What are bibbs? In my new house in northern NY, the builder put in Zip-R sheathing, 2x6 walls with spray foam, and 10.5" rigid foam panels in the roof. It works great (at least in the winter, not so sure about the summer). Triple-pane Marvin windows.

"Fiberglass bibbs is another option which is fiberglass blown in like cellulose."

As David suggests, your house orientation and window size and type is an important consideration. As I frequently post, having your major public spaces--living, dining, kitchen and perhaps MBR--all oriented to the south is the first and ideal step in passible solar design.

Orienting these spaces and their windows to the south allows for the use of roof overhangs, trellis/arbor and similar architectural features to be designed so as to keep direct summer sun out of the house, while permitting winter sun to penetrate when it's desirable to do so. A west orientation is the absolute worst orientation, and should be avoided at all costs.

The issue with using external rigid insulation panels (which have numerous advantages as mentioned) is that substantial thicknesses mandate "sleepers" or furring strips be added to the outer face so that siding materials can be reasonably fastened and not attempt to penetrate 4-6 inches of rigid insulation before contacting solid sheathing. IOW, anything over 1-2 inches of rigid insulation may require extra work and extra cost before exterior siding can be installed.

Closed foam cell foam is a good solution when trying to achieve an high R value in a small space. Insulating the walls of a 100 year old house is a good application. Installing foam requires the highest skill level and is the most expensive.

The first decision is whether the walls will be 2 X 4 or 2 X 6. Once you establish that that you can design your layers of insulation and vapor barriers.

The other decision is whether he attic be conditioned space.

I highly recommend to install the HVAC equipment and duct work within the conditioned space. I personally would make that a high priority if the funds are limited.

Specifier - there is absolutely no reason to use closed cell foam or radiant barrier in the roof if you don't have ducts up in the attic. The attic can be 160 degrees but putting R-60 on the attic floor is easy and cheap. "Most important decision" - crazy talk.

Closed cell is way more expensive than open cell. I have quotes upwards of $15k to spray foam in a roof line.

And BTW - how do you propose a radiant barrier and foam? Radiant barrier requires an air layer to be effective.

Have you considered an adobe house for your local? There is a reason those were built before the days of AC.

If it faces north, then that means your rear elevation faces south? If so that is ideal especially with the windows.

Make your overhangs larger. I live in sunny and hot FL and my overhangs are 32" which shades my windows quite nicely in summer.

If I were in your neck of the woods, I'd go for adobe. A lot cooler naturally.

Yes - foam in the roof brings the attic into conditioned space. Cellulose in the walls and foam in the attic is helpful if you have ducts in the attic.

Air sealing is with caulk, foam, gaskets, etc. Zip wall sheathing is an air barrier - seams are taped and there is a plastic coating on the OSB. Gaskets are used at the junction of framing and foundation.

We foamed where there were large penetrations from wires/plumbing etc. We also foamed behind outlets since that is a break in the drywall air barrier. We foamed under the drywall at the subfloor/drywall barrier. There are lots of ways to be air tight. Some focus on making 1 really good and others focus on a little bit of 2 barriers. We will do both.

I have an architect friend building a house in Sedona. I've seen the plans, but I don't remember the specs on the insulation. I remember the AC load was around 4 tons for 3000 sq ft. I'll ask when I see her this weekend.

Roof mounted a/c? Really?

Ok that seems like a bad idea but perhaps that decision has come and gone. Open cell is a perfectly fine air barrier. Closed cell is a more dense product with a high R value per inch. This doesn't matter in the roof line.

In NC, if we do foam it is generally open cell. The frustrating thing is that they typically shoot for R-30 on the roof line. That is not technically code but they get away with it. When I did the calculations, the roof line was 50% larger than the ceiling plane. So if you just do R-30 then it is comparable to a ceiling plane of R-20 when our code specifies R-38. The foam people like to argue that foam is so much better than fiberglass (or other options) that R value doesn't matter. That just flat out isn't true.

Yes - it air seals. And yes that is important. But R value is R value. So you are much better to seal appropriately - and you can use 1 inch of foam - and then focus on R value. At some point, air does not move and making it not move more doesn't help.

You should know that the 2018 IECC codes allow you to bury ducts in insulation and have a prescriptive way of doing that. What we are doing is R-76 blown fiberglass. All ducts will get buried by R-18 at least - in addition to being R-8 ducts. This actually works pretty well. In a dry climate, this is a no risk thing to do. Doing R-76 vs R-38 is about $500. Ducts in conditioned space is better but if that ship has sailed, then you can do dramatic amounts of fiberglass for cheap. You can also encapsulate the ducts in foam for $1500 or so. All of which are effective and cheaper than foaming the roof line.

Why would roof mounted equipment be less maintenance? I think it would be more since the tech has to climb on the roof each year for the preventative maintenance check.

Save the money on foaming the roof deck, it is not necessary unless you want the extra living space. Build a utility room for the air handler and install the condenser on the ground.

Has a load calculation been performed?

Yep...post all the HVAC gear up on the roof in constant 105-110 degree weather. Great planning...

PS: Check your equipment warranty and your checkbook for operating costs...

