When designing homes in a hot climate, be sure you’re looking at the right loads. Apologies to readers if this is a bit basic, but it was one of the first things that came to mind recently when I was asked about common mistakes made in hot climate homes. So, it might be useful to understand the exterior loads and how our designs account for them.
Lower ΔT = lower R-value requirements
We all have looked at the building code tables, and have seen that prescriptive R-values for insulation are lower in warm climates and higher in cold climates. For example, here are the prescriptive R-values for wood-framed wall assemblies in the current International Residential Code (2018):
- In Climate Zones 1 and 2, the minimum R-value for cavity insulation is R-13.
- In Climate Zones 3, 4, 5, and Marine 4, the minimum requirement for cavity insulation is R-20, or R-13 cavity insulation plus R-5 continuous insulation.
- In Climate Zones 6, 7, and 8, the minimum requirements are R-20 cavity insulation plus R-5 continuous insulation or R-13 cavity insulation plus R-10 continuous insulation.
The different requirements for different climate zones is a function, of course, of the indoor-to-outdoor temperature differences in each area (delta T or ΔT). In buildings, insulation slows heat transfer from one side of the wall (or slab, floor, or roof) to the other. If there’s less temperature difference, insulation is doing “less work” and is therefore less effective.
Let’s put some numbers to this.
We’ll choose Phoenix—one of the hotter spots where large numbers of folks live in the US. ASHRAE design temperature for Pheonix is 110°F— for reference, Las Vegas is 108°F. (Yeah, it’s a dry heat, but so is my oven.) Anyway, the design ΔT is 35°F (110 – 75), for one of the hotter locations we have to design for stateside.
What would that same ΔT look like as a winter design temperature?
70°F indoors – 35°F = an outdoor wintertime design condition of 35°F.
That’s a “nothing” winter design temperature—the kind of design temperatures that you see in Florida.
The takeaway: I wouldn’t say that insulation doesn’t matter in hot climates, but perhaps insulation matters less than in colder climates, or something like that.
Why windows matter more
So what are the loads we’re worried about? Yep, you know it—windows and roofs.
John Straube did a very nice analysis of windows versus walls—one of his very accessible monographs is “Can Highly Glazed Building Façades Be Green?” John compared how windows and walls behave as environmental separators for heat flow. First, for conduction (U-factor), a typical, good residential window at U-0.33 is compared with a relatively high performance wall at R-20 (true R-value, not nominal). For reference, an average 2×6 stud wall is ~R-12 with typical wood framing. So that’s a factor of 6 or 7 difference of heat flow between walls and windows.
The term QC means “heat flow due to conduction.”
Its also important to compare how much solar gain goes through these assemblies. An opaque wall has a solar gain coefficient of about 0.015. So for a SHGC = 0.60 window, that’s a factor of 42 worse performance (greater heat flow) in bright sun. If we went with low solar heat gain and crazy good (R-8) glass, we could reduce this to only a factor of 10 worse performance.
The term QS means “heat flow due to solar gain.”
It’s also interesting to note that solar gain through an opaque wall hit by direct sun (QS = 3.5 Btu/sf·hr) is greater than the conduction through an R-20 wall experiencing a 60 degree ΔT (QC =3 Btu/sf·hr).
So obviously, windows really, really matter in hot, sunny climates. But of course, people design the same glass box high rises all over the world. Ah well…
The other thing that matter for thermal loads are roofs, that giant plane of enclosure area that faces the sky (and often, the sun). And they’re often dark in color, to maximize solar absorption.
That’s a discussion I’ll leave for another time.
-Kohta Ueno is a senior associate at Building Science Corporation.