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Insulation for Unvented Roof Assembly in Hot-Humid Climate

Ashley Hoynowski | Posted in General Questions on

After reading through the forum and GBA proper I am still struggling with my roof assembly a bit and thought I would ask for some advice/clarification.

I am in Canton- Ga, Zone 3A I am doing a full renovation of our little farmhouse in the next year or so I have to replace roof and siding down to the studs (though I plan to make interior improvements after the envelope is taken care of). I have read multiple articles about warmer and mixed-humid climates and I have decided against a ventilated attic due the high humidity here.

I know green builders advocate R-60 for a roof in my zone which I was planning to do with split insulation- 6″ rock wool on the underside between the rafters and make up the rest in exterior rigid mineral wool. However, as I started thinking about it I am wondering if in my situation this is overkill given:

1. I plan to have a metal roof and solar mounted to the standing seams. Does these choices reduce the need for so much insulation to keep the attic cool? Is there a way to calculate how much this would impact things? (If I understand correctly the desire for high insulation in my zone is due to heat coming in from the attic)
2. I feel like I have a pretty low HDD (2815-3397 with base temperature 65-68) and CDD (285-575 with a base temperature of 78-74) however with climate change this could radically shift the CDD number
3. It is expensive and while I am not doing projects to save money I want to be sure that I am not spending money (and effort since this will be a DIY job) just to spend money

However, the roof is arguably the most important part of my energy efficiency plan (except, perhaps the windows) and I don’t want to mess it up or cheap out. I’ve included a modified drawing of my plan below, however it doesn’t include a few details such as exposed soffits/rafter tails and the barrier/underlayment choice since I have not figured that out quite yet.

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Ashley,

    I’m only a few miles from you and so understand the climate. You might want to read Carl Seville’s article, which was just posted on the GBA home page. I think Carl’s recap will give you some insights into what is practical in our climate. For example, a vented attic can be a great option in some cases. Also, an R-30 roof is sometimes good enough if that’s all your assembly or budget will allow.

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