Roof-mounting is common in the Phoenix area. Don't ask me why.

Why would roof mounted equipment be less maintenance?

Typically there is more dust and dirt at ground level. Tree clippings, yard debris by lawn crew and so on. I'm sure though you could argue 'What lawn? - it's a desert here?!'

Provided of course you're not near cotton wood trees. I don't know cotton wood trees could survive a climate like Arizona.

In some of these areas they place package units on the roof. You can cut out the higher skill need to install split system as the package unit is prefilled with refrigerant with no refrigerant work all you have to do is hire a crane to place the unit, attach duct work, hook up electrical and gas connection (if needed) and run your condensate run which is really low skill for Arizona climate because it's bone DRY.

Call me crazy but it seems obvious that for a one story house on a slab the HVAC ducts should be in a sealed conditioned attic with the HVAC equipment up there too and the comp/cond on the ground for ease of maintenance and possibly shade. For a trussed roof it makes sense to foam the underside of the roof for ease of installation.

This will cost more but the cost is offset since it eliminates the need to seal roof and ceiling penetrations, insulate ducts (except for condensation prevention) and gets the comp/cond out of direct sun.

And a sealed attic should be advantageous in high winds too.

And it also gets 6 awful looking louvered vents off the facades.

If insulation were to only be in the attic floor, foil-backed roof sheathing would be a cost effective way to lower the temperature of the attic air.

Hi, Marshall,

You'll be well served to consult with local builders and energy raters in your area. They will know what works best in your climate and will have a knowledge of cost and availability of various insulation materials and methods.

I live in Phoenix. I have a roof mounted AC unit on top of my two floor townhouse that I rent out. I just replaced a 20 year old unit that was up there. All the townhouses in the subdivision have them on the roof and there are no issues. Roof mounted AC units are not that unusual here. That one disadvantage is that they are not very pretty to look at, but the condos are so tall that you cannot see them. For my house, I put the units 60 feet away on the ground so I couldnt hear them running.

I believe we are doing spray mounted foam into my addition, but I can ask.

I am with your wife for the large windows, but if possible, make sure you have large overhangs. I live in a midcentury home that has walls of glass, but I have very large overhangs which help a lot.

The roof thing reminds us that lots of things are local. What is funny for people outside of AZ is that putting the unit in the area with the most sun seems like a bad idea. Electronics don't like heat. Heat hurts the efficiency of a/c units.

Shade is preferable but I understand that the efficiency hit is pretty small. Given the shorter lineset run helps efficiency, it may be a wash. It is done in commercial buildings all the time here.

Just imaging bringing a crane in to replace a unit on a house or having techs climb onto a house seems foreign to us. But we also may have more 2 and 3 story structures with higher pitched roofs.

Either way - the location of the HVAC is irrelevant. Local info on insulation maybe helpful vs national advice here. Certainly in areas where foam is more common, it is cheaper. And on average install/construction it is superior to fiberglass. There are just better ways to spend money sealing a structure - IMO.

I thought everyone in Arizona had swamp coolers and not air-conditioning. That's why they are installed on the roof.

To follow up, I spoke to my friend about her build. She called it 'value engineering', in that she saw it as effective but not super pricey.

Inside wall surface will be simple drywall. 2x6 construction with R15(?) fiberglass. Outside covered with 1/2" plywood, then 1" foam sheets, then "that spirally Tyvek stuff", then adobe. Total will be around R25.

Attic will be R54, fiberglass bats, though she said she might start with 1" foam foam against the roof. (Roofing is concrete(?) tiles.)

There is no basement, but the house is off the ground. She said the perimeter will be sealed with spray foam. Garage will be either 1" foam or fiberglass, depending on budget.

For HVAC, they are going with two mini-splits, each with separate 'heads' for each room or area. Unused rooms will have the temps set high which should keep the units off. There will be an ERV.

They are also considering solar, but some time down the road. Five to 10 kilowatts.

It is possible to achieve R25 with 2X6 walls. But you have to use the more expensive fiberglass insulation and rigid foam. I would not trust the builder to do that unless it was in writing.

If the attic floor has R54 insulation, then what would be the purpose of 1 inch of spray foam on the roof deck? I also don't understand the 1 inch foam or fiberglass depending on budget comment. The cheapest fiberglass batt will have a higher R value than 1 inch of foam. I think there is a misconception that foam is always better than fiberglass. That is not always the case.

Is there something about mini splits your friend likes? Two mini splits might be able to keep a small open floor house cool in Arizona. I think a ducted split system would be a better choice and probably less expensive.

I think the one inch of spray foam would be applied to the ceiling/attic floor for an air barrier which can be a great way to go if being careful with recessed lighting and attic access. A well designed small home could probably get by with one minisplit, as long as you keep interior doors open during extreme weather. Ducted is always more expensive and less efficient, but most prefer being able to shut interior doors during hot or cold weather which changes things. Well distributed outdoor air exhange is also tough without ducts.

So the 2x6 fiberglass is likely R-19.

The garage should just be batts filling the framing.

Just some advice for her - if her solar gain is well controlled, she might be pretty oversized. Has a real engineering firm done the Man J?