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Attic conditioning

deerefan | Posted in General Questions on

My wife and I are building a home near Austin, TX. The ductwork will be placed in the attic of a low pitched roof. The roof covering will be standing seam metal. I would like to ask several clarifying questions regarding this:

1. My thought is to place open cell foam insulation against the roof deck with rigid foam on top to decrease the potential for condensation on the roof deck. Joe Lstiburek’s book suggests that you can also place closed cell foam directly against the roof deck instead of the other 2. This would seem to limit the ability of the deck to dry to the inside and is against other articles I have read. Can someone please explain what is the best design to go with and why?

2. Since the ductwork is in the attic, I read that it is best to condition this space. When I asked the HVAC designer regarding this, I was told that this is typically not done in residential construction. My builder also looked at me very funny. Am I being crazy by wanting the attic space to be conditioned, is the cost of this too high for the benefit given that the attic is already insulated?

3. Where should I put my water lines. Most people here put them under the slab, but given that the attic will be insulated, it would seem practical to put them there and avoid potential for having to break the slab in the future to fix a leak. The slab will form large part of the floor in our house and the look would obviously be destroyed if we had to have a big patch put in it. Any recommendations on this.

Thank you all for your help. I have read a lot on these topics over the past several months, but some of the things that seem to make sense aren’t what people are commonly doing and I appreciate your help with these dilemmas. 

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  1. User avatar
    Deleted | | #1


  2. User avatar
    Walter Ahlgrim | | #2

    Do yourself a favor and make room inside the conditioned space for your HVAC equipment and ductwork.

    Yes I know that everyone in Texas has there ducts in the attic, That fact does not make it a good idea.

    Attic duct work will increase your operating cost by about 30%. All ducts leak no matter how carefully installed. That means the supply ducts are leaking conditioned air outside of the conditioned space, depressurizing your house forcing unconditioned air to find the gaps and leak into your home.

    A conditioned attic is only slightly better than ductwork in an unconditioned attic. In my opinion conditioning an attic is the least bad option for an existing home with HVAC in the attic. By conditioning the attic you have increase the surface of the house by 25% and limited your insulation options.

    Buried Attic ductwork is also risky in some climates, the warm moist air in the attic will want to find cold AC ductwork and condense into liquid water in your attic.

    Take the time to look at the links below

    I say put the pipes wherever if costs the least to install today. A sub slab leak is unlikely with PEX and copper should lasts 40 years even with somewhat acidic water. At that point the pipes will get relocated into the walls and attic.


  3. User avatar GBA Editor
    Martin Holladay | | #3

    Deere Fan,
    I'm not sure why Walta / Walter thinks that you aren't planning to locate your ducts inside your home's conditioned space. Clearly, you are.

    1. In general, when spray foam insulation is installed on the underside of roof sheathing, builders and homeowners don't worry about the fact that the roof sheathing is unable to dry in either direction. As long as the roof sheathing is dry on the day it is installed, the sheathing is generally OK.

    2. The exception to the above rule is when open-cell spray foam is installed on the underside of the roof sheathing, without any rigid foam above the roof sheathing. In that case, moisture problems are possible. For more information on this issue, see "High Humidity in Unvented Conditioned Attics."

    3. If you are building an unvented conditioned attic, it's perfectly OK to put your water pipes up there. For more information, see "Creating a Conditioned Attic."

  4. User avatar
    Jon R | | #4

    > what is the best design

    IMO, the best designs have warm sheathing and can dry in at least one direction. And allow sheathing to be easily replaced if the roof ever leaks.

    You might look at designs that allow overhead conditioned duct space without conditioning the entire attic.

  5. User avatar
    Walter Ahlgrim | | #5

    Martin I am afraid the once Deerefan sees the I do not want to do this price tag for a conditioned attic, most people are going to say OK just do like all the other houses.

    Be sure to consider plenum trusses

    I use to visit a relative’s home in Phoenix AZ at after 40 year the copper pipes under the slab began to leak and were abandoned for new pipes in the attic. Cold water from the kitchen sink became a luxury that requiring wasting lots of water and waiting patiently for it arrival.


