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  1. #1
    Join Date
    Sep 2009
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    Arnold mo
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    3,955

    Manual J:Encapsulated foamed attic vs. traditionally vented/insulated

    Could one of the pro's do a quick comparison test with their manual j calcs to answer a question I have? Take any house that you have a manual j done on and leave everything the same except for the attic (with no ducts or air handler in attic) and see what happens to the load when you change from a vented attic with traditional insulation levels of fiberglass or cellulose on the attic floor(whatever the R-level is for your area) to an attic spray foamed at the roof with 3 inches of foam (closed cell and/or open cell). Which method results in a higher cooling load? Remember, no ducts in attic. Thanks to anyone willing to do this.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  2. #2
    Join Date
    May 2009
    Posts
    264
    There is a button on the wrightsoft software called encapsulated. It always reduces the load in Florida heat to eliminate duct gain completely. R-8 flex reduces substantially the load on a conventionally insulated home.

  3. #3
    Join Date
    Jul 2002
    Location
    Yuma, AZ
    Posts
    2,361
    It also depends on the sealing level of the air ducts and equipment.
    Another factor is the climate.

    The Manual J v.7 which I use for retrofits, assumes a 15% duct heat gain for ducts in the attic. That served me well for the past 30 years. I did get bit one time on a flat roof house that I downsized 1/2 ton. The problem was excessive duct leakage and the ducts were not accessible. I had to replace the unit with the original size. (before Aeroseal.)
    "I have never let my schooling interfere with my education."
    Mark Twain
    More at: http://www.quotationspage.com/subjects/education/

  4. #4
    Join Date
    Dec 2010
    Location
    Down by the river
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    1,646
    ummm, he said nothing about ducts, well the op did. there are no ducts nor equipment in attic.
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  5. #5
    Join Date
    Sep 2009
    Location
    Arnold mo
    Posts
    3,955
    Quote Originally Posted by tipsrfine View Post
    Could one of the pro's do a quick comparison test with their manual j calcs to answer a question I have? Take any house that you have a manual j done on and leave everything the same except for the attic (with no ducts or air handler in attic) and see what happens to the load when you change from a vented attic with traditional insulation levels of fiberglass or cellulose on the attic floor(whatever the R-level is for your area) to an attic spray foamed at the roof with 3 inches of foam (closed cell and/or open cell). Which method results in a higher cooling load? Remember, no ducts in attic. Thanks to anyone willing to do this.
    ?
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  6. #6
    Join Date
    Dec 2010
    Location
    Down by the river
    Posts
    1,646
    Quote Originally Posted by tipsrfine View Post
    ?
    that's what I was screaming, you said no ducts or equipment in attic, and have 2 answers that factor duct leakage in the attic. People are quick to answer yet fail to read and comprehend.
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

  7. #7
    Join Date
    Sep 2008
    Location
    New York
    Posts
    131

    attic insulation

    1300 sf attic
    R38 in the ceiling S=12705 btuh L= 2916
    R35 encapsulated S=12172 L= 2916

    Climate zone 6A
    Attached Images Attached Images

  8. #8
    Join Date
    Sep 2009
    Location
    Arnold mo
    Posts
    3,955
    Quote Originally Posted by tipsrfine View Post
    Could one of the pro's do a quick comparison test with their manual j calcs to answer a question I have? Take any house that you have a manual j done on and leave everything the same except for the attic (with no ducts or air handler in attic) and see what happens to the load when you change from a vented attic with traditional insulation levels of fiberglass or cellulose on the attic floor(whatever the R-level is for your area) to an attic spray foamed at the roof with 3 inches of foam (closed cell and/or open cell). Which method results in a higher cooling load? Remember, no ducts in attic. Thanks to anyone willing to do this.
    R-35 with 3 inches of foam sprayed at the roof???????
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  9. #9
    Join Date
    Sep 2008
    Location
    New York
    Posts
    131

    3" R6 foam

    1300 sf attic
    R38 in the ceiling S=12705 btuh L= 2916
    R18 encapsulated S=12637 L= 2916 (3" of R6 foam)

    Climate zone 6A
    Attached Images Attached Images

  10. #10
    Join Date
    Sep 2009
    Location
    Arnold mo
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    3,955
    Quote Originally Posted by tlj000 View Post
    1300 sf attic
    R38 in the ceiling S=12705 btuh L= 2916
    R18 encapsulated S=12637 L= 2916 (3" of R6 foam)

    Climate zone 6A
    Thanks, I appreciate it. It really makes me think I need to buy the software and learn it well. It seems one could use it as a sort of learning tool simply by making small changes such as you did with this, and then investigate WHY the program made the changes to the loads.
    I wonder how the program interpreted "3 inches of foam". Does it consider that the raftors are completely encased in 3 inches also, and not factor in the conductive gains through them? I mean, take all the raftors and put them together and you have a large area that only has an R-1.
    I can see why this tool, like all other tools, is only as good as the person using it. I certainly can't see relying on a calc prepared by someone who has little experience using it. Thanks again.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............
    http://www.mohomeenergyaudits.com

  11. #11
    Join Date
    Sep 2005
    Location
    Atlanta GA area
    Posts
    21,014
    Interesting thread... I have done a few 'foam' homes.

    The homes I did had either 2x10 or 2x12 roof rafters, filled. The difference was amazing!

    One thing to be careful with foam homes: MUA! Is there enough air to allow the fireplace to draft? Is there enough air to allow the kitchen vent to exhaust? Is there enough air to allow the bath fans to exhaust? When one opens or closes a door, will it fight you because the house is too tight?

    We had to use ERV's (one for each system) to make up for an over tight house... worked well.
    GA-HVAC-Tech

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  12. #12
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,331
    Quote Originally Posted by ga-hvac-tech View Post
    Interesting thread... I have done a few 'foam' homes.

    The homes I did had either 2x10 or 2x12 roof rafters, filled. The difference was amazing!

    One thing to be careful with foam homes: MUA! Is there enough air to allow the fireplace to draft? Is there enough air to allow the kitchen vent to exhaust? Is there enough air to allow the bath fans to exhaust? When one opens or closes a door, will it fight you because the house is too tight?

    We had to use ERV's (one for each system) to make up for an over tight house... worked well.
    When dealing with very air tight homes, I would suggest using make-up air ventilation instead of balanced ventilation to give the clothes drier, kitchen hood, and bath fans a chance of getting some air to function. Balanced flow ERV exhaust as much as they supply. This does nothing to help exhaust devices.
    Most occupants do not realize the importance of continuous fresh air ventilation for purging the pollutants and renewing oxygen. They tend not operate the ventilating during the warm months when it is needed the most. Properly ventilated homes will be damp during the shoulder seasons unless you have a whole house dehumidifier to maintain <50%RH during low/no cooling loads.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  13. #13
    Join Date
    Nov 2001
    Location
    East Texas
    Posts
    896
    These foam houses really do not impress me so far. Especially if u need an ERV or even worse a dehumidifier. Seems u just need more equipment to deal with the tightness of the home. Then u got to maintain / repair this additional equipment.

    If I built again dont think I would use foam. JMO

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