Using reflective foil as a vapour barrier

by Katie Daniel | January 8, 2018 3:16 pm

[1]

By Sorin Pasca, M.Sc.
Space-heating accounts for more than 60 per cent of the total residential energy consumption in Canada[2], but for the coldest parts of the country, it represents more than 70 per cent of the total household energy use. Heat is lost from homes through a combination of conduction, convection, and radiation pathways. While conduction and convection can be controlled through insulation and airtightness, losses from the third mechanism are almost impossible to be contained, as radiant heat travels away from any warm surface and is absorbed by anything solid.

Vapour barriers are required by building codes in many countries, including Canada. In cold climates, the vapour barriers resist diffusion of inside moisture through the gypsum board and so prevent condensation and wetting of the insulation layer inside the wall system. Vapour barriers are usually made of sheets of polyethylene plastic with no reflective or insulation value. In colder climates, vapour barriers are placed just behind the gypsum wallboard.

The College of New Caledonia (CNC) in Prince George, B.C., and Twin Maple Group have been collaborating to research the use of the aluminum reflective foil insulation as replacement for vapour barrier in wall building systems. The objective was to quantify the potential energy savings as a result of reducing radiant heat losses. (This research would not have been possible without the support from Natural Sciences and Engineering Research Council of Canada ^[NSERC] through an Engage Grant for Colleges. This author would also like to express thanks to Twin Maple for its in-kind contribution to this project, with respect to materials and technical expertise. CNC departments participating in the study include the School of Trades & Technology, International Education, Facilities Services, and Applied Research and Innovation.)

Twin Maple has used reflective foil in many commercial building projects such as attics, crawlspaces, and HVAC ducts and pipe wraps. However, one of the most successful applications has been the installation of the low-emissivity (low-e) ceiling product in more than 160 curling and hockey arenas across Canada. (This type of energy-efficient retrofit was acknowledged by BC Hydro as eligible for rebates under the PowerSmart incentives program.)

This project used reflective foil rather than ordinary polyethylene plastic as a vapour barrier component of the wall system for residential building applications. Besides a small contribution to the overall R-value of the wall, the major advantage of the proposed technology consists in reducing the radiant heat loss almost entirely, while still providing its main role as a vapour barrier, given its impermeability attribute.

In order to be effective, an air gap was left between the drywall and reflective foil by placing horizontal 25-mm (1-in.) straps perpendicular to the studs. This adds to the overall insulation and also contributes to reducing conduction losses, because of smaller bridging (contact points between drywall and studs). The expectation was even convectional heat loss would be affected because the straps are placed horizontally, thereby obstructing vertical air movement behind the drywall.

The calculated energy savings were five per cent, showing the proposed building technique can reduce energy costs related to space-heating and help new homes quality for Energy Star certification. The author expects a significantly higher impact when applying the technology on existing homes, especially those located in cold regions. (For colder climates, the savings are more significant because ‘percentages’ are eventually converted into ‘dollars.’ In other words, 10 per cent of a $3000 annual heating bill for a Prince George house is more than 10 per cent of a $1000 heating bill for a Vancouver/Lower Mainland home.)

[3]

The experiment
Following the codes and standards for regular wall systems in the northern B.C. residential building sector, two identical sheds were constructed by the Carpentry Department of the CNC School of Trades & Technology. Shed 1 had the reflective foil stapled to the studs (Figure 1), strapped horizontally with 25 x 50-mm (1 x 2-in.) wooden straps and then drywalled. Shed 2 was built with a regular poly vapour barrier and gypsum boards attached directly to the studs.

The sheds were placed in the CNC Residence parking lot (Figure 2), and exposed to similar environmental conditions, such as insolation, wind, and shade. Each shed was heated by a 700-W capacity electric shop-heater, plugged in through an energy logger. The inside temperature in each shed was controlled by a thermostat. Temperature loggers recorded the inside and outside temperatures. Relative humidity (RH) meters were placed in each shed.