  6. deerefan | | #6

    Thank you all for the valuable responses. This is exactly the type of feedback I am looking for.

    Although I have been trying to make the ducted system work for our house, it really seems to be a backwards approach. The house is really not suited to adding soffits because of modern relatively sleek lines, plenum trusses will pose additional difficulties and a conditioned attic is likely quite expensive - out of interest does anyone know the approx additional % energy spending for conditioning an attic (assuming a 12/1.5 roof slope)?

    If you had a choice, would you install a VRF ductless system instead of ducted units in a new construction? This was my initial preference but my wife said no to the wall units - I think she is over this now. This would definitely improve flexibility for the 4 bedrooms, office, and family room temperatures.

    One problem is that we have a large room 30x55'. What are ductless options for this? Could we use a ceiling cassette - this would be less obtrusive in her mind. I have heard these are too loud for bedrooms.

    Thank you again.

  7. User avatar GBA Editor
    Martin Holladay | | #7

    Deere Fan,
    If I were building a new house, I would certainly choose ductless minisplits over attic ductwork. But clearly, your wife gets a vote.

    A ductless minisplit can certainly keep a large room comfortable, as long as (a) the unit is sized to meet the anticipated load, and (b) the house has a good thermal envelope.

    The better a home's thermal envelope, the easier it is to heat and cool, and the more likely residents will be comfortable. A good thermal envelope has insulation R-value that exceeds code minimums; above-average attention to air sealing (with performance confirmed by a blower door test); and high-performance windows.

    In general, it makes sense to spend more money on your thermal envelope details, and less money on ductwork and HVAC equipment.

  8. deerefan | | #8

    Upon further review, ductless may not be the best way to go given the size of the house and number of rooms. Given that the house has mainly linear components, I am considering using plenum trusses instead, but would like to ask whether you think this is worthwhile. The roof is fairly low sloped, the total volume of the attic is approximately 8000 cu', compared to volume of the living space which is approximately 70000 cu'. Will the cooling/ heating difference be significant between the 2 approaches to warrant the extra difficulty of executing the plenum truss option?

    Second question, the total BTU of the ac units as currently designed is 130000. I am not sure if this is a good question but what are the usual requirements for a well performing home per unit area in Central Texas?

    Thank you again.

  9. User avatar
    Jon R | | #9

    It depends on the numbers. But consider that a conditioned attic adds surface area, needs much more expensive insulation and often has higher moisture risk.

    Also consider buried ducts. See:

    Large rooms are fine - it's small, closed door rooms where you are likely to have problems with ductless.

    Don't even ask about load - get a Manual J/S/T/D.

  10. User avatar
    Walter Ahlgrim | | #10

    I like your new plan.
    The only way to know the difference in surface area is to do the math. My guess is a5-12 roof would be over 15%.
    There is no safer bet than a well ventilated attic.

    I like graph in this article on HVAC.


    1. User avatar
      Dana Dorsett | | #13

      >There is no safer bet than a well ventilated attic.

      While generally true in heating dominated climates it's not universal.

      In the Gulf Coast states a ventilated attic usually adds more moisture to the structural wood than it removes, and increases the uplift forces on the roof decking during hurricanes.

      But Austin isn't Houston either, not quite as humid as at the coast, and far enough inland that the worst of the hurricane wind forces are somewhat blunted. Still, a DIFFUSION vented SEALED attic is still probably a better bet than the generic "...well ventilated..." prescription.

      Diffusion venting also reduces some (but clearly not all) of the performance issues of HVAC equipment being in the attic above the insulation- it's still inside the pressure boundary of the house, if not the thermal boundary.

  11. deerefan | | #11

    Thank you for the replies. That brings up an interesting question. The mechanical engineer never asked about window types, insulation values or anything of the sort when picking equipment sizes - at least not to my knowledge. Can equipment be properly selected without this?

  12. User avatar GBA Editor
    Martin Holladay | | #12

    It is impossible to calculate your heating and cooling loads unless you know your window specifications and insulation R-values. If your "engineer" chose equipment without knowing the window U-factors, the window SHGCs, and the insulation R-values, then your "engineer" is a fraud.

