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Creating conditioned space in the attic of a minimum pitch truss system

Jason L | Posted in Energy Efficiency and Durability on

Wow! I have spent the last 2 hours reading on this site and the depth of knowledge is astounding. Most of what I have found about the subject has been in cold climates, but our concern here is heat not cold.
I will soon be building a home with a mix of 4:12 pitch and 3/8:12 pitch (flat) sections. I want to create conditioned space in the attic and achieve energy star certification. I was planning to use the flash and batt system (1 inch closed cell foam, R38 Batt) on the bottom of the roof deck, but after some reading here, I am a bit concerned. Is this a viable option for a dry hot climate (Southwest Texas), or should I skip the flash and just batt? Additionally, I am concerned about cooking the roofing. Is this a concern, and if so, what should I do about it?
Additional information:
2×4 wall construction with bib insulation.
tyvec house wrap with window wrap
4:12 sections will be roofed with concrete tile
3/8:12 sections will be roofed with a 3 ply hot mop system capped with rolled asphalt roofing
Elastomeric Stucco System
Climate zone 3B

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jason,
    Have you read this article: Creating a Conditioned Attic ?

  2. Expert Member
    Armando Cobo | | #2

    Jason,
    As a residential designer in NM, TX and the Southwest I can tell you it’s fine to do a conditioned attic in southwest TX, but I would skip any batt insulation, it’s never installed correctly. Make sure you get a full R30 min. under the roof sheathing or split between the underneath of the roof sheathing and rigid insulation above it. To get R38 throughout the whole ceiling with any blown-in insulation, you must install raised heeled or energy trusses.
    On the flat roof sections, I would rather use a bituminous membrane that is torch applied. They usually hold better and you can get some light colors to reflect most of the UVs. I would also use foam under the sheathing AND rigid insulation on top to get your R38.
    On the walls, I would recommend 2x6 @ 24” o.c. with blown cellulose, again no batt insulation. Also, you must install 2 layers of WRB, like #15 paper or Tyvek StuccoWrap.
    I should add that the 2009 IECC recommends installing R3 insulation for thermal bridging. It’s a good idea, plus some brands serve as WRB taping all joints.

  3. Aj Builder, Upstate NY Zone 6a | | #3

    John "Texas" Brooks...... This is your baby.

    Armando, great post, I should have mentioned you too knowing the southwest is your area. I like the specs you posted.

  4. Jason L | | #4

    Thank you for your input Armando! After your suggestions, I did a bit more research on rigid foam insulation above the deck as well as the 2009 IECC and Energy Star V3 requirements. I like the idea of rigid above the deck with open cell underneath. I have looked into using SIPs, but they are too expensive. What process do you recommend that will not break the bank?

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