool insulation in a recovery system helps improve the roof’s overall durability and thermal efficiency if the existing insulation is in poor condition and gaps are evident. Placing a 52 mm (2 in.) layer of rigid stone wool insulation with a total added RSI value of RSI 1.34 (R-7.6) is a cost-effective measure to improve thermal performance, fire safety, and sound attenuation.
In a full re-roof application, the new insulation system can be either full-depth stone wool or a hybrid roof design, incorporating a layer of stone wool insulation over a base layer of polyisocyanurate insulation. This solution can also comply with updated wind load and fire testing requirements. Full re-roofing is optimal for buildings with a high roof-to-wall ratio where most of the heat loss occurs through the top, and there is a favourable cost-benefit.
Project spotlight: The Ken Soble Tower in Hamilton, Ontario
The Ken Soble Tower project sought to rehabilitate a postwar apartment building in Hamilton, Ontario. The building was completely upgraded, inside and out, to achieve Passive House standard. The building overhaul would include nearly every facet of the tower, from the building envelope, mechanical systems, electrical, plumbing, and safety systems to interior upgrades to its 146 units, to support aging in place, accessibility, comfort, and overall improvement of the occupant experience. The cladding design includes a 152-mm (6-in.) thick stone wool exterior insulation and finish system (EIFS). Architects chose this system for the project due to the non-combustibility (important given the vulnerability of the senior-aged occupants), the excellent moisture control offered by both the stone wool and the unique, built-in drainage layer cut into the back side of the insulation, as well as the liquid-applied water-resistive barrier used in the EIFS composition.
In all, 4645 m2 (50,000 sf) of stone wool insulation was incorporated into the new façade, helping to realize the RSI-6.69 (R-38) effective RSI value required to achieve EnerPHIT Passive house certification, while reducing greenhouse gas emissions by an impressive 94 per cent.
Alejandra Nieto is a sustainability manager for ROCKWOOL. She is a graduate from the master of building science program at Ryerson University; with a background in construction science and management, and architectural technology from George Brown College. She has experience in design and research of the methods and materials involved in sustainable and energy efficient buildings and systems, and is an active member in multiple ASHRAE technical committees.
Construction Canada received the following Letter to the Editor:
Dear Editor,
The article was very interesting and informative. Could the author comment on the affect this type of roof energy design has on the design roof snow load? The reason I ask is research in the United States indicates the roof load is significantly higher than for roofs retrofitted with R-30 insulation than older buildings. It has been observed that freezer buildings have significantly greater depths of snow on roofs than on the ground. I have also seen the same condition in my work in Canada. The commonly accepted explanation is the surface of the roofs are cooler than the ground and snow does not melt as normally expected. Up to this time, the ratio between ground to roof snow loads is based on extensive field studies done in the 1960s, so the reduction of snow depth on the roof is based on how roofs were insulated during this period. A discussion of increased snow loads on building roofs is well presented in the article “Snow Thermal Factors for Structural Renovations” in Structure Magazine (https://issuu.com/structuremag/docs/structure-jun19-zmag/24). The latest version of ASCE-7-22 has incorporated these factors. Also, for low sloped roofs, structural engineers are now required to calculate the roof deflections when checking for ponding of rain. With more snow on roofs, the roof deflects more which encourages ponding of rain during spring melting. Care needs to be taken to improve drainage off roofs to ensure ponding is less likely.
David P. Thompson, M.Sc., P.Eng.
Principal
KTA Structural Engineers
David,
Thank you for the comment and the interesting article that you shared.
The engineering design of the roof is outside my area of expertise, and generally as an insulation manufacturer. For this application, we always recommend to work with a structural engineer to ensure the roof system can withstand the upgraded insulation. That said, I do believe that we should not compromise energy efficiency due to structural concerns. When dealing with a roof retrofit, different approaches can be taken both in terms of the type and amount of insulation added, and whether updates are required to the structure. For example, if the existing roof structure is not being reinforced and you need to limit the added weight, one can opt for a hybrid retrofit solution where you can use a layer of foam plastic insulation and then add the non-combustible and dimensionally stable stone wool insulation as a cover board. Stone wool roof insulation can also be tapered and available as a cant strip to assist with drainage requirements.
I would be happy to connect you with our building science team if you have any additional questions, or you can reach our technical service team at http://www.rockwool.com/contactus .