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  1. #1
    Join Date
    Apr 2012
    Posts
    19

    Manual J, attic insulation

    Just quick question, when measuring the rooms, windows, etc., to come up with proper sizing for geo, should the contractor be looking at attic insulation also? I had 3 guys over so far and none of them went into the attic or even asked about the amount of attic insulation. That seems like it would be a huge factor in determining heat/loss and tonnage?

  2. #2
    Join Date
    Feb 2009
    Location
    Portland OR
    Posts
    2,001
    Yes that and floor insulation are a factor. While many contractors will measure windows make sure they actually do a proper calculation with the information they take. I have heard and seen many contractors measure windows just to show they o it and then they make up sizing, that's why I always show my manual J but don't give out the data until contract is signed and payment is made.
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  3. #3
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    As an engineer who does load calcs, I have to measure all surfaces that can transfer heat to an unconditioned space. For example, exterior walls, windows, doors, slab perimeter, thermal mass, roof, floor, and even interior walls like between garage and living space.

    A lot of the time having good insulation or good windows can lower the unit by a ton or two.

    Purpose of manual J is so you don't oversize your unit.

    Even if they don't know the insulation valves, the State of California has a list of default values based on averages of when the house is built. Link provided below:

    http://www.energy.ca.gov/2008publica...Appendix_B.pdf


    http://www.energy.ca.gov/2008publica...Appendix_B.pdf

  4. #4
    Join Date
    Jun 2012
    Location
    Virginia
    Posts
    36

    Another point of consideration...

    Dealing with a contractor than not only does the manual J but also understands thermal loading from pressure barriers, as well as, infiltration and exfiltration loss/gains.

  5. #5
    Join Date
    Jun 2012
    Location
    Florida
    Posts
    41
    Quote Originally Posted by gregbig2 View Post
    Just quick question, when measuring the rooms, windows, etc., to come up with proper sizing for geo, should the contractor be looking at attic insulation also? I had 3 guys over so far and none of them went into the attic or even asked about the amount of attic insulation. That seems like it would be a huge factor in determining heat/loss and tonnage?
    They technically should be counting appliances, plants, etc. It can take hours depending on how thorough the calculation is. Numerous studies have shown units are oversized across the United States. I would say the estimate of a 2 ton reduction to be a tad aggressive and not usual although I've seen some significant downsizing occur when true calculations are done. As I've worked with Wrighsoft and other load calculation software, I was surprised to find attic insulation to NOT be as huge a factor as I thought.

  6. #6
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,378
    Only count the appliances that will be used during peak demand conditions. People CAN use common sense and do things like run dryer during off peak times...

  7. #7
    Join Date
    Jun 2012
    Location
    Virginia
    Posts
    36
    There is a certain level of subjectivity in the Manual J that allows for a person to fudge the results to fit what they suspect based on the lifecycle of the unit tonnage they aim to replace. Even John Proctor one of the founding fathers of the long form Manual J admits to the subjective shortcomings of load calcs. For example, I've used Elite and Writesoft for years and when doing loads on split level homes if you don't account for roof on the downstairs as zone 1 and account for roof type and R value in the upstairs zone 2 the resulting calculation does not account for the stack-effect of the the heat gain year round from the downstairs zone. The upstairs cooling/heating load should be affected but Elite and Wrightsoft don't reflect the thermal differences between upstairs and downstairs zones. I've contacted Elite and they recommended adding sensible and latent heat to the upstairs zone to account for the higher coolong load for an upstairs zone. I still believe the Manual J is a great tool for sizing but being aware of the shortcomings has made me less confident that the load calc will be accurate to within a +/- 1000 btu range....

  8. #8
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,378
    You bring up a good point about the upstairs downstairs thing. Around here if the downstairs is covered 100% by the upstairs very little A/C is needed. 1 ton per 1,000sq ft to handle the latent and internal heat generated is all that is typically needed.

    Upstairs may require as much as 1 ton per 350 sq ft since it will be doing most of the work. On the heat side the smallest furnace that you can buy is plenty for upstairs in most cases, but oversizing is generally "forced" to get enough blower capacity. Some builders are running 10KW of heat strips on a large air handler for upstairs and it's plenty of heat for our typically mild winters. The gas furnace downstairs is doing 80% of the work.

  9. #9
    Join Date
    Jun 2012
    Location
    Florida
    Posts
    41
    Good points on the subjectivity of the various softwares and methods. You mentioned confidence, but are you comfortable with 1k worth of b.t.u. either way? Most of the companies I see select based on manufacturer and the majority of time the selection won't change.

  10. #10
    Join Date
    Jun 2012
    Location
    Virginia
    Posts
    36
    Thanks Fellas...."Confidence" may be overstating....I would more realistically call it "reluctant comfort" with the range and even the 1k may be a little lite because in the case of a moist crawlspace nothing in any available load calc software accounts for latent loading from the leaky infiltration of the stack effect caused by pressurizing the conditioned space on a cooling call. You have to use some benchmarks of the Manual J modeling though otherwise we're back to thed school 600 sf/ton or the fingersizing.

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