Roof of Glass House – R-Value Beyond Which Makes No Difference
17whippoorwill
| Posted in General Questions on
Hello,
Thanks to everyone who asks and answers questions, and bloggers … lots of good info.
Question: If one were to build a glass house (aka Philip Johnson), or on a less grand scale, a standard pitched roof 1,600 SF house with normal walled rooms in climate zone 6 or 7 (or maybe anywhere), with at least 40% of the exterior being double pane glass windows, is there a roof R-Value beyond which there would be no benefit, or is there a calculation that can be made inputting exterior wall R-Value, window R-Value, Wall, Roof, and Floor areas?
On the same example house, should exterior wall insulation be increased from R21?
Thanks
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Replies
The heat loss through the roof depends on the on the inside and outside temperatures, the area, and the R-value, and is not affected by the number or quality of windows.
You know Philip Johnson called The Glasshouse his four-bucket house (one in each corner). It was more a social space than a home. Of course, technology has evolved and you could make that type of enclosure a lot more livable.
If I ignore the glass part of the question R60 is the normal answer as a rule of thumb.
Your question has a lot of variables some are know today and others are like the cost fuel 15 years from now are wild guesses.
If you know the cost per square foot and the R values for the different options you can build a BEopt computer model of the building and select the best option.
Seems to me that a glass house is a big statement to the world that the cost to build and operate this house is irrelevant to its owner and this is what they want no matter how impractical it may be.
https://www.nrel.gov/buildings/beopt.html
Walta
With that much glazing, I would suggest that triple pane, euro style (u factor in the range of 0.1) windows would be worth it. And the walls should definitely be more than R21 in zone 6 or 7. It's still over half the vertical surface area. Maybe R40.
There is always a benefit to more insulation. Always. There are two questions you should be asking instead. The first is whether it is cost-effective. The second is whether it is the most cost-effective improvement you can make, or whether there are other more cost-effective improvements you can make.
As Charlie Sullivan points out in #1, the heat loss through each surface is independent of the other surfaces. So the most cost-effective level of roof insulation is independent of wall construction. And the most cost-effective level of wall insulation is independent of window construction, and the most cost-effective level of windows is independent of roof and wall.
Construction costs are so dependent on location, and energy costs are so dependent upon climate, that there is no rule-of-thumb answer about what is cost-effective.