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  1. #1
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    Sep 2008
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    Default most important place to insulate

    I know that roof/ceiling insulation is the most important area to get the most bang for your insulation buck - but not sure if it applies here.

    Client has a finished bonus room over an unheated garage. Room has average insulation in ceiling (R-19 batts?) but no insulation in floor. The room is tight, but is super cold because it's over the garage. It's heated by a single zone baseboard heater so it's not heated continuously in winter.

    Because of budget client won't go for new sheetrock top and bottom. room is gable roof with kneewall storage alonge the sides. Wonder if putting R-38 in floor will do more for occupants than upgrading the 20 year old insulation in cathedral ceiling (with crawlspace, no-access vented attic above).

  2. #2
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    Default Re: most important place to insulate

    Quote Originally Posted by concisedesign View Post
    Wonder if putting R-38 in floor will do more for occupants than upgrading the 20 year old insulation in cathedral ceiling.
    Fill out your profile so we know where you live and work. Climate makes a difference.

    If the garage ceiling is accessible, and there's room for R-38, it would certainly be more cost-effective to insulate the floor. And, while you're doing it, you can finish the garage ceiling with 5/8" type X drywall to create the code-required fire-rating.

  3. #3
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    Default Re: most important place to insulate

    What is the floor finish in the bonus room? If it is going to be carpeted consider installing homasote carpet backer board over subfloor before carpet.If it is a hard surface floor then good luck!

    If you are going to remove existing drywall on garage ceiling make sure you foam or caulk all holes in subfloor above. Perhaps batt insulation with a layer of 1" iso board installed on bottom of floor joists before drywall would provide max. value.

    Also check behind knee walls to determine quality of the insulation there and add at least an air barrier if there is nothing to back up the batts. Upgrade access doors to storage areas or add foam insulation to back side of door and weatherstrip stops and sill.

    Last, if the room is at the furthest point from the boiler you may have to add supplemental conditioning, insulation only retains heat, it doesn't make it.
    "ALS IK KAN" - Stickley

  4. #4
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    Default Re: most important place to insulate

    The way it was explained to me, we put R-13 or 19 in walls because that's all that will fit, and we put R-38+ in attics (as much as R-60) because it's easy and cheap to keep adding it. But it's not that more flows through the ceiling, just that it's easier to insulate.
    Those garage rooms are often a disaster. If the insulation is fiberglass batts, you'll typically find:
    --floor batts have fallen down away from the subfloor, leaving a space for cold wind to blow all the way under the floor
    --knee wall batts are open to the cold, and convective currents reduce their effectiveness to <R-1 according to infrared thermography--these infrared photos are really scary
    --cathedral ceiling sections are underinsulated and the batts are often not tight against the drywall, permitting constant flow of cold air against the drywall, major windwashing through the batts too

    Add in the huge surface area and distance from the heating plant and they are often cold.

    To solve the insulation issues:
    --floor, I like dense-packed cellulose, solves both air flow and underinsulation issues. Can be hard to find a good dense-packer; hint, let them know you want 3.5lbs/cu ft and will calculate the bags needed along with them, that should scare off the cheaters. Alternately, block off the joist bays right below the knee walls with something air tight like foam board & great stuff, or plastic bags with fiberglass batts inside, but if the insulation has fallen down from the floor this only helps so much.
    --Knee walls, cover the back of the wall with something relatively air tight like foam board, therm-a-cardboard, or supposedly tyvek though I have trouble imagining the tyvek sealing as well as the foam board.
    --cathedral areas, consider dense-packing them but be sure you are cool without the ventilation. You can also reduce wind-washing by blocking the bottom end of the rafter bays either all the way or part of the way (depending on how you feel about ventilation).

    For the heating, some kind of zoning where that space calls for heat separately from the rest of the house may be the only way to ensure comfort.

    Good luck

  5. #5
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    Default Re: most important place to insulate

    Quote Originally Posted by ThingOfBeauty View Post
    But it's not that more flows through the ceiling, just that it's easier to insulate.
    Heat, by conduction and radiation, flows equally in all directions. But hot air rises if there is any infiltration of cold air or air cooling at windows, and this convective heat movement can significantly raise the temperature of the ceiling and hence cause more heat loss.

    --Knee walls, cover the back of the wall with something relatively air tight like foam board, therm-a-cardboard, or supposedly tyvek though I have trouble imagining the tyvek sealing as well as the foam board.
    Tyvek was invented to prevent air (and water) infiltration and to be highly permeable to water vapor. Putting XPS or Polyiso foam board on the back of the knee walls is creating a wrong-side vapor retarder. (Don't bother telling me that some manufacturers rate their XPS at 1.1 perms per inch and so it is not a vapor retarder - 1.1 is not significantly different from 1.0, a permeable material has a perm rating of 10 or more).

  6. #6
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    Default Re: most important place to insulate

    Right you are Riversong, but in our climate we put OSB with perm rate <1 on the cold side of leaky 2x4 walls all day long with no moisture problems. So, take a 2x4 wall that's supposed to have R-11 insulation but actually performs <R-3 due to convection through the fiberglass due to lack of cover on outside face of wall. Add 1" XPS at R-5.5.
    Now you have 75% poorly installed fiberglass with 25% R-3 wood framing, total R-value about 6ish, covered by R-5.5 semi-permeable foam. I know the CMHC guys don't want a vapor retarder more than 1/3 of the way from warm to cold, but we aren't in Canada here, in fact our vapor drive reverses quite noticeably every summer, so 50/50 might be closer to ideal around here, meaning R-6 in and R-5.5 out might be just about perfect.
    I'm cool with it--in my climate--anyway don't see how it's worse than OSB which seems to work just great.

