Exterior Retrofitting – Does it work? Is it Worth it?
Alot of clients ask if there really is that much of an improvement or cost benefit to upgrading insulation and retrofitting older homes. This is an article I came across in BOMA Magazine.
As you can see these guys did their homework and the benefits can be seen by a 65% reduction in gas usage after the retrofit was complete. Also condensation on windows was eliminated – preventing mold and mildew problems and increasing the level of comfort in the building.
So to answer the question: read on for yourself… but it’s a big YES!!!!!
Building Exterior Retrofit and Its Impact on Energy Performance – A Case Study
(Nick Trovato, M.Eng., P.Eng. Read Jones Christoffersen Ltd.)
The building envelope plays a key role in the energy performance of a building and the comfort of its occupants. Read Jones Christoffersen Ltd. (RJC) recently completed a major retrofit of an existing 1960’s office building. This building had a very poorly performing building envelope. Water and air leakage was occurring, resulting in high energy consumption. The entire building was stripped to the structure and totally renovated. The building envelope was upgraded and a new mechanical system installed. The new mechanical system used “chilled beams” system – one of the first applications in Alberta.
This paper will discuss the improvements made to the building envelope and the mechanical system and how the integration of the two systems has significantly improved overall occupant comfort and reduced energy consumption. We will present a comparison of current energy consumption data with historical data obtained prior to the retrofit.
Read Jones Christofferesen Ltd. was the building envelope and structural consultant for the retrofit of a two-storey 1960’s office building (photo 1). The author was retained to investigate the causes of excessive condensation problems in the building and to provide recommendations for the building envelope retrofit.
The existing building exterior was found to be poorly insulated and the wall assembly did not have an effective vapour retarder. Thermal bridging was evident and condensation would form on the building interior.
The client had specific requirements and there were a number of site restrictions that impacted the overall design of the retrofit program. Using an integrated approach to the design, the client, RJC and the mechanical consultant reached a solution that addressed all of the project requirements and provided a highly effective and well performing building.
This paper will present a description of the existing building, the retrofit work performed and the impact on the building performance and gas consumption.
Typical Exterior Cladding
DESCRIPTION AND HISTORY OF THE BUILDING
The two-storey concrete building in Edmonton, Alberta was constructed in 1962 and was used for office occupancy. It has two levels above grade and a single-storey basement. The building was constructed using precast concrete elements. The suspended floor slabs are precast concrete single tees supported on precast concrete beams and columns on the interior. The exterior walls are load-bearing precast concrete walls supporting the floor tees.
The original building envelope relied on the face-sealed principal for water management. The precast wall elements, fixed sealed glass units and sealant materials were the only line of defence to prevent water entry. Any flaw or failure in these components could result in water leaking into the building.
A paper-backed fibrous insulation was applied to the interior surface of the precast to provide the thermal resistance and to prevent vapour movement through the assembly (photos 2 and 3). Our review determined that the joints between the paper backing were not sealed and there were areas where the insulation was not continuous.
It is not clear whether the original designers intended to create the air barrier with the concrete panels, caulked joints and glazing, or whether the paper backing was to be sealed to prevent air flow. It was evident through our site review that the paper backing was not applied in a continuous fashion and therefore could not act as the air barrier.
We noted the concrete columns that were integral with the exterior panels extended from the exterior to the interior of the building. These columns provided a continuous thermal bridge at each vertical rib, contributing to a significant area where heat loss could occur.
Typical ceiling space with paper-backed insulation. Note concrete column creates thermal bridge from exterior to interior.
View of paper backed insulation. Note water stains on surface and dirt in insulation (indicative of air movement).
The windows were a double glazed sealed unit set directly into the opening in the precast panels. The frame was a solid metal bar cast into the precast. A preformed rubber gasket was used to secure the glazing into the assembly. Since the frames and glazing were proud of the line of wall insulation, the glazing assembly was on the cold side of the wall, creating additional thermal bridging at the frame.
Although the face-sealed exterior did perform as intended, it relied on workmanship and regular maintenance to prevent water entry. Significant water staining and mould growth were noted on the interior of the building. Our review concluded that the damage observed related primarily to condensation on the inboard surface of the exterior precast panels. Although the precast concrete could be considered an effective air barrier (provided all joints are configured to also provide an air seal), the lack of an effective vapour retarder on the warm side of the building and the lack of sufficient insulation resulted in condensation forming on the interior surfaces of the exterior precast panels during the cooler months of the year. Warm humid air from the interior would flow past the paper-backing and fibrous insulation and contact the cold interior surface of the precast. The moisture would condense onto the concrete, flow onto the wood framing members around the window openings, resulting in mould and wood rot.
