Conclusion
This study showed replacing the ordinary poly vapour barrier with the reflective foil would lead to energy savings of at least five per cent (i.e. more was expected for existing homes). These are potential energy savings associated with space-heating structures built by the latest standards for wall system insulation (nominal R-22) in the residential building sector for northern British Columbia. However, savings are expected to be significantly higher when upgrading old homes. (Five per cent savings represents the equivalent of R-1 added insulation to, say, an R-20 effective wall thermal performance. If retrofitting existing walls [i.e. 30 years or older] at effective R-10 or less, then R-1 extra insulation can lead to savings of 10 per cent or more.)
Surprisingly, the researchers did not record any significant energy savings as a result of the 25-mm (1-in.) air space left in conjunction with the poly vapour barrier. Although the air gap is critical in making the reflective foil effective, its role as provider of extra insulation could not be proved. This is why the recorded savings are considered to exclusively be the result of the reduced radiant heat loss through the use of reflective foil. (To clarify: Shed 1 [second week] had poly vapour and 25-mm air gap behind drywall; Shed 2 had poly vapor both weeks and no air gap. The expectations were Shed 1 [second week] would lose less heat than Shed 2 because of the extra air gap. The recorded heat loss in Shed 1 [second week] was the same [actually slightly higher, probably due to differences between the two sheds] than the heat loss recorded in Shed 2 for both weeks. [See Figures 4 and 5.])
The average exterior temperature during the trials was well above the average winter temperature in Prince George (i.e. −5.4 C [22.3 F] for December through February). More accurate results might be obtained for a bigger temperature differential and also if data had been recorded over a longer period. Some material insulation properties may be sensitive to temperature as well. (Some insulation materials exhibit better thermal performance as temperatures get colder while some materials exhibit worse thermal performance as temperature gets colder. For more, visit buildingscience.com/documents/information-sheets/info-502-temperature-dependent-r-value.)
Due to Prince George’s relatively cold climate, the calculations for heat loss and potential energy savings are simplified, since our homes require space-heating most of the time. In this region, the ratio between heating degree days and cooling degree days is over 99 per cent. For milder climate jurisdictions, a second reflective barrier may be installed at the exterior of the wall system and in the attic as well, to prevent heat from entering the house from outside during the warm season. However, energy needed for heating have to be assessed separately from those needed for cooling.
Twin Maple and CNC plan to expand their collaboration with a second project to establish a more complete evaluation of the potential effect on energy efficiency from using reflective foil as a replacement for vapour barrier in wall systems. This can be obtained by scaling-up the experiment in actual residential building structures. Potential energy savings should be put in a broader context of the economics of the proposed upgrade, either as a part of the new house building technique process or as a renovation job in existing houses.
This author and other researchers are proposing a cost-effective retrofit solution for old homes by placing the reflective foil directly on the existing gypsum boards of all exterior walls and on the ceiling of the rooms in direct contact with the attic. New drywall would then be installed and finished after strapping accordingly. (In basic terms, the experiment proposes applying [i.e. stapling] the reflective foil directly on the existing gypsum wallboard, strapping it with 25-mm [1-in.] wooden slats, and then attaching new gypsum wallboard while the older drywall remains in place, untouched. [This is because removing gypsum board in existing houses older than 40 years may pose a risk to asbestos exposure.])
A second objective of the future proposed collaboration will focus on implementation of a standard method of calculating the TTR index in the residential building sector. A standardized TTR index can be useful for new homebuyers in comparing the energy needs between listed properties. The proposed method can also benefit owners or sellers who may want to improve the thermal performance of their house-for-sale by making energy efficiency upgrades. Conducting exact heat loss measurements can be useful for right-sizing the usual highly oversized heating equipment and to better assess the solar heat gain, by comparing between night time heat loss and day time heat loss.
Sorin Pasca, MSc, CEM, is a research associate with the Applied Research and Innovation program at the College of New Caledonia in Prince George, B.C. He has been actively participating in several projects in areas related to energy efficiency and green energy technologies. As a business energy advisor with LiveSmart BC, BC Hydro, and FortisBC, Pasca has managed numerous building energy efficiency retrofit projects throughout the small business community in Central and Northern British Columbia. He can be reached at pascas1@cnc.bc.ca.
