Proposed Change 1541

From: National Research Council Canada

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Page: Last modified: 2020-02-18
Code Reference(s):
NECB15 Div.B
Windows, Doors and Skylights
Allowable Areas for Fenestration, Doors and Skylights
This proposed change introduces a reduction in the allowable vertical FDWR (fenestration and door area to gross wall area ratio).


[] Fenestration and Door Area

[1] 1)The maximum allowable total vertical fenestration and door area to gross wall area ratio (FDWR), determined in accordance with Article, shall be as follows:
FDWR = 0 . 40 for HDD 4000 , FDWR = 2000 0 . 2 × HDD 3000 for 4000 < HDD < 7000 , and FDWR = 0 . 20 for HDD 7000 ,
FDWR = 0 . 30 for HDD 4000 , FDWR = 1700 0 . 2 × HDD 3000 for 4000 < HDD < 7000 , and FDWR = 0 . 10 for HDD 7000 ,


= the heating degree-days of the location of the building determined according to Sentence
[2] 2)The total skylight area shall be less than 52% of the gross roof area as determined in Article

Note A- Total Vertical Fenestration and Door Area to Gross Wall Area.

Table A- shows a sample of maximum allowable FDWR for various HDD.
Table [] A-
Maximum Allowable FDWR for Various HDD
HDD Maximum FDWR
< 4000 0.430
4000 0.430
4250 0.328
4500 0.327
4750 0.325
5000 0.323
5250 0.322
5500 0.320
5750 0.218
6000 0.217
6250 0.215
6500 0.213
6750 0.212
7000 0.210
> 7000 0.210



The NECB 2017 allows maximum prescriptive FDWR values by climate zones, which often exceed the typical FDWR values of actual building designs in a climate zone. By reducing the maximum allowable prescriptive FDWR values so that they correspond more closely to the FDWR values of typical buildings in a climate zone, it ensures that only buildings with a low-enough typical FDWR value for the climate zone utilize the Prescriptive Path. Building designs with a FDWR value higher than the maximum allowable prescriptive value should demonstrate compliance using either the Part 3 Trade-off Path or the Part 8 Performance Path.

Justification - Explanation

Given that the fenestration U-value is much greater than for opaque assemblies, and more costly per unit area, lowering the maximum allowable prescriptive FDWR of a building will reduce the overall U-Value of the above-ground building envelope, reducing:

  • the amount of uncontrolled thermal transfer through the building envelope
  • the demand and consumption of energy for heating and cooling
  • the capital cost of the envelope and equipment.


Impact analysis

Related to Article

Fenestration is typically an expensive element of the above-ground building envelope, and in some cases is more costly per unit area than opaque above-ground building assemblies. Reducing the prescriptive allowable area of fenestration may lower the capital cost of the building and the equipment used to condition it.

The NRC conducted a simulation study on the estimated energy savings from adopting the proposed reduction in FDWR. The study used OpenStudio and NRCan CANMETEnergy’s building technology assessment platform (BTAP) to generate 15 NECB 2017 compliant archetypes and run annual simulations. The savings presented are a reduction of the whole building’s annual energy consumption (includes heating, cooling, lighting etc.) relative to the 2017 NECB. Figure 1 provides an overview of change in energy usage of the 15 archetypes in each of the six climate zones, illustrating the range and trend of the energy savings. Table 1 shows the detail energy savings of each archetype-location pair, as well as the average savings of each archetype across the climate zones. Note the negative values for climate zones 4 and 5 (Victoria and Windsor, respectively) reflect an increase in energy usage by the buildings after adopting the proposed FDWR (which are higher than NECB 2017 values for zones with HDD ≤ 4000).

The proposed changes generally reduce FDWR limits which essentially amounts to replace glazing with opaque insulation (wall), which results in reduced solar heat gains and reduced opaque envelope heat loss. The net effect on heating and cooling depends on the time of day and the season. For example, during the winter, the reduced solar heat gain will result in additional space heating during the day, but the increased opaque areas will reduce night time heating. The majority of the energy saved for most archetypes is due to reduced HVAC fan power consumption (associated with reduced heating/cooling demands manifested as reduced runtimes for the HVAC system). For zones with a higher proposed FDWR, the additional glazing allows greater sunlight during the day but increases heating, cooling, and fan consumption resulting in an overall increase in energy usage for the associated climate zones.

Figure 1 Overview of climate zone averaged energy savings

Overview of climate zone averaged energy savings


Table 1 Energy savings from proposed FDWR and skylight changes

Archetype Climate Zone
CZ-4 Victoria CZ-5 Windsor CZ-6 Montreal CZ-7A Edmonton CZ-7B Fort McMurray CZ-8 Yellowknife Archetype average
Secondary School -0.7% -0.9% 2.5% 2.7% 2.9% 2.4% 1.5%
Primary School -1.2% -1.2% 4.1% 4.4% 4.0% 3.3% 2.2%
Small Office -1.7% -1.6% 5.3% 6.4% 6.8% 7.0% 3.7%
Medium Office -3.0% -2.4% 9.1% 11.7% 13.1% 10.8% 6.5%
Large Office -1.8% -1.7% 5.2% 6.6% 6.1% 5.3% 3.3%
Small Hotel -1.1% -0.9% 3.3% 3.2% 3.3% 3.3% 1.8%
Large Hotel -1.4% -1.2% 4.2% 4.0% 4.2% 4.3% 2.3%
Warehouse -2.5% -2.3% 6.7% 7.9% 8.4% 6.2% 4.1%
Retail Standalone -1.5% -1.6% 5.7% 6.4% 6.3% 5.6% 3.5%
Retail Strip mall -1.7% -1.7% 4.6% 6.6% 6.6% 5.2% 3.3%
Quick Service Restaurant -0.9% -0.9% 2.5% 2.7% 2.8% 2.2% 1.4%
Full Service Restaurant -0.3% -0.3% 1.2% 0.8% 1.2% 1.0% 0.6%
Midrise Apt -1.4% -1.1% 4.1% 4.0% 4.8% 4.9% 2.5%
Highrise Apt -0.9% -0.8% 3.3% 2.1% 2.6% 3.2% 1.6%
Hospital -0.8% -0.7% 2.2% 2.4% 2.5% 1.9% 1.2%
Climate zone (simple) average -1.4% -1.3% 4.3% 4.8% 5.0% 4.4%  


Additional documentation for this proposed change is available on request at

Enforcement implications

There would be no change in the effort to comply with the Prescriptive Path by designers, nor for an AHJ to validate Prescriptive, Trade-off or Performance Path submissions.

For building designs without skylights complying using the Part 3 Prescriptive Path, there are no enforcement implications.
For building designs with or without skylights complying using the Part 8 Performance Path, the modeller and AHJ would simply validate that the reference model has no skylight area defined.
For building designs with skylights intending to comply using the Part 3 Prescriptive Path, these will require that the Part 3 Trade-off Path be used for the horizontal roof areas, and that the AHJ confirm the correct application of the Trade-off Path for the roof areas. Given that many buildings do not include skylights, there is little to no impact expected for the enforcement of this proposed change.

Who is affected

The owner would be affected as there would be reductions to the maximum allowable prescriptive areas of FDWR.

Others that may be affected are the designers, engineers, architects, building officials and manufacturers/suppliers.




[] ([1] 1) [F92,F99-OE1.1]
[] ([2] 2) [F92,F99-OE1.1]
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