The roofing assembly was a conventional built-up tar and gravel roof with 2” of rigid insulation. The roof assembly was performing adequately however it was weathered and had reached the end of its anticipated life.
The mechanical system consisted of a hot water heating system with induction units located at the perimeter at the base of the walls. Air was circulated through the induction units and a return air plenum was situated in the ceiling space. The original boiler system had been upgraded to a 1.8 million BTU standard efficient unit in 2004.
The building had been recently purchased by the current owners with the intent of performing a major retrofit of the entire building. The building would still remain as office space, with an open office concept. It was critical to the client to maintain good climate control. Also, since the existing windows were narrow,the client required that the project was to maintain as much of the window opening as possible, to provide the maximum light entry into the building.
We examined options that involved removal of the precast, recladding of the precast and considered a double façade system (such as curtainwall) over the existing exterior precast. The client decided that it was important that the exterior cladding remain to maintain the building’s current appearance. The client was aware of the limitations imposed by this decision, however, they were determined that the architectural features of the building exterior must remain.
BUILDING EXTERIOR UPGRADES
The above criteria had significant impact on the decisions made regarding the design changes for the project. Since the building exterior appearance had to remain, any improvements to the wall assembly had to be made from the interior. The following issues had to be addressed:
- Reduce vapour diffusion
- Significantly reduce or stop air leakage
- Improve thermal resistance
- Reduce impact of thermal bridging
- Prevent water leakage
- Minimal reduction in vision area
After a review of the assembly and options to upgrade the exterior, it was decided that increased insulation and an air barrier/vapour retarder would be applied to the interior of the precast wall (Figure 1).
Generally the exterior precast elements were in good condition with minimal cracking evident and no signs of water leakage. The exterior of the precast was cleaned of grime and staining and a clear penetrating hydrophobic surface sealer applied to reduce the risk of future water penetration through the concrete.
A minimum of 75 mm (3”) of spray applied urethane foam was applied to the wall panels to provide an air barrier/vapour retarder and to provide increased thermal insulation (approximately R-20) to the building (photo 4). The foam was easily applied and around the many corners and irregularities in the wall surface. Also, the foam was fully bonded to the inside surface of the concrete. A spray applied cementitious fire-proofing material was used to cover the spray foamed areas and provide the required fire protection. A steel stud wall was constructed between the concrete ribs, prior to the application of the spray foam. A 25 mm (1”) gap was provided between the stud and back of the precast concrete panel to allow the foam between the stud and concrete to reduce thermal bridging. The gap also allowed for irregularities in the construction of the wall and permitted the installation of a straight finished interior wall. A vented air space was provided between the foam and interior gypsum board to allow for air circulation in the wall cavity, in the event that some water leakage occurs through the precast joints or window perimeters (photos 5 and 6). A continuous gap was provided at the base of the wall to permit water drainage and venting. A grille was also provided at the upper portion of the wall to improve air circulation.
Since the precast panels were continuous from grade to roof, the only precast joints were vertical and were located at adjacent precast panels. These joints were located within a small pipe chase that was formed by the precast ribs at the edge of each panel (photo 4). These joints were sealed from the exterior with a silicone sealant and backing rod. The void in the shaft was fully insulated with a combination of spray-applied urethane foam and a semi-rigid fibrous insulation around any piping placed in the shaft. As there may be some slight thermal movement of the panels, a flexible air barrier membrane was applied on the exterior surface of the interior side of the shaft.
Foam application at head. Note membrane tie-in provided for future installation of window.
In order to reduce the impact of the thermal bridging at the columns and still provide continuity of the air/vapour barrier, a rigid insulation and membrane was applied. Insulation was limited to 25 mm (1”) in order not to reduce the widths of the vision portion of the windows. The entire assembly was clad with gypsum board to provide the required fire protection at the columns.
Insulated column. (Note insulated wall space below window. ) Vented wall enclosure (grille not shown)
The existing windows were secured directly to the precast. Although it would have been preferable to install the new windows directly onto the new interior insulated walls, this would have created a deeper sill condition at the precast. Also, the embedded steel perimeter bars that were part of the original window assembly could not be readily removed therefore a cover piece would have been required to conceal the bars. This would have adversely affected the appearance of the building and was not an acceptable solution to the client.
