Process for Choosing Windows and Window Costs

By Matt

One of the important features of our house are the windows. The house is oriented with one of the long dimensions facing south with large windows on the main floor in order to allow the winter sun to heat the 4″ thick cement slab in the living room and kitchen.  The 3D SketchUp of our house on the left is the south-facing facade. The image on the right shows the north-facing facade.

With a proper passive house design, solar gains can account for 20-50% of the yearly heating cost. Ideally, the south facing windows should be an absolute maximum of 12% of the overall floor square footage.  Any more than that and you will end up with over-heating during the day and excessive heat loss at night. If you’re not carefully designing the structure to handle the heat load, a maximum of 8% is recommended. Some of the over-heating during the day can be mitigated by providing a thermal mass to absorb the heat, evening out the heat gain, which can be radiated during times of shade or late in the day. 70 to 85% of the windows should be south facing to capture the most winter sun which would be lost to windows facing north, east or west.

Several design iterations were executed to optimize the number and size of the windows to obtain the right ratio of windows to slab size. After factoring out the frames of the window so that glazing is correctly calculated, we have 273 square feet of glazing, with 196 (72%) square feet located on the southern face. The value is on the low side of the suggested value as other factors were considered, such as including operable windows upstairs on the north side to facilitate air-flow in the summer to reduce cooling costs.

To make a proper calculation of the volume of the house to be conditioned, spaces such as the ‘open to above’ area and basement is counted in the overall square footage. Our total living space will be 1,732 square feet, but the total area of the house used for calculating conditioned space is 2,151 square feet. This number includes a 280 square foot below grade basement (below 1/3 of the house) and the ‘open to above’ area in the dining room.  As such, the square footage ratio of south facing windows to overall floor area is 9.1%.

To optimize solar gains and minimize thermal losses the windows should have the lowest U-factor (Inverse of R-value) possible and highest solar heat gain coefficient (SHGC). We thought that triple pane windows would be out of our price range so our initial design plants incorporated Marvin Integrity windows. However after thinking about the form of heating and cooling we were going to utilize in our build (two outdoor minisplit compressors paired two indoor units, centrally located, one upstairs and one downstairs), we thought it essential that we upgrade to triple pane to minimize heat loss and convection currents which can make a room feel colder than it is.

We used the National Fenestration Research Council (NFRC) certified product guide to get some ideas of which companies produce triple pane windows which at least meet the version 6.0 of the Energy Star Guidelines for northern climates (U<=0.28, SGHC>=0.32). We sent our designs to Marvin, Zola, Wasco, and Accurate Dorwin and had them send a quote. The specific product lines quoted were:

• Marvin Clad Ultimate, a clad-wood (pine) window with extruded aluminum exterior, double pane Low E2 glass with argon fill;
• Wasco Geneo line, with frames made with a uPVC/fiberglass co-extruded material (RAU-FIPRO), argon filled insulated triple pane glass;
• Accurate Dorwins fiberglass triple pane, argon filled, sungate 400; and
• Zola Thermo Clad pine wood triple pane argon filled windows.

The Marvin Clad Ultimate were relatively expensive for the meager U-values obtained (~0.28) and whose cost was mostly dictated by the use of wood for the frames. Aesthetically, on the interior space where the wood would be exposed they would have been amazing but we couldn’t justify the extra cost and insufficient U-values. Similarly the Zola Thermo Clad windows were slightly more, again due to the inclusion of wood, but have the best thermal properties with a U-value of 0.14, twice that of the Marvin Clad Ultimate. In addition the cost of shipping ($4000 – $5000) added 20% to the cost of the window budget. While beautiful, Zola windows were out. The Wasco Geneo line and Accurate Dorwin windows were similar in price with Accurate Dorwin being slightly less expensive but with slightly poorer thermal performance, U=0.15 for Wasco and U=0.20 for Accurate Dorwin. Accurate Dorwin had a version of the window with a U=0.14, but this sacrificed the SGHC and would only be appropriate for non-south facing windows. On the negative side, the Wasco Doors were 30-60% more expensive than the Accurate Dorwin doors. See the table below for a selection of doors and windows specifications and prices.

In the end, the thermal properties of the Wasco windows and doors and the 20% final discount on the entire bill led us to choose the Wasco Geneo line. In addition, Wasco is produced in Milwaukee Wisconsin, in my hometown, so we could go and visit the manufacturing facility and showroom. Mike Wilson from Wasco windows gave over an hour of his time showing us how Wasco windows are manufactured. We were able to see and touch the product lines which gave us confidence in our selections.

Final Numbers

The total rough opening square footage was 354.5 sq. ft, of which doors were 83.4 sq. ft. and the garage windows were 28.7 sq, ft, (double pane). The total cost was about $17,000. Approximately $8000 of that was for the three doors. The cost overall was about $48 per square foot, while if one only looks at the cost for the triple pane fixed and operable windows the cost was about $33.5 per square foot.  This is a totally reasonable price for the excellent thermal properties the Wasco windows provide.

The costs mentioned here do not reflect shipping. Accurate Dornin came with free shipping, Wasco shipping was about $500 and Zola was going to be $4,000 – $5,000 adding about 20% to cost of project. Also, Wasco provided a 20% discount (not included in calculations above).