I routinely recommend R-50 to R-60 attic insulation retrofits, even in hot climates. I heard it again a couple weeks ago. “R-38 attic insulation is all you need in Phoenix.” Or Miami. Or Sacramento. Or Las Vegas.
The context was retrofitting existing homes, in particular, those that have already have 3-4 inches of insulation (not R-38 already). And the premise was that it isn’t cost-effective to raise levels to more than R-38.
That’s silly. Is R-38 a magic number? Or does R-38 represent 12″ thick fiberglass batt? And since you’re blowing in insulation, do you have to hit a number that was determined yesteryear based on the R-value of a batt? It is true that DOE used to have charts that recommended only R-30 in climate zones 1-2. But even those charts have been updated (although they still precisely and miraculously determine that optimal levels match r-values of typical fiberglass batts.)
I asked that question. The answer was that we could round to R-40. But apparently I was missing the point–insulating to R-50 or even R-60 as I had mentioned just isn’t cost effective. It doesn’t make sense.
Or does it?
I’ll answer my own question. Yes, if the attic construction allows it, and you’re upgrading the attic insulation anyway, pushing to R-50 (or even R-60, but let’s take this one step at a time) is very reasonable.
Let me give some of my assumptions. First, it gets hot in attics in the summer–we’re talking about hot climates here after all. The indoor/attic delta-T rivals a cold January day in Burlington, Vermont, albeit with the gradient in the opposite direction. Insulation slows heat transfer. And slowing heat transfer out of a hot attic into cooler living space is a good thing.
Second, if we’re talking adding existing insulation, you’re airsealing the attic plane before you add insulation. (You are air-sealing, right?) This is labor intensive and it represents a big part of the cost of upgrading an attic. Other costs include marketing, sales, and getting the truck and crew out to the address. Yes, you’re onsite already. That being the case, the additional cost (not the price, that’s a separate issue) to add an extra R-10 is small.
Don’t believe me? Do the math. Let’s assume a 1,500 s.f. attic. You’ve already prepped the attic. The truck is in the driveway. The tarps are laid down. You’ve hung plastic in the daughter’s bedroom where the attic hatch is. You’ve air-sealed. And you’re set to add insulation to get up to R-40. How much does it cost to bump that up an extra R-10? We’re looking at incremental costs only because we’ve already covered everything else.
You’ll have to use your own numbers, for insulation cost, coverage, labor costs, lbs/hour of fiber your machines can move, etc., but it will look something like this. If a 25lb of cellulose costs $8 and gives me about 100 s.f. at R-10, I’m going to need 15 bags to cover that 1,500 s.f. attic. That’s $120 of material. Now, a Krendl 2300 is rated to move 3000 pounds of in an hour. I’ve got 25 lbs x 15 = 375 lbs to move. The guys are already crawling around the attic blowing, remember, so all they need to do is stay a bit longer in each position–and not too much longer if you’ve got a good blower. Let’s call it an extra 30 minutes. (Don’t like that? Figure out your number.) Two guys, each at $75/hour (that’s what you pay your insulation techs, right?) for 30 minutes, that’s…$75.
Now I add the materials to the labor and I’m looking at $200. Remember, I’m talking cost, not price. But on an incremental basis it doesn’t cost that much. If you’re focused on cost-effectiveness, make sure you crunch the right numbers for that incremental R-1o. And if your customers care about comfort, resilience (how cool does the house stay in August, or January, when the power goes out?), or even taking the extra step to save a bit more energy just because they want to (they drive a Prius and want to buy solar) be sure to factor that in, too.
Now, the math looks very different if the customer has R-38 already, and you’re not going to otherwise be in the attic doing things like air-sealing. But if you are there, taking the extra step often does make sense. You don’t have to stop at R-38. It will never be cheaper to bump that up to R-50 than while you’re there already pushing insulation through a hose into the attic. So do it now.
Next, let’s talk about adding wall insulation. In Phoenix.