  13. User avatar
    Armando Cobo | | #14

    Deere -
    Low pitch attics should not be vented. I recommend metal roof here in TX for this application. I specify 1.5" R9 Polyiso over the roof decking and 8.5" R30 sprayed open cell foam under the roof decking. The reason for the 1.5" polyiso is to match the 2x4 nailer at the perimeter of the roof. I would contact Kidd Roofing.
    I would use the conditioned attic to run my duct work for a single story house or the second floor attic. For the first floor of a two story house, I use open web trusses with dedicated chases for the duct system. The ducts are all metal, wrapped in R6 insulation and sealed with mastic. A conditioned attic needs to be ventilated at a rate of 50cfm per 1000sf of ceiling area minimum.
    Your conditioned attic is a great place to run you water pipes. I would use an on-demand recirculating pump.
    If your builder does not understand high-performance houses in Austin, its time to get a new builder. Call Matt Risinger.

  14. deerefan | | #15


    Thank you for your response. This appears to be against several pieces of advice above, can you explain. Once again, the conditioned attic adds volume to be cooled and heated. Why do this and not put the ducts in a plenum truss? What you suggest is what most around here do, but that doesn't necessarily make it best. I appreciate your time but please elaborate. Thank you.

  15. User avatar
    Armando Cobo | | #16

    deere -
    1. Joe Lstiburek and Peter Yost, who used to work for BSC, have done research for ventilation in low pitched roof for awhile, and found that 3/12 pitch (low-pith definition) is minimum requirement for air to move in ventilated roof assemblies.
    2. ACCA Manual J uses surface area to calculate loads (walls, roof and floor areas).
    3. "Conditioned attic" is CONDITIONED! By code, they must be ventilated.
    4. Sealed and insulated trunk and branch duct system, with the shortest possible branches, is IMO, the most efficient way to go. You get less static pressure and maximum airflow, allowing to install smaller equipment. Higher cost per lineal foot but less ducts to install. Flex ducts are 30-50% more inefficient than metal ducts, plus are rarely installed correctly. No flex ducts for me, EVER!
    5. I don't know your plans, nor your truss lay-out, but I always use open web trusses in those attics without a plenum built into.
    6. Just because builders and trades in TX have "done it this way for 30 years" does not means they've "done it" right for 30 years. We have terrible construction quality in TX, and building officials don't help much. I deal with these issues every day!
    7. I design Zero Energy homes only, and I'm a DOE instructor. My suggestions are for best practices, and I live, work and design homes here in TX, among other states.

  16. deerefan | | #17


    I am confused by your response. You suggest a conditioned attic, but then say it must be ventilated. This seems contradictory.

    Second, are you saying that because my roof is low pitch 12/1.5, it cannot be ventilated and so it has to be conditioned?

  17. User avatar
    Armando Cobo | | #18

    1. A conditioned, sealed, unvented attic must have mechanical ventilation from the HVAC system. A duct in the attic must have a supply vent delivering at least 50cfm per 1000sf of ceiling area.
    2. Yes. I mean no soffit vents or vented channels.

  18. James Howison | | #19

    Two local recommendations to assist with this planning:

    I just worked with New Results to install the new Mitsubishi ceiling mounted one-way mini-splits (and a ducted dehumidifier) in Austin. Definitely not too noisy for bedrooms. There is definitely some sound but the fan is constant, not turning on and off like a central system. There is zero noise from the condensate lift, there is some very quiet "crackling" from coolant moving but we are very happy with the units, even in bedrooms. The outside compressor is very quiet, I've never heard it inside the house and that includes being about 2 feet away through crappy windows.

    Visually the ceiling mounted units are much better than the wall mounted. Although Mitsu could make them better simply by taking their damn logo off them. And they have white leds that can't be turned off (even the "sleep" mode has them on, and that's not even accessible via the KumoCloud app). Nothing that a bit of black electrical tape on the top side of the little access panel can't take care of, but give them user the ability to turn them off via software, come on!).

