I just want to reiterate what you said “Spray foam insulation isn’t a miracle product. In fact, it often suffers the same problem for which spray foam contractors bash fiberglass: poor installation. Worse, bad spray foam installers can create more serious problems for homeowners than bad fiberglass installers.”

I never would have thought that bad spray foam installers could create more problems down the road than bad fiberglass installers because you would think that spraying would be an “easier” job.

We actually started our own insulation company and have our website in the works. I will definitely use your articles as a resource for information to let our customers know an “outside perspective.”

You’re welcome! If you intend to go with Performance Path ( CBECC-RES software modeling), be sure to have a HERS rater to run the model early in the process so you can change directions if necessary. For example, due to diminishing returns, you may discover that simply doubling up on ceiling insulation may not be sufficient to get you below the Title 24 whole-house energy budget.

Also, regardless of what the HERS energy model says, if you go with a vented attic, make sure the ceiling is properly air sealed. It’s not just a matter of sealing all the penetrations — the joints between the ceiling drywall and interior wall top plates need to be sealed with spray foam or other suitable sealant such as Knauf Ecoseal Plus.

If you go with prescriptive Option B (high performance vented attic), you must install R-19 on the underside of the roof deck (to dampen duct gains). In my opinion, it would be silly to use SFI in that case. Option B also requires an air gap between shingles and roof deck. That’s easy to do with tiles or metal roofing but I think a second roof deck would be necessary to support shingles. Ouch.

BTW, I’m not sure why you questioned the benefit of a radiant barrier in the context of Option B since it’s not required (see JM table). As you noted, radiant barriers provide no benefit if covered with insulation.

You can ignore the prescriptive requirements and instead demonstrate compliance via CBECC-RES software modeling (i.e., performance path). In that case you can install the shingles directly on the roof deck, omit the R-19 roof insulation and go heavy on the ceiling insulation. I’m not convinced a radiant barrier would be cost effective in your climate, but if you insist, go with TechShield roof sheathing, which has an integral radiant barrier. Separately installed radiant barriers are never cost effective.

All of that being said, it’s always best to to accommodate HVAC inside the thermal envelope (Option C). Depending on the home’s design, it’s usually possible to accomplish that without resorting to an unvented (encapsulated) attic, which adds unnecessary cost. But if that’s the only way to bring the ducts and air handlers “inside”, you can still have a whole house fan. For example, Tamarack manufacturers a line of ducted whole-house fans.

This is new single story construction in CA CZ 4 – a semi-arid/Mediterranean low humidity climate. In July and August, it is not unusual to experience a 40 deg F temperature swing from high to low. Hence the reason for using a whole house fan in a vented attic to draw in cool outside air starting around 8 pm and exhaust it through the attic. In winter, this cycle can be reversed in the morning.

CZ 4 is also ideal for a heat pump, either ducted or ductless or some combination. The refrigerant lines can be run to attic air exchangers, then ducts into the rooms or the lines run straight into ceiling or wall-mounted units. Any ducts would be R8 and meeting the leakage requirement.

Option B is the data that I have been looking at, but I do not understand why a radiant barrier roof sheathing will work with a fiberglass shingle roof. To reflect heat back out of the attic, there needs to be an air gap to break conduction heat transfer. If SFI is tight up against the decking, the heat will conduct through.

The other consideration is cost – there is a significant labor cost to insulate both the roof deck and the ceiling (especially with SFI). I thought that as long as the Title 24 energy CBECC-RES calculations (and perhaps Manual J) passed CA requirements, you could insulate a vented attic without following a prescribed option – which for me would be radiant barrier decking with all of the insulation in the ceiling (R50+).

You wrote: “The discussion above seems to be an either/or choice: either you insulate the roof deck on a unvented/conditioned attic or you insulate the ceiling only for a vented attic.”

That’s not correct. Vented attics in CZ 4, 8-16 must have insulation on the roof deck AND ceiling if ducts are in the attic (or follow performance path). If you don’t live in these CZ’s then I agree with what you wrote. (I’m guessing you live in CZ 4, 8-16 since the prescriptive path requirements you quoted only apply to those climate zones.)

BTW, although unrelated to your question, the JM pdf has an error of omission…
JM’s Prescriptive table omits the following requirement to qualify for ducts in conditioned space (Option C) : duct leakage to the outside must be verified at <15 cfm. Otherwise the JM table is a very good representation of the 'uglier' Figure 3-23 in the 2019 Title 24 Compliance Manual.

(1) Prescriptive Path –Vented Attic : R19 min below roof deck. R38 min ceiling.
(2) Performance Path – Unvented Attic: R22 min below roof deck.

The discussion above seems to be an either/or choice: either you insulate the roof deck on a unvented/conditioned attic or you insulate the ceiling only for a vented attic.

For a vented attic (I am considering using a whole house fan in our area), I would be interested to know your thoughts on insulating both the ceiling and the roof deck vs. a high R-value (e.g. R50+) ceiling only (assuming the house passes the Title 24 calculations). One other consideration – my understanding is that radiant barrier roof sheathing will not work either with either spray foam roof deck insulation or fiberglass since the insulation provides a conductive heat path into the attic.

@LeeH, yes, just a single 1-ton system for the entire house. It’s a Carrier 18VS 5-stage heat pump. To answer your other question, the house tested out at 0.52 ACH50.

BACKGROUND: Professionally I design mechanical systems and envelope specs for high performance homes, so 2000 to 2500 ft2 per ton are routine for my projects. But 3300 ft2/ton was a record even for me. I designed this house specifically to work with the 1-ton 18VS — it’s the only non-mini-split inverter based heat pump available under 2 tons.

Although my load calc modeling and all of my experience said this would work, admittedly I was a bit nervous. But I had a fall-back in case it didn’t work: I can cut the roof load in half by applying a high reflectivity coating to the flat roof. Fortunately that won’t be necessary (it would cost ~$2k). The first summer we were in the house, it hit 104 degrees two days in a row. The main level slipped 1 degree above the 77F setpoint, pretty much what I expected.

We live in rural SE Arizona at 4400 MSL. Because of the higher elevation, we’re not as hot as Tucson but we typically see 100 several times each year. As it turns out, the system will maintain 77 degrees at 100 outside. Since I let our bedroom zone float during the day, it takes 2 to 3 hours in the evening to reach our preferred nighttime setpoint of 73. In winter, the heat pump alone can hold 69 or 70 when the ODT drops into the mid-20’s. I installed a 3kW strip kit as it occasionally drops into the low 20’s and teens.

David, just for clarification you have a 1 ton unit serving your entire 3300sqft or 4-1 ton units? Even the latter is commendable… Also did you have a blower test done?