Window Thermal Comfort
In any high-performance building, both the energy and occupant comfort impacts of the glazing should be carefully assessed. For a building seeking any of the PHI or Phius certifications, a very stringent evaluation and quantification of this thermal comfort impact must be executed. This comfort evaluation looks at both the overall occupant comfort as a result of air-temperature and relative-humidity but also the localized thermal discomfort caused by cold-surfaces and drafts at the glazing surface.
In particular, the localized discomfort which results from a radiant temperature asymmetry (a difference in the temperature of the surfaces surrounding the body) must be assessed in order to evaluate the possibility of eliminating perimeter heating. If the radiant temperature asymmetry exceeds a certain level, a compensating heat source may be required in order to offset the potential discomfort. Typically this would mean a radiator of some form, installed beneath the window.
Many engineering reference standards suggests that ideal thermal comfort is found when there are temperature differences of less than 7.6°F [4.2°K] between all the surfaces around a person’s body. The windows are important in this respect, as this is where the coldest surface temperatures in the building will typically occur during the winter months.
For (Blaine County) Idaho Zone 6(B)’s winter comfort-design weather conditions [ -16°F / -8.9°C ], Passive House certification guidelines for this climate would recommend that as long as windows with an Installed R-Value higher than 5.6 hr-ft2-F/Btu [ U-Value less than 0.18 Btu/hr-ft2-F ] are used this will lead to surface temperatures which PHI finds acceptable for thermal comfort. Note, for smaller windows with a lower ‘view-factor’ (less visible to the occupants) slightly lower R-Values may still achieve the desired thermal comfort targets.
Window Products
For the project here, the initial code minimum variant uses windows which just meet the ID ECC 2020 / Zone 6(B) limits (U-0.30 Btu/hr-ft2-F)
As discussed above, the Passive House frameworks recommend using windows which are at least less than Uw-installed of 0.18 Btu/hr-ft2-F in order to satisfy winter thermal comfort goals. For the project here, in order to achieve the challenging heating and cooling annual energy demand limits, this project should utilize windows with an installed U-Value of less than 0.18 Btu/hr-ft2-F, and follow the Passive House recommendations where possible.
While this is a challenging requirement to achieve, there are now several cost-competitive high-performance window products available in the North-American market at this point. Some good options for this project which can provide windows with a U-Value of 0.18 Btu/hr-ft2-F or better include:
Note: The window frames and glass specification are critical to the proper performance of the building. Before finalizing any window or door order for the project, please submit all window quotes and shop-drawings to bldgtyp for review and approval.
Site Shading
The energy balance of the windows are critical on any high performance building project. The goal for this climate should be to take full advantage of wintertime solar gains in order to reduce overall heating energy need, while always being cautious about the potential for overheating and increasing cooling energy need. This can be particularly challenging with highly glazed rooms or space which feature large amounts of south or west facing glass.
All radiation values presented consider the local shading context. Where relevant, this context is created using satellite images from google maps and plot-lines from OpenStreet Map and CadMapper. The site shading and orientation includes the following:
Orientation / Sun-Path Diagrams:
Taking into account the climate, orientation, and shading, in the results below we have assessed the average seasonal (winter / summer) solar radiation falling upon the windows in the project. The radiation levels will vary by orientation and as can be observed, shading obstructions also have a strong effect on the final level of solar radiation any individual window receives.
Winter Radiation
- Good winter-time radiation is observed on the east/southeast facing windows. In particular in the kitchen, and the upper levels. The deep roof over the home’s living room reduces the amount of radiation received by the large glass wall, but some early morning sun is likely.
- As the primary exposure of the home is east/southeast, the main solar gain will be early morning when the sun is low in the sky. While this is unlikely to cause any overheating issues, this may still cause discomfort-glare problems for occupants. It is recommended to ensure that all east/southeast facing units are outfitted with robust interior blinds/shades in order to help the occupants manage this potential glare.
- No west/northwest facing windows experience any useful solar gain during the winter period. One potential performance improvement would be to reduce the north- and west-facing glazing area by +/- 10% in order to reduce the overall heating energy need.
- One other possible improvement would be to reduce the number of clerestory windows under the large roof in the main living room. While some clerestory units are beneficial in order to improve the interior daylighting, consider reducing the number of units to the minimum needed in the space.
Summer Radiation
- The large roof does an excellent job of reducing the solar radiation on the primary glass wall. This will help significantly to reduce any possible summer overheating issues.
- The highest levels of radiation are observed on the unshaded east/southeast facing units. These are the primary bedroom windows, and the ADU bedroom windows. It is strongly recommended to add some form of shading for these windows in order to help manage overheating risk within these smaller spaces. This shading could be some form of horizontal shading (~12" - 18" deep), or could be shutters / shades. At a minimum, all east/southwest windows should have robust interior-side blinds, however this may not be sufficient to manage interior overheating during some peak cooling load time periods.
- Where possible, ensure that some or all of the west-facing units are operable in order to allow for natural ventilation and night-time cooling.
- Consider the use of deciduous trees or other vegetation such as vines on trellis elements to provide shading to the any east or west facing units. This type of shading can be particularly effective with low-angle sun such as is experienced in the morning and afternoons on the east and west.