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Manual J

alcoprop | Posted in General Questions on

I got back my Manual J report from my local HVAC supplier. Please critique it and see what I should do. I am not good at reading these yet and not for sure if he did a good job or not. I did notice that he got the insulation value wrong as he put R23 and R12 on the basement walls when it is actually R12 of exterior insulation and R23 interior and he put semi tight and I plan on being 1ach50 or lower. I don’t know if this changes things much or not. Let me know what you guys think and if possible what system I should go with? This is a remodel and the duct work is already in place and would like to keep it. If more info is needed on the duct system I can get that.

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  1. MattJF | | #1

    First thing I notice is the indoor design temps should be 70f not 74f for heating and 75f not 70f for cooling.

    The infiltration is 58cfm and should be more like 15cfm. This is a rough conversion going from ach50 to a heat natural. Ach natural equals ach50/20.

    The ventilation can probably be half of what is listed as well. 133cfm sounds like max cfm on an ERV for a house like this.

  2. MattJF | | #2

    So when I first looked at the manual J I thought it was 3200 conditioned above ground. I am looking again and you are 1600sf above, and about half that conditioned below.

    What is your above ground wall construction/insulation and what are your window U values? I see window u values listed at .49, when they should be more like .3.

    I suspect you should be under 24kbtu/hr, but with more info we can get a better estimate.

    Also are you looking to do ducted heat pumps or a gas furnace?

    1. alcoprop | | #3

      Above wall construction will consist of 2x6 framing, r23 mineral, 7/16 zip, 3” of comfort board for r12, 1x4 furring, siding

      Correct I am about 1600 above ground and about 800 or so finished basement with the remaining underground conditioned crawl.

      Windows are Marvin Signature casement with a u-factor of .28

      I was thinking ducted heat pumps but I am open for ideas. This client does like the idea of being able to add solar in the near future and go zero energy or close to it.

  3. MattJF | | #4

    Here is my quick load calc. 13kbtu/h upstairs.

    I actually haven't learned how to calculate basement loads so I sort of just guessed on those. I need to go read that part of manual J. What are you planning for floor insulation in the basement and crawl spaces? I have the basement at 6.3kbtu/h with my guesses.

    I have code ventilation loads in currently, but you could drop those a bit per this guidance:

    1. alcoprop | | #5

      It is a remodel so basement floor is concrete and staying concrete unfortunately and I highly doubt it is insulated. Crawl space walls and rim joists will be getting closed cell. I don’t know what I am going to do on the basement walls yet. Maybe just continue the closed cell or run mineral batts down there also.

      So any ideas on a system that we should go with?

  4. MattJF | | #6

    For the basement walls, you want rigid foam board right against the wall. Look to see if you can find reclaimed polyiso foam. 3" gets you to above R15 code without the need for fluffy stuff. Check craigslist for distributors. Put a strip of 1 or 1.5" eps foam on the floor under the polyiso to prevent it from soaking up water in the event of water ingress. With thick foam, you can install horizontal strapping to mount drywall. It may make sense to use polyiso board for the crawl spaces if you can find reclaimed foam. Make sure to use HFO blown ccSPF for the rim joists.

    For HVAC, the Fujitsu Slimduct series is excellent.

    At 1 ton unit for upstairs and 3/4 ton unit for the basement would take care of things.

    1. alcoprop | | #7

      Thanks for the info. I am going to make another post asking for System recommendations! Appreciate the help.

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