The first week of measurements took place between April 13 and April 20, 2017. (The reasons for the timing of the study were purely administrative—the project was approved in January for six months and the college’s carpentry instructor and students could build the sheds only after the semester was done [i.e. March 31]. For a comparative study, temperature difference was not an issue. Heat loss would have been more dramatic had the work occurred in winter, making the results a little bit more accurate.) After the first week, Shed 1 had the reflective foil replaced by poly vapour barrier but the 25-mm straps were kept in place. There was no change applied to Shed 2. Heat loss measurements were replicated for another week, between April 23 and April 30. The energy savings were calculated by comparing the total heat losses over one week, adjusted to a common temperature differential of 20 C (36 F), and to the same interior surface size.

Shed dimensions and nominal R-values
Each shed was 2.4 m (8 ft) long, 1.8 m (6 ft) wide, and 2.4 m tall; the tip of the roof was 3 m (10 ft) from the ground. The walls were built using standard 2×6 frame construction, while the ceiling was filled with 305 mm (1 ft) of insulation. As a result, the interior dimensions of the two sheds were:

• Shed 1 (2×6 studs, reflective foil, 25-mm [1-in.] straps, and 12-mm [½ in.] drywall): 2.06 m (6 ft
  9 in.) long, 1.45 m (4 ft 9 in.) wide, and 2.09 m (6 ft 10 ½ in.) tall for total interior surface area: 20.65 m² (222.25 sf); and
• Shed 2 (2×6 studs, poly vapour barrier, and 12-mm [½ in.] drywall): 2.11 m (6 ft 11 in.) long, 1.50 m (4 ft 11 in.) wide, and 2.12 m (6 ft 11½ in.) tall for total interior surface area: 21.62 m² (232.69 sf).

[4]

Since Shed 1’s interior surface area was 4.5 per cent smaller than that of Shed 2, a correction factor of 1.047 was used to adjust the recorded heat loss in the latter before being compared to the heat loss in Shed 1 for evaluating potential energy savings.

The estimated nominal R-values of the walls, ceiling, floor, and door are:

• walls: R-22;
• ceiling: R-40;
• floor: R-1.5; and
• door: R-5.

(However, the effective R-values of the various assemblies may be different.)

Recorded data
Temperature was recorded using LogTag temperature loggers, with data downloaded through that company’s Analyzer software. The interior loggers were placed in the centre of the sheds, at 0.46 m (1 ½ ft) from the ceiling and 1.7 m (5 ½ ft) from the floor. Interior temperature was recorded at 10-minute intervals and averaged over the entire reading period of one week (for a total of 1008 data readings). Recorded interior temperature was in the range of 25.5 to 28.5 C (77.9 to 83.3 F).

Exterior temperature was recorded at one-hour intervals and averaged over 168 hours—the equivalent of one week time. The average exterior temperature was 5.6 C (42.1 F) in the first week and 7.6 C (45.7 F) in the second week. The maximum exterior temperature never exceeded the minimum interior temperature—an indication heat transferred from interior to exterior at all times.

Energy consumption was measured using Onset Data Loggers connected to individual 700-W heaters in each shed. Each heater was coupled to a thermostat, which was placed at the same location as the temperature data recorders. Energy data was downloaded using the HOBOware software and recorded at 10-minute intervals.

[5]

Results and discussion
Figure 3 shows the recorded temperatures, energy consumptions (heat loss), and the calculated adjustments during the first week of measurements.

After one week of data readings and all the adjustments made, energy savings associated with upgrading to reflective foil were 1.959 kWh. In other words, the shed with reflective foil as vapour barrier required 4.8 per cent less energy for space-heating than the shed with poly vapour barrier.

To avoid potential errors from differences between the two sheds, the experiment was replicated for a second week by replacing the reflective foil with regular poly vapour barrier in Shed 1. The straps were reattached—therefore, the 25-mm (1-in.) air space between the drywall and vapour barrier was still in place.

Figure 4 shows the comparative results after recording data for two subsequent weeks in Shed 1. The energy savings, calculated after adjusting the heat loss to a common DT = 20 C (36 F), were 2.147 kWh or 5.4 per cent—relatively close to the initial calculated energy savings of 4.8 per cent, recorded after one week, between Sheds 1 and 2.

