In their predictions for US high performance buildings in 2014, Hammer & Hand, recipient of the 2014 Green Home of the Year Award from Green Builder magazine, states that US window manufacturers have pushed performance as far as they can with existing “American” profiles, so more will move to a European sash and frame design, with better glass, better hardware, thicker frames to accommodate thicker insulated glazing units and better thermal breaks. So were we able to source a high performance window from North America, or did we have to source it from Europe? For our bake-off, Steven Theodore selected high four performance custom window manufacturers – Marvin and Duratherm from US, Loewen from Canada, and Intus from Europe. We included Duratherm as it is a Maine company that makes contemporary windows with the sleekest contemporary lines using a wide variety of woods from mahogany to teak. We couldn’t consider other European windows like Optiwin because they didn’t have a dealer in Maine. The table below compares the performance metrics from the four vendors for the Lily Pond House. All of the windows have triple pane glazing, and the performance metrics are shown only for the South elevation sliding doors that will be used passive solar heating of our thermal mass cement slab. The actual comparison is pretty complicated as each elevation (South, West, East, North) and configuration (e.g. door vs. window) have different specs from the same vendor.
South facing sliding doors performance comparison
The National Fenestration Rating Council (NFRC) is a nonprofit organization that sets the standards for measuring the performance of windows and doors in US. The metrics include U-Factor, Solar Heat Gain Coefficient (SHGC), Visible Transmittance (VT), Air Leakage (AL), and Condensation Resistance (CR). For reporting, AL and CR are optional for manufacturers. Comparing the vendor quotes is instructive for assessing each vendor’s approach to performance. Both Loewen and Intus provided the whole window ( including glazing, frame and spacers) U, SHGC and VT values for each window and door. Duratherm provided two sets of numbers – the center of glass U value and SHGC for the South facing and other windows while Marvin did not include any performance specs in their quote although Marvin does provide an interactive calculator on their Web site. The U-factor indicates the heat loss through the window assembly. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating properties. The reciprocal of the U-factor gives the R value of the window assembly. Referring to the table, in thermal performance, Loewen has an R-4 value whereas the Intus provides an impressive R-10. SHGC is the fraction of solar energy transmitted through a window, which was about 60% for Duratherm, 50% for Marvin and Intus, and 40% for Loewen. VT is an optical property that indicates the fraction of visible light transmitted through the window. For this metric, the range was a low of 45% for Loewen to a high of 70% for Intus. The higher the VT, the more the window maximizes daylight. AL measures the infiltration through cracks in the window assembly, and is defined as the cubic feet of air passing through a square foot of window area. The lower the AL, the tighter the window assembly. CR measures the window’s resistance to condensation on the inside glazing under humid conditions. The higher the CR, the better the window resists condensation.
The price in the table is normalized by the lowest cost Intus windows and shows the increase was 23% for Loewen, 55% for Marvin, and 300% for Duratherm, with some of the difference attributable to the exterior/interior clad used in the frame. Intus does offer wood and aluminum cladding as well at higher price points. Unlike PVC, unplasticized polyvinyl chloride (U-PVC) is rigid and recyclable. U-PVC windows are the most popular windows in Europe. It was clear that the Intus windows had the best performance specifications, which we ended up going with.
-
- From Lithuania to Maine
-
- Shrink wrap on a crate
-
- South facing sliding doors
-
- South facing windows
Our builder ordered the windows and doors through the Portland based Intus dealer – Performance Building Supply. We visited the store twice to look at the in store installations. The dealer could not arrange a site visit to see the windows/doors in the field. In ordering Intus windows, we thought we were making an early adopter decision, and our experience validated that. Intus order says that the delivery time for windows is approximately 12-14 weeks but ours arrived a month later than expected. To top it off, the windows got delivered in a shrink wrap on a crate. Unfortunately, one of the panes in our largest window was cracked during transit. On the up side, our builder’s assessment after installation is that the quality of the windows and doors is top notch. We think they look great with their sleek contemporary frames. Plus they remind us of our time in Europe where every window opened inside. We’ll report on the performance and level of service by our dealer after the blower test.
Lily Pond House 