    Now foil faced polyiso like R-Max is sketchier, being less permeable and (typically) thinner so lower R-value, believe 1/2" is about R-3. But before I found that out I put a bunch in, in fact probably once a year we take apart a house entirely sheathed with 1/2" R-Max, don't see any problems yet. Might not be a good plan if your name's on it, definitely think twice if you're in a colder climate than 5000 heating degree days!

  7. #7
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    Default Re: most important place to insulate

    Quote Originally Posted by Riversong View Post
    Tyvek was invented to prevent air (and water) infiltration
    Tyvek doesnt prevent air infiltration.
    The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt. ~Bertrand Russell

    wausaubuilder.com

  8. #8
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    Default Re: most important place to insulate

    Quote Originally Posted by greentree View Post
    Tyvek doesnt prevent air infiltration.
    That is one of the primary functions of all weather-resistant barriers (housewraps), and it is why all seams must be taped. The second is to prevent or minimize water penetration. The Third is to allow water vapor to escape by diffusion.

    The air infiltration rate of Tyvek is 0.007 cfm/ft²@75 Pa, for Typar it's 0.005 cfm/ft²@75 Pa. In other words, virtually nil.

  9. #9
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    Default Re: most important place to insulate

    Quote Originally Posted by Riversong View Post
    That is one of the primary functions of all weather-resistant barriers (housewraps), and it is why all seams must be taped.
    good advice for OSB users

  10. #10
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    Default Re: most important place to insulate

    Quote Originally Posted by Riversong View Post
    That is one of the primary functions of all weather-resistant barriers (housewraps), and it is why all seams must be taped. The second is to prevent or minimize water penetration. The Third is to allow water vapor to escape by diffusion.

    The air infiltration rate of Tyvek is 0.007 cfm/ft²@75 Pa, for Typar it's 0.005 cfm/ft²@75 Pa. In other words, virtually nil.
    I thought only vertical seams were supposed to be taped.
    http://www.lavrans.com

    "He uses statistics as a drunken man uses lamp posts; for support rather than illumination." -Andrew Lang

  11. #11
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    Default Re: most important place to insulate

    Quote Originally Posted by ThingOfBeauty View Post
    Right you are Riversong, but in our climate we put OSB with perm rate <1 on the cold side of leaky 2x4 walls all day long with no moisture problems.
    CDX and OSB have perm ratings of 0.75 when dry, but when they get damp it increases to 3.5 for CDX and 2 for OSB. This is a great safety factor - as the RH increases behind the wooden sheathings, their permeability increases and the assembly can dry to the exterior (in winter). Rigid foam board, with the exception of EPS (Perm = 3.5), will not do this.

  12. #12
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    Default Re: most important place to insulate

    Quote Originally Posted by Lavrans View Post
    I thought only vertical seams were supposed to be taped.
    Not if you want it to function as an air barrier. If you're using 9' rolls, then any potential benefit of the "shingling" pattern is virtually nil (which is why 3' wide rosin paper or 15# felt drains better).

    A vapor barrier that is 95% intact will be 95% effective at preventing vapor diffusion (which is why Kraft paper on fiberglass, even when interrupted at each stud, is considered an effective vapor retarder). But an air barrier that is 95% intact might be less than 50% effective at preventing air infiltration with a high pressure gradient.

    If you're using plastic housewraps, then all seams should be taped and gables (even with unconditioned attics) must be covered (or wind-driven rain will get behind the cladding and then run down behind the WRB).

    It's also important to make sure the WRB is tight to the sheathing. Any air pockets between sheathing and WRB (as is possible with lap siding) can create condensation behind the WRB, which cannot pass through and will dampen the sheathing.

    This is, perhaps, one more reason for using vertical battens and a rainscreen, as the battens will help keep the WRB tight to the sheathing.
    Last edited by Riversong; 11-29-2008 at 09:44 PM.

  13. #13
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    Oct 2008
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    Default Re: most important place to insulate

    To the OP:

    Foam it.

    Use spray foam to insulate all accessible areas (floor joists, knee walls, etc.). In my opinion, there simply is not a better way to create a tight, well insulated space.

    You can use batts, but if you are charging them for your labor, it will be just as expensive by the time you install blocking, backing, caulking, and draftsealing. A competent foam contractor takes care of all the steps with the foam gun.

    Be sure that the kneewall doors are weatherstripped and tight.

    You know why it's called a Bonus Room?

    'Cause you get bonus heat loss, bonus expense, and bonus headache!

    After the room is foamed you will see an improvement in the conditioning. If it is still not good enough, you could consider a ceiling upgrade. Maybe next winter the homeowners' budget will allow it. Leave the existing ceiling in place, and install foamboard covered with a new layer of wallboard. Just be careful with screw length and placement to ensure adequate attachment to the framing.

    There may be vapor issues associated with this method, depending on your location. So be sure to consult an expert in your area first.

    Matt Clark

  14. #14
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    Default Re: most important place to insulate

    Quote Originally Posted by greentree View Post
    Tyvek doesnt prevent air infiltration.
    Greentree, what is your experience with Tyvek that leads you to believe it does not perform the function of an air barrier??
    "ALS IK KAN" - Stickley

  15. #15
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    Default Re: most important place to insulate

    Consisedesign, Although you did not mention what the lighting source in the room is, it does make sense to cover that issue as well. If you have can lights and only 6" batts in the ceiling, chances are there is no insulation over the lights. Check them out for insulation contact and replace if required to allow for a tighter envelope in the ceiling plane.

    Maybe a visit from an energy auditor utilizing infrared would be a good idea in this case since you cannot really attest to the quality of insulation in the ceiling and gable walls!
    "ALS IK KAN" - Stickley

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