    Like all mini-splits you have to work hard to place them where a constant (albeit light) stream of air won't be annoying, with the ceiling mounted ones that can mean re-doing the framing if the framing doesn't run the direction you want. That's not really hard, but your HVAC contractor probably won't want to do it. Basically you want them pointing down a hallway, or pointing at the door of a small room. Don't believe the "air skips along the ceiling" bs, even on the ceiling setting you'll feel air standing ~5 feet from the unit.

    But the likely loads in a new house make these expensive, especially since Mitsubishi is very hard to pin down on minimum capacities. I'd say that visually even the ceiling mounted one-ways are large for small bedrooms, they look great in large rooms.

    The ventilating dehu is awesome, especially now when it's cool and humid. The house is comfortable and the ventilation is great, we really notice that cooking smells dissipate more quickly (for what that's worth!).

    So my recommendation would be the ceiling mounted units in the main living areas, with a cooling and dehu/ventilation boost into the bedrooms (via your conditioned attic) using the Ultra Aire SD12

    But give New Results a call and get them involved in your build?


  19. deerefan | | #20

    So, it looks like I ended up where I started from, except with better understanding of the subject.

    To review, because my roof is low pitched (1.5/12), I cannot easily ventilate it well and so I should condition it. Since I am conditioning it, I will plan on running ductwork through it and there is no need for plenum trusses. As far as roof deck insulation, will plan on blowing open cell 8.5" under and 1.5" polyiso over. Can the experts help me confirm this? Thank you.

    1. User avatar
      Dana Dorsett | | #21

      8-8.5" o.c. foam (~R31) on the underside and 1.5" polyiso above the roof deck (~R9) should work great in Austin. It's R40-ish center cavity R beats an IRC code min R38 (zone 2) by R2-R3, but due to the R9 thermal break over the rafters improves it's U-factor by a bit more than the mere center-cavity R might imply. It's cutting the heat transfer of the framing fraction by about half.

  20. User avatar
    Armando Cobo | | #22

    Deere -
    You dont mention your wall assembly, window specs, water heating and distribution, electrical loads/specs, nor expected building tightness goals, which are all required to design a proper HVAC system. When you said your "living space which is approximately 70000 cu'", that tells me is a big house, therefore, finding the right HVAC contractor is of huge importance.
    IF, and only if, your specs mean that you want a high-performing house, your HVAC system should be in the neighborhood of 1 ton of cooling per 1,500+ sf of living space.
    That is just a good guess (starting point) not knowing a whole lot of your house, but it should give you a good point of reference to do a simple math... xxxx heated sf /1500 per ton = AC tons. Typically, here in TX, you'll see 1 ton per 500 sf. If you see that, get an HVAC contractor that knows what is doing.
    I would have thought a good experienced Designer or Architect or Builder would have advised you of all these issues... Hummm?!

  21. deerefan | | #23


    Thank you again. Yes, we are definitely interested in making this a high performing house. No one ever asked these questions , and I paid A LOT for the design and engineering.

    My builder is a great individual with a lot of experience building homes locally. I know he is very flexible and willing to do what will work best for us. My intent was to use 5.5" of open cell foam in the stud cavity and 2" XPS foam outside. Most walls are 2x6 framed, some are concrete mass walls with 2" XPS in the center. The roof insulation will be as you suggested. We will be using thermally broken aluminum windows with high performance glass. We will be using a VRF system, with mostly ducted components and likely a couple areas of ductless cassettes. When I added the tonnage of our heat pumps it is 12, or pretty much exactly ton/500 sq' - FAR from a high performing home.

    So, what should I do now? I appreciate your advice. I really want to make this house as good as it can be.

  22. James Howison | | #24

    First thing I'd do is ask the mechanical engineer for the Manual J report (it's code required, at least in Travis County). Then I'd use to get a second opinion, it's entirely usable for homeowners. One area where things get sticky is that because VRF modulates down, there's a lot of conflicting opinions on how VRF selection should relate to Manual J loads, especially with ductless mini-splits.

