# New build in San Diego coastal… U value matter much?

| Posted in Energy Efficiency and Durability on

We’re getting ready to break ground on a 2 story 4000 sq ft house a few hundred feet from the ocean, in San Diego. Our weather is….cool. 90% of the time it’s between 55F and 70F. In homes we’ve rented, we’ve barely ever even needed a/c, and used heat sparingly. These were not spectacularly well built nor efficient homes, to say the least.

As I’m looking at our various window/door choices, U factors range from about .45 for all-aluminum contemporary thermally-broken windows, to .30 for wood/aluminum clad, etc. On the one hand, an approx R-value equivalent of 2 or 3 is pretty much the same when the walls are 16+. On the other hand, the U of .3 is 50% better. Put that way, it sounds like a lot!

We hate to be cold, solar is pointless because it’s so cloudy so often, and so we want to ensure we’re doing what we can to have a tight and efficient envelope.

Should we really be concerned about the difference between .48 and .30 U values?

Thx for opinions!

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### Replies

1. | | #1

I hear you. I live in the PNW and we go entire months without heating or cooling.

A professional should check my math but here is a fun way to look at this:
Let's assume that you have a wall that is 75% wall and 25% window.
Let's assume your wall is R-16 and you are considering an R-2 or R-3 for windows.

Using some arbitrary units (you would need to scale based on heating and/or cooling degree days in your area):
Using R-2 windows:
200 units of heat loss through windows
75 units of heat loss through the rest of the wall

Using R-3 windows:
133 units of heat loss through windows
75 units of heat loss through the rest of the wall
That is about 75.6% as much heat loss vs the wall with the R-2 windows. A 25% reduction in your total energy lost through that entire wall.

Another bit of math:
Let's assume you wanted to use the R-2 windows but achieve the same efficiency for the entire wall as if you had used the R-3 windows. Your wall assembly would need to have an R-value of 150.

The inverse impact of R and the parallel nature of heat loss means that it often makes sense to attack the weakest links.

2. Expert Member
| | #2

While all of this is true, and it is an instructive way to look at the problem, this is where energy modeling provides a much more complete picture. While a reduction of 25% in whole-wall heat loss sounds good, it becomes meaningless as the total heat loss approaches zero, which is nearly the case in San Diego. A heat load analysis can incorporate the efficiency and fuel costs of your chosen HVAC equipment on an annual basis so that you can do the final dollar comparison. The payback on the U=.3 windows could be decades, in which case they make little sense.

Aside from the energy cost issues, the more efficient windows will "feel" warmer because of the lower energy loss. This could be enough of a factor to install them even if the payback isn't great. But again, maybe not so much of an issue where the temperatures never go below freezing.

1. | | #5

And of course, a wall is just a wall. Also need to consider the roof, the floor, air infiltration, etc. An experienced builder and an energy modeler will be able to advise on where you will get the best bang for your buck.

3. | | #3

Thanks for the comments. I just discovered that CA requires .30 anyway. The bummer is the state also requires a low SHGC, which is the opposite of what we want. Our winter is really sunny, and solar gain is critical.

1. | | #4

Hi Tim,

That's true if you are following California's T24 code prescriptively. If you instead hire an energy modeler to complete the performance approach, you can install whatever windows you like so long as you pass code.

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