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 NYC Zone 4(A)’s winter comfort-design weather conditions [ 8.6°F / -13.0°C ], Passive House certification guidelines for this climate would recommend that as long as windows with an Installed R-Value higher than 5.9 hr-ft2-F/Btu [ U-Value less than 0.17 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 NYS Energy Code 2020 / Zone 4(A) limits (U-0.32 Btu/hr-ft2-F)
As discussed above, the Passive House frameworks recommend using windows which are at least less than Uw-installed of 0.17 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 0.17 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 0.17 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 is 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:
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
- Assuming that the street trees lose most of their leaf canopy during the winter months, the primary south-west facade receives fairly high winter solar radiation. This solar radiation will help to offset winter heating energy need.
- While winter-time solar radiation is helpful in offsetting heating energy need, on days with a lot of sun there may be some overheating risk even in winter time. We recommend ensuring that ALL south and south-est facing glazing be outfitted with robust interior blinds to ensure that the occupants can control the solar exposure of the spaces.
- Note that the large glazed area in the bulkhead does receive a significant amount of solar radiation in winter. This, combined with the internal air stratification may lead to risk of overheating in winter. It is recommend to reduce the amount of glazing here, and/or ensure that this glazing has both good external shading (overhangs / trellises / canopies, etc..) AND durable interior user-operable blinds for all south-west glazing.
- No useful winter time solar radiation of any form is observed on the north and west facades. One good strategy for improving the performance of the building would be to reduce all north and west facing windows by +/- 10%. This would help improve comfort and energy performance in the building.
- Another option that would help improve performance of the north windows would be to change from a ‘double-hung’ style window with 2 distinct sash elements to a single tilt-turn style window. This reduces the amount of frame in each window which helps improve energy performance, air-sealing, and thermal comfort.
Summer Radiation
- Assuming that the street-trees leaf out fully in summer, they will provide decent solar shading for the lower-levels on the south/south-west facade. Even with this shading, it is strongly recommended to ensure that all south/south-west glazing be outfitted with durable, user-operable interior blinds to allow for solar control.
- The bulkhead appears to be fully unshaded and high exposed to summer radiation. It is strongly recommended to reduce the size of the bulkhead glazing and to ensure that it is well shaded. This should include both good external shading (overhangs, trellises, canopies, etc…) but also durable interior blinds on all south bulkhead glazing. Due to the high levels of solar radiation and the rooftop position, this space is at high risk for overheating.
- On the north/north-west facades there is some amount of solar radiation observed on the upper levels. This later-afternoon sun will be very low in the sky and so will represent a possible discomfort glare risk for the occupants, in addition to the solar heat gain issues. It is strongly recommended to ensure that all upper level windows on the north/north-west facades have durable interior blinds installed which are user-operable.