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 the 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 Massachusetts Zone 5’s winter comfort-design weather conditions [ 3.2°F / -16°C ], Passive House certification guidelines for this climate would recommend that as long as windows with an Installed R-Value higher than 5.0 hr-ft2-F/Btu [ U-Value less than 0.20 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 Massachusetts Energy Code 2021 / Zone 5 limits (U-0.30 Btu/hr-ft2-F)
As discussed above, the Passive House Institute recommends using windows which are at least less than Uw-installed of 0.20 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 0.20 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 0.20 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:
Site Shading Elements:
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 un-shaded south-facing windows. In particular the in the kitchen, and the upper levels. The front porch depth effectively eliminates any beneficial wintertime solar radiation on the lower level south-facing windows.
- West-facing units do receive some beneficial solar radiation during the winter period.
- No north or east facing windows experience useful solar gain during the winter period. One potential performance improvement would be to reduce the north- and east-facing glazing area by +/- 10% in order to reduce the overall heating energy need.
- One other possible improvement would be reduce the size and/or number of glazing units in the north bridge / hallway space. Prioritizing glazing in the primary living areas rather than the circulation spaces is a good strategy for a building in this climate, as long as it does not negatively affect the architectural expression and character.
Summer Radiation
- Significant solar radiation is observed on the south, and west facing units which indicates likely overheating issues.
- It is recommended to included horizontal shading (~12" - 18" deep) over all south-facing glazing in order to reduce the summer season solar gain and improve thermal comfort inside the space.
- The west windows receive the most solar gain of any orientation, and should be shaded in order to reduce the summer season solar gain and improve thermal comfort inside the space. Given the low-angle of the sun in the summer, vertical shading elements are more effective than horizontal shading elements.
- It is recommended to included robust window-blinds for all west-facing units in order to help the occupants control the solar gain and improve thermal comfort inside the space.
- 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 west-facing units. This can be particularly effective if the vegetation is located on the west side of the building, and is allowed to grow tall enough to shade the upper levels of the building.