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Community and Q&A

Insulation Strategy for climate zone 2

FrozenTexan | Posted in General Questions on

I am in southeast Texas and currently building a 16’x32′ workshop with a 10/12 gable roof framed with 2×8 rafters. I’m planning to condition the space with a DIY mini split unit. My question is with regards to insulation. I’ve been searching and have got myself down to two main options but am kind of stuck as to which is more reasonable. 

1) use rockwool in the walls and ceiling, leave the attic vented and call it a day

2) spray foam the walls and roof deck and bring the attic into the conditioned envelope.

With option 1, it’s simpler and cheaper but an unconditioned attic in Texas in the summer will melt just about anything you put up there. And while the space is too small to really be habitable it would be nice to have storage.

Option 2 is more complicated and I have more concerns. In zone 2 I think I would want to use open cell to not put myself at increased risk of roof rot. Would filling the rafters even give me enough insulation to meet code? If not how much thickness do I need to add? Being able to use the space for storage would be nice but if I have to shrink the space down much further its going to lose some usefulness. Is moisture going to become an issue due to being in zone 2? Would I need to add a mini split unit in the attic to deal with the moisture?

An insulated attic sounds nice but I’m wondering if the pay off would be worth the cost.

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  1. Expert Member
    PETER G ENGLE PE | | #1

    Attics are for running wires, not for storage. You'll have a nice amount of shop space, so don't complain about missing the attic storage space. Go with option 1. You'll save enough money to makes the shop bigger if you really need the space. Rockwool or FG are also better for the planet than SPF. Cellulose for the attic if you can get it locally. You can also DIY batt and blown-in insulation.

  2. jameshowison | | #2

    Perhaps consider a relatively new option code approved option, "Conditioned Unvented Attic With Vapor Diffusion Port"

    There are options in there for insulation at the roofline (not spray foam, fibreglass or rockwool) as well as on the attic floor.

  3. exeric | | #3

    I would definitely go with Peter Engle's advice. I will add that if you go that route you should use radiant barrier backed roof sheathing, such as this:

    It makes a real difference in a cooling climate like yours. It might even allow you to still use the unconditioned attic space for storage as it will be much cooler in the attic then. No guarantees though that it will be enough for that. But it will still be worth using it as it will allow you to downsize the mini split size from what it would otherwise need to be for a given level of attic insulation.

    1. exeric | | #5

      Radiant barrier sheathing in most of California, where I live, is part of the prescriptive building code, along with cool roof low IR absorbing shingles. If you have to match a new roof to an existing roof that doesn't have cool roof shingles, then radiant barrier roof sheathing is the method. People get confused about this, especially in cooling climates. There have been so many false claims that radiant barriers have insulation value that it causes people to misunderstand that radiant barrier are very effective at blocking the transmission of heat. They just don't work as insulation. Don't let that fact prejudice you on their use!

  4. walta100 | | #4

    If you do not vent the attic you will need to condition the attic to more or less the temp and humidity as the rest of the house.

    The roof line and gable end walls will need to be insulated they will have almost double the surface area of the attic floor. Even if you managed to insulate the roof and gable end walls to the same level as the attic floor it will lose twice as much energy thru the top of your home day in day out forever.

    In my opinion the only reason to condition an attic is when the owner is unwilling to fix truly silly choices like putting the HVAC equipment in the attic or trying to live in the attic, even then the conditioned attic is only marginally better than the truly stupid choice of a vented attic full of HVAC equipment.

    Is the benefit of the ability to store junk in a conditioned attic really worth the cost of 6X your insulation budget plus adding 10% to your HVAC budget for larger equipment plus the 20% higher operating costs forever?

    Seems to me you could have R60 on the attic floor for half the cost of R38 spray foam maybe less.

    Congratulation you may have the only attic in Texas that is not full of HVAC equipment. LOL


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