Randy George is in the final planning stages for a new house he will be building this summer in Vermont, and from the sound of it he won’t have much trouble staying warm through those long winters.
In addition to R-45 walls, an R-65 roof and R-20 slab, the house will have air infiltration rates lower than one air change per hour at 50 pascals of depressurization. Although not quite meeting the Passivhaus standard, that’s extremely tight construction.
George wants his windows to have a U-factor of no more than 0.2 (equal to R-5), but his contractor tells him he doubts that windows meeting that criterion will be worth the expense. “All along I’ve felt that one of the high-performance Canadian windows would be critical to the performance of our house,” George writes in his Q&A post at GreenBuildingAdvisor.
But after meeting with the contractor, George went back to the heat-load calculator he’d been using for another look. While his original calculations showed that he’d need a heating system capable of producing 20,000 Btuh, reducing the R-value of his windows from 5 to 3 would only boost that to 22,500 Btuh. Projected heating costs would go up only $150 a year. Even after juggling the numbers slightly — boosting the required Btu and adjusting the cost of propane upwards — the difference in heating costs would be about $200 a year.
“I’m assuming that the cost difference between these windows would be several thousand,” George writes. “This is making me question the wisdom of the good windows. Can anyone tell me what I’m missing in all of this, or is there a little too much hype surrounding the ‘good’ windows?”
Payback is a more complex than it looks
GBA senior editor Martin Holladay doesn’t find much to challenge in George’s number-crunching,…
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