[6]

Introducing a new energy performance rating index
The EnerGuide rating system (EGRS) was introduced by Natural Resources Canada (NRC) to help builders and homebuyers better understand and implement upgrade opportunities for increasing the energy efficiency of houses. This can translate into energy and money savings, along with significant reduction in greenhouse gas (GHG) emissions. Since EnerGuide’s objective is to increase the energy efficiency of new residential building structures, its recommendations must be applied at the design stage and incorporated during the construction phase.

Software has been developed to assist energy consultants with modelling the energy usage of individual house units[7] based on several key indicators, including:

• home’s dimensions;
• building envelope and technique;
• HVAC systems; and
• climatic related data.

Although processing this type of information can be time-consuming, it is significantly facilitated by the available input on building practice, material, and product choice from existing building plans.

Unfortunately, rating old homes for energy performance has always been more of a challenge—especially when analytical methods are employed. In most cases, building plans are no longer available, so details of the building practice are missing.

Some of the material insulation characteristics may have been altered by time or renovation. Airtightness may have also been affected by continuous usage of windows and doors. Measuring the interior house’s dimensions can be extremely time-consuming, given the absence of the building plans. Employing past utility bills for assessing energy consumption can be an option, but rough estimates on heating systems efficiencies (and also on the energy use behaviour of individual occupants in regard to their preference for thermal comfort) must be addressed. This can significantly affect the accuracy of the results. Moreover, any energy calculation based on past consumption needs to be normalized with weather data for that specific period.

The study highlighted in this article introduces an inexpensive empirical method for determining the total thermal resistance (TTR) index of a building by calculating it as a result of measuring the exact heat loss over a short time and recording interior/exterior temperature difference for that same period. The TTR index is calculated with the formula:

TTR = ΔT/Q

Where:
ΔT is the temperature difference in C; and
Q is the heat loss rate in kWh/h.

For example, if the recorded exterior temperature over a span of 24 hours is −20 C and the measured heat loss/consumption is 240 kWh in order to keep the interior temperature at 20 C, then the TTR index, say, for a 40-year-old house in Prince George, would be calculated using the equation above as:

TTR = ΔT/Q = (20 – [−20]) / (240 / 24) = 4

The proposed TTR index accounts for the combined R-values of all building components, air leaks, and dimensions. It can provide a very good indication of the energy needs of a specific house. A high TTR index can mean a well-built home, with good insulation and airtightness, but it can represent low energy needs associated with a smaller house as well.

This works similarly to the rating of cars for fuel consumption. Sometimes, high engine performance alone can be deceiving when assessing the overall consumption of the car. A brand new pickup truck can have very good engine fuel efficiency, but it still may consume a lot of gas because of its size. Nowadays, many energy-conscious people are equally interested in energy efficiency and downsizing their own energy needs. TTR index can be easily converted into actual energy consumption for space-heating by estimating the energy efficiency of the heating system, the exterior temperature, and the desired interior temperature.

For the existing Canadian residential building sector, the research project’s very rough estimates suggest the values of TTR indexes would be on a scale of 1 to 10, with TTR-1 being the most energy-demanding house and TTR-10 the least energy-demanding. The results of this experiment showed Shed 1 with reflective foil as vapour barrier had the highest TTR index (TTR-90) of all shed configurations. This corresponds to the least heat loss or energy needs for space-heating. Shed 2 with poly vapour barrier had the lowest TTR index (TTR-82).

It is evident the high values of TTR indexes in this experiment are the consequence of a small interior space in conjunction with good wall insulation. However, a house may require 20 times more heat than an insulated shed. Thus, the TTR index of a house may be 20 times smaller than the TTR index of a shed—probably in the expected range from 1 to 10.

The calculation of the TTR index should be independent of temperature difference, as it is only a measure of insulation and size of the building structure. For example, replicating the measurements in the same Shed 2 for a second week generated the same value for the TTR index (Figure 5), although the temperature differential was different (DT decreased by approximately nine per cent).

There was no difference in recorded RH between the two sheds (i.e. reflective foil versus poly). In fact, the recorded humidity was quite low—a subsequent study may include humidifiers installed in each shed, which may affect heat transfer as well.

[8]

[9]

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[10].)

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[11].

Source URL: https://www.constructioncanada.net/using-reflective-foil-vapour-barrier/

---