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Community and Q&A

Cost-Effective Wall and Roof Assembly for Hot Climate

Pearl_Matt | Posted in GBA Pro Help on

Hi everyone!

I’m a builder in South Texas in climate zone 2. We are FINALLY building our own personal home and with the price increases it has me picking my brain on how to construct my envelope.

The home will be mostly a single story with a bonus room over the garage, living sq ft around 3000. 60% of the home will be brick veneer and the rest cedar siding over a rain screen.

Now to the envelope…. and insulation

I’m toying around with the combination of 1″ polyiso on the exterior on top of either Zip or OSB. Oddly enough I can get Zip for around $33 a sheet and regular OSB for $31 so thats almost a no-brainer.

For the roof however the prices shoot up for true 1/2″ or 5/8″. We plan to use shingle roofs so I imagine I wouldn’t be able to put exterior insulation on the roof unless I put yet another layer of sheathing which is going to get rather expensive.

The exterior walls will be 2×6 24″ OC. What insulation would you choose if you were building the home yourself? If I can get the envelope rather tight then I was thinking dense pack cellulose. Open cell might be just as cheap though (we use it on all the other homes we build).

We already have the HVAC setup figured out to accommodate the fresh air, ventilation and humidity.

Any help on what you would do to save on cost yet yield a tight and good-but-not-high-performance home please share!

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  1. Pearl_Matt | | #1

    Thanks in advance!

  2. Expert Member
    BILL WICHERS | | #2

    I would probably use dense pack cellulose in the walls, but open cell spray foam might be a good option for you if you can get it done cheaply (“hey subcontractor, score some brownie points foaming the boss’s new house for cheap”). I’m a big fan of exterior rigid foam, so I’d absolutely put up a layer of exterior polyiso.

    You can sometimes use exterior rigid foam on a roof without a double layer of sheathing, but using two layers is more common. If you haven’t already checked, check plywood prices too — plywood has been cheaper than OSB during all these crazy materials price increases. If you’re going to have a vented attic then I’d just use blown cellulose here — blown cellulose is the cheapest way to insulate, and about the best way too for a vented attic.


    1. Pearl_Matt | | #6

      Good info. We’re going to have a concealed attic with rafters insulated. We’ve only ever sprayed foam on the rafters. Would you recommend another insulation here?

      1. Expert Member
        BILL WICHERS | | #7

        If you are insulating the underside of the sheathing (no vent channels), then I would used closed cell spray foam here.


  3. JC72 | | #3

    Assuming a proper application of ZIP-tape for excellent air sealing I would opt for grade 1 install of fiberglass batts as the least expensive option. After that I would go with whatever you feel the installer is really good at because imo the competency of the installer is as important as the choice of insulation (rockwool, cellulose, spray foam, etc).

  4. walta100 | | #4

    I know you said you had the HVAC under control but being in TX that almost certainly means the HVAC is in the attic this is a very bad plan. If you can avoid this mistake it will lower your electric bills 25 to 50% by keeping the ceiling flat and the attic full of cheap R 60 cellulose.


    1. Pearl_Matt | | #5

      Totally true. The attic will be conditioned and the rafters will be insulated. This is how we build all of our houses now.

      1. Expert Member
        PETER Engle | | #8

        But just because that's what you're most familiar with doesn't make it the best option. With 3000 sf of floor space, you should be able to manage a utility closet for the mechanicals. Use a plenum truss to get the ductwork inside the thermal envelope and then put loose cellulose on the attic floor. This is by far the cheapest and most effective way to insulate, and the cellulose has a negative CO2 climate impact. Putting mechanical systems and ductwork in attics is one of the biggest mistakes we've made as an industry in building houses. Insulating the rafters helps, but it is still a band-aid solution for poor design choices.

        If you must insulate the rafters to create a conditioned attic, you could go with dense-packed cellulose filling the cavities, with vapor diffusion ridge venting per new IRC standards. This is an easy, rugged and inexpensive solution. We should all be trying to eliminate spray foam insulation as much as possible due to its global warming effects.

        1. Pearl_Matt | | #9

          I agree with eliminating the use of spray foam as much as possible but I don’t understand the benefits of a vented attic. Why allow heat in above your living space and then try to combat it?

          We don’t have basements so running ductwork only makes sense overhead so why not put in a conditioned attic. Why let all that space go to waste?

  5. walta100 | | #10

    Heat loss is determined by the surface area and R value of the surface.

    When you put the insulation on the rafters you have just doubled the surface area that will lose heat!

    When you put the insulation on the rafters you are all but forced to use expensive spray foam making it very likely someone will skimp on the R value they are willing to pay for. So you are more likely to end up with an R25 roof than an R60.

    As long as there is an R60 blanket between the attic and the living space the attic temp is irrelevant.


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