Why Better Windows Matter
Right now, it’s the coldest week of winter here in Portland. With temperatures dropping down to the low to mid-20s, it’s hard not to notice the furnace kicking the heating bill higher than the last month.
With our Northwest Passive Panel, we build highly insulated, airtight buildings. But what about all those holes in the wall? Yes, we’re talking about windows!
“What’s wrong with windows?” you might ask. Windows are of course wonderful. They let in light, allow us to see the scenery around us, yet are the weakest link in a wall’s total insulating performance. Take, for example, the NWPP, which has a thermal resistance of about R-37. Let’s have a 10’x10’ section of wall and use an average 5’x5’ window, which gives us a sensible 25% window to wall area ratio. If we use the average North American window, which has a thermal resistance of R-3, that severely downgrades the total wall R-value to R-9.5. Yikes.
What if we triple the R-value of the opaque wall and keep the same window? We have an impractically 3-foot-thick wall that’s only improved to R-11. Now if we keep the wall at R-35 and double the window performance to a triple paned R-6, we get a total R-15 performance.
As you can see, even with a limited number of windows on a building, they have an outsize impact on keeping the heat in and the cold out. In addition to picking the right windows, a myriad of other factors in the installation are important. These include air and water sealing around the window frames to prevent drafts and any potential window leaks from damaging the wall. A major factor in affecting installed window performance in the wall is the linear thermal bridge coefficient between the window frame and our wall. Our strategy of mounting compatible windows in the middle of the wall and over-insulating against the window frame allows this transition to outperform standard details.
Knowing these key elements in the physics of how third-party components interact with our product allows us to work with our clients to ensure that the windows and doors installed in their projects meets or exceeds the criteria needed to achieve their energy efficiency targets.