    Also strongly consider a ventilating dehumidifier, good discussion at

  23. User avatar
    Armando Cobo | | #25

    Deere -
    IMO, any HVAC contractor that comes to me with a design of 500sf/ton on a system is not worth dealing with; that just tells me they don't have a clue of what they are doing. ACCA Manuals J, S, D, and T need to be done correctly to design a proper system, but they can be manipulated to read anything. That's why is so important to have a knowledgeable team, one you can trust.
    Austin is probably the best town in TX for high-performing houses, and there are many good and credible builders and contractors, but you must do your homework and research who to use. Interview them, ask for recommendations of their clients, check for registrations to green programs at high levels, etc. You could hire an experienced consultant... check with Risinger or Peter Pfeiffer, or they may give you a better suggestion.
    Wall assemblies come in many fine and different ways, and your specs don't appear to be out of line. I would replace the open cell foam cavity for dense pack cellulose for many reasons, but at the end of the day it's all about details and quality of craftsmanship.
    One thing I've found in my 35+ years in the business is that our building industry is not the most educated and honest of all trades, and to make matters worst, most folks spend more time researching a car purchase (a depreciating asset) or a set of golf clubs than they do researching about houses, builders and contractors. It blows my mind every time.

  24. deerefan | | #26

    Do the tonnage requirements change based on whether it is a conventional or VRF system?

    A large open room in our house will have a flat ceiling with about 16" tall trusses, is this enough for insulation and duct work or would it be best to use a ductless ceiling cassette in this area?

  25. User avatar GBA Editor
    Martin Holladay | | #27

    Deere Fan,
    Q. "A large open room in our house will have a flat ceiling with about 16" tall trusses, is this enough for insulation and duct work?"

    A. Maybe. You are in Climate Zone 2, where you need a minimum of R-38 ceiling insulation. That much insulation requires about 10.5 or 11 inches of fluffy insulation -- more is better. Many HVAC ducts are 8" high rectangular ducts, although a 6-inch round duct can serve an individual register. I suppose you could make it work with spray foam.

  26. User avatar
    Armando Cobo | | #28

    16" tall trusses are usually built with perpendicular 2x4 top and bottom cords, which leaves a 9" space in between, at the low-end, if the trusses are built with a 1/4" or 1/2" slope for drainage (that means that it starts at 16" in one end and X" taller on the other end, depending on the slope) or they are parallel trusses which are built to two separate wall heights, but you still have 9" space. Is really not that much more to go with 24” tall trusses with a 24” chase in the middle to run a metal duct system all the way thru.
    An 8” rectangular trunk duct requires 10” min. space to wrap it with R6 insulation, so really, you need taller trusses. This is why you need a correctly performed Manual D, to know the duct sizes so you know the clear space between the top and bottom cord of the trusses.
    Your roof assembly should be a conditioned, non-ventilated attic space, and I would recommend you install rigid foam on top of the roof decking and open cell foam under the roof decking. How much of either insulation, it depend on how big is your duct system and the space needed, but at least 1” over the roof decking and 9” open cell under the roof decking.

  27. deerefan | | #29


    So your recommendation is to go with increasing the truss height to 24" - I will look at how that affects the plan.

    Can you expand on your recommendation of using dense packed cellulose vs open spray for walls? Is there a risk of the material settling over time, leaving a gap on top? From a health perspective - there will be open cell in the roof - so the downside will be there already. Thanks.

  28. User avatar
    Armando Cobo | | #30

    First, you have to know what HVAC and duct system you are using, then you know what space you need in the trusses. I think you are undecided between heat pumps (my choice) and a VRF ductless system, make a decision and the you know which trusses you need.
    Second, dense packed cellulose, if installed at 3.5 lbs/ft³, it should not settle. Cellulose is recycled material, hygroscopic and cost less than open cell foam between studs. Borates in it provide control against mold, fire and insects. Also, Cellulose has the lowest embodied energy of all the popular insulation types.
    I would also add at least 1/2" taped rigid foam on top of the taped wall sheathing, 1" better, to control thermal bridging.

    1. deerefan | | #31


      The plan is to use VRF ducted heat pumps. There are a total of 4, maybe 5 zones. The question is regarding 2 of the areas with lower flat roofs that have 10' tall ceilings. one is a large area which makes me think it may be better to increase the truss height and use a long plenum with short side branches. The second is a small office, which would share an outside VRF unit with a master suite. The master suite is supplied by a small ducted ceiling unit. Thank you again.

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