In order to isolate the new windows from the cold precast, rigid insulation was installed between the frame and concrete. Also, a high performance, thermally broken curtainwall frame was used. Thermal modeling of the window assembly and concrete columns determined that 25 mm (1”) of rigid insulation would be suitable to minimize the risk of condensation (Figure 3). A 4 element glazed unit was used to provide a thermal resistance of R-8 at centre of glass
In addition to the wall retrofit, the existing roof was replaced with 100 mm (4”) of insulation and an SBS sheet membrane.
MECHANICAL SYSTEM MODIFICATIONS
The existing mechanical system was totally replaced. The design of the new mechanical system was affected by the limitations of working within an existing building. This included low floor to ceiling heights and restricted penetration sizes through the structural precast beams. The client also required an aesthetically pleasing system and one that had a number of zones to permit occupants to regulate the environment at their work spaces.
A number of different options were considered by the mechanical consultant including central VAV systems, heat pumps, fan coils and variable flow refrigerant systems. Ultimately a modular active chilled beam system was selected. Work consisted of:
- Installation of high efficiency gas fired boilers.
- High efficiency free cooling outdoor chiller.
- Heat recovery systems.
- Duct distribution through existing openings.
- Building management control systems.
- Horizontal ceiling mounted induction units (chilled beams).
Chilled beam unit
There were several advantages to using the chilled beam unit. Both heating and cooling are provided by the unit. It provides a thin assembly that can easily fit as part of the suspended ceiling system. The system operates at a low flow rate, thereby reducing the acoustic impact to the space. It also allows for energy recovery options and very good temperature control of individual zones or office spaces. The chilled beam system does not require large ducting and could be fit into the restricted ceiling spaces in the building.
The improvements to the building envelope allowed for reduction in mechanical equipment sizes (Table 1). Also, the significant reduction in air leakage and the use of high performance windows allowed for the elimination of perimeter heating at the exterior walls.
Table I: Equipment Design Capacities
|Description||Original Capacity||New Capacity|
|Boiler||1,825 MBH||1,000 MBH|
|Chiller||60 tons||54 tons|
|Air handling units||12,500 cfm||5,300 cfm|
The improvements to the building envelope and mechanical systems resulted in a well performing, building. The building typically operates at a temperature of approximately 22 deg.C. (72 deg.F.) and humidity levels are approximately RH 20% to 30%.
The building has been regularly monitored by building staff for signs of moisture entry, condensation and tenant comfort from November 2009, when the building was open for use. To date there have been no reports of condensation or water entry. The client and the occupants were extremely pleased with the comfort with the space and there were no complaints raised by the occupants with regard to drafts, temperature or discomfort.
A comparison of building energy consumption was performed based on billings obtained prior to the renovation. Since the client had recently purchased the building, data for the period the building was occupied was limited to January 2008 to March 2009. The building was under renovation starting in April 2009 and was re-opened in November 2009 therefore consumption data for this period was not used. Comparison of data was made for the period after the building was renovated and occupied (November 2009 to October 2010).
January 2008 to November 2008 – 2110 GJ
January 2010 to October 2010 plus November 2009 – 743 GJ
Overall reduction in gas consumption: 65% reduction
The majority of the savings were noted during the winter months. Gas consumption during the summer months remained fairly consistent at 35 GJ to 80 GJ per month between the months of June to September.
The client has noted that the existing boilers were operating at 30% capacity during the previous winter months. Also, no condensation was noted, even during an unusual winter period, where temperatures were as low as -50 degrees Celsius.
The renovation to the building exterior and the mechanical system has resulted in significant improvements to the building performance and a considerable reduction in energy consumption.
The reduction in energy consumption is attributed to the replacement of standard efficiency boilers with higher efficiency boilers as well as the significant improvement to the air tightness and increased insulation for the building envelope.
It is difficult to separate the amount of the savings that has resulted from the change in mechanical systems. However, the improvements to the building envelope enabled the mechanical consultant to downsize equipment while still providing a comfortable work environment. Improving the air-tightness of the building exterior has also reduced drafts in the building. This enables the mechanical system controls to work effectively and allows occupants to control their surrounding environment.
The client and the building occupants are extremely satisfied with the completed project. The work has met or exceeded all the client’s requirements and resulted in a 65% reduction in gas consumption when comparing 2008 to 2010 data.
The integration of the building exterior design and the mechanical design enabled the design team to work to achieve a solution that met the client’s needs. The reduction in mechanical equipment sizes and elimination of perimeter heating/cooling resulted in mechanical cost savings.
Nick Trovato, M.Eng., P.Eng., is a principal with the Read Jones Christoffersen Ltd., a national consulting engineering firm. He is the manager of Building Science and Restoration in the Edmonton office and has extensive experience in design as well as evaluation and restoration of buildings.