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Community and Q&A

Supply Registers in Conditioned Attic

deerefan | Posted in Energy Efficiency and Durability on

Hi all,

I am currently constructing my own home in Zone 2A. It is a 6000 sq’ single story ranch home. The roof slope in 1:12 monopitch. The roof assembly is 3/4″ advantech decking, covered with wip 300 ht membrance, 4″ rockwool cavity rock insulation covered by deck armor, cascadia clip system with rails on which is attached standing seam metal roof.

I understood that with this low slope of a roof, I could not ventilate the attic and thus it is part of the conditioned space. My HVAC ducts run through the attic. My question is: do I need to add a supply register to the attic? My recent review of greenbuilding advisor articles suggested that if the roof is covered with closed cell foam (effectively my situation, given that the deck is covered with wip 300), then I could skip the registers. I want to make sure this is correct, given that I do live in a humid climate and do not want to develop moisture issues in the attic. If I do need registers, does anyone have suggestions regarding the size and number. My HVAC engineer who designed this several years ago was not helpful at all regarding this issue. thank you

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Replies

  1. JC72 | | #1

    What does your local code allow?

  2. Expert Member
    PETER G ENGLE PE | | #2

    Trial and error might be a viable option here if the attic has adequate access. There is no real "right" answer, because moisture buildup in attics seems to be very dependent on local climate, the microclimate within the house, and other HVAC issues. Even the tightness of the ducts in the attic comes into play. I would consider just installing a temperature/humidity monitor in the attic (or a few since that's a really big attic) and see what happens. If the attic is truly airtight, it won't take much conditioned air to keep it dry. And, since you will probably be tapping right off the trunk, even a small register would provide quite a bit of airflow. A single small supply at one end and return at the other could very well do the trick.

    FWIW, 4" of rockwool is less than half of current code requirements for roof R-value in your zone. Even if there is no code enforcement in your area, you should still aim for at least code minimums to control your energy costs and CO2 footprint.

  3. walta100 | | #3

    Rockwool has an R value 3.3 per inch. 3.3x4=13.2. In zone 2 R13 is silly low you need something closer to R60.

    Seems to me you need about 10 inches of reclaimed XPS on top of your Avantech.

    I think large amounts of spray foam in new constructions is a poor way to spend your money. You will get more Rs for fewer dollars if you select any other insulation product.

    Walta

    1. Expert Member
      BILL WICHERS | | #4

      I think you're a little low there Walta, as 3.5" thick mineral wool is usually rated as R15, which gives about R4.3 per inch.

      I do agree that the OP's roof assembly is way underinsulated though. Even in CZ2 you're supposed to have R38, and just about anywhere North of CZ2 needs R49 -- and those are MINIMUMS.

      Bill

  4. deerefan | | #5

    Thank you for the replies. I did not complete the description of the roof assembly. There are also 5.5" Rockwool batts in between the trusses. This results in an effective R value of 32. I think this is not bad, I certainly am very opposed to using foam so that is not an option. If anyone has other suggestions, I would be open to this.

    The house has 3 wings, each one with its own heat pump, main trunk line and branches. What exactly is a "small register". Also, should I put one in each wing or just one for the entire attic? Thank you again.

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  6. Expert Member
    PETER G ENGLE PE | | #7

    Again, trial and error might be the best approach. install humidity monitors in at least one place in all three wings to see whether any dehumidification is necessary. If so, start with the most central wing and see how it affects the others. By "small," I would start with a 10' length of 3" spaghetti duct on the supply and return side. This will allow you to put them right where you want them in addition to giving you some needed back pressure coming straight off the trunk.

    1. vidkidruss | | #8

      Hi Pete, I'm needing to do a similar thing as the original poster but much smaller home 1600 sq ft. and I've got open cell foam on a 5/12 pitch roof. I've got a wifi temp/humidity monitor and I'm getting about 65% humidity and need to drop it about 10%. When you say 3" spaghetti duct, what is that? Just regular flex duct like used for all my other registers? Also, could I start with only the supply and no small return in the attic or do I need to do both?
      Thanks, RH in Austin

  7. matthew25 | | #9

    I am surprised no one has mentioned the suggested air supply rate of 20 CFM per 1000 sq. ft. of conditioned attic and/or crawl spaces.

    https://basc.pnnl.gov/home-improvement-expert/checklists/vented-unvented-attic#:~:text=HVAC%20supply%20and%20return%20air,the%20amount%20of%20living%20space.

    1. vidkidruss | | #10

      Thanks Matthew, I had not seen this article. That helps as it seems I need about 30 cfm of supply and return. The 4" will obviously get that on the supply. Without getting too complicated I'll try the same for the return.

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