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Insulating a shed ceiling or cathedral ceiling. What is the proper way?

Post in 'DIY and General non-hearth advice' started by Don2222, May 16, 2012.

  1. Don2222

    Don2222 Minister of Fire

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    I have R4 foil, Faced R19 Fiberglass crisscrossed with R30 unfaced fiberglass for a total of R53 in my ceilings and when I blocked the gable ends with insulated plywood doors I made, the whole house felt warmer on a windy day. That windy cold air that whips into the attic and displaces warm air! Then what happens to the warm air in your house? It has to displace that cold air that came in! My wife could really feel the difference!

    The cold air coming in the soffit everyday is also displacing warm air unless it is enclosed in the rafter vents. You really do not know until you try it!

    I bet in Alaska it would save you some real good money in your heating bill ! ! !

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  2. Don2222

    Don2222 Minister of Fire

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    X2 I agree!
  3. woodgeek

    woodgeek Minister of Fire

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    You, the previous owner of my house and about 10 million folks in the 70s made the same mistake---insulating your house attic without airsealing first. Your observations are correct, stack effect and wind pulls warm air right through that nominal R60 insulation, so you lose heat as if it weren't there. Rather than the ideal solution, pulling up the insulation, airsealing, and putting the ins back down, you have realized a complicated but workable alternative--airsealing the rafter/gable side above the insulation. You did this in a way that ensures ventilation of the decking (good), but still creates a condensation risk in your attic, because your air/vapor barrier (the reflectix) is on the 'cold side' above the R60.

    Of course, if you have no visible condensation under your foil, you're good to go. But your solution is not preferable to the current retrofit best practice of v carefully airsealing the attic floor and then dumping R-60 on top. And when that has been done correctly, sealing the gable and other vents has no effect on the heat loss.
  4. Don2222

    Don2222 Minister of Fire

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    Not Quite.

    I have 2 layers of foil encased with polypropelyne. See > http://www.reflectixinc.com/
    See http://www.homedepot.com/h_d1/N-5yc...053&langId=-1&keyword=reflectix&storeId=10051

    The first layer is under the insulation in the attic floor right against the sheet rock right up tight to the floor joists. Before I put this foil in, I filled all the wire holes with a can of spray foam insulation. This layer keeps the heat in the house in the winter. This is the best air sealing I can do in the old house without pulling up all the sheetrock ceilings.

    The second layer of foil is in the rafters stapled over the rafter vents. This layer keeps the heat out in the summer.

    This has been in there 3 years now and no moisture at all. I can easily lift up the insulation and foil to work on wiring or inspect it! ! ! ! ! :)

    Since it is not a perfect world, shutting the gable vents in the winter will always stop some heat loss! ! !
  5. woodgeek

    woodgeek Minister of Fire

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    Divide the square footage of your attic floor by 60 (R-60) and multiply by a temperature difference, say 60 (for a nice cold night), and you will get the BTU/hr that should be going through that attic insulation. For a typical house, this will be 1500 sqft and 1500 BTU/hr = 440W. Not a lot. Now you close up your gables, and say your rafters are R-5 (reflectix+sheathing), versus R-0 with the wind whipping through there beforehand. You now have R-65 up there, and your loss is 400W. Net energy loss decrease for an airsealed R-60 attic on a 10°F, windy night from closing the gables: 40W.

    Since you are certainly seeing some significant energy savings closing the gables, and get a warmer attic when you do, something doesn't add up. The most likely explanation: the poly laid 'tight' across the drywall is not airsealing, and you are not getting R-60.

    Of course, I'm an idiot and figured I had finished air-sealing my own attic three times (over three years), only to then discover other huge leaks. Net savings from ~25 tubes of caulk and some foam board: equivalent to ~200 gallons oil/yr, and a **cold** attic with the roughly R-25 FG job the previous owner did. And about half the A/C days.

    Not trying to bust you Don, love your posts...but IMO your house attic job is non-standard, and could cause others problems if they tried to replicate it.
    StihlHead likes this.
  6. Don2222

    Don2222 Minister of Fire

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    Hi Woodgeek

    I agree that air sealing is very important. It was 93 Deg F outside in the Sun today and the Attic was boiling hot!

    So I opened up the gable vents and WOW withing seconds it felt 20 Deg F cooler!

    So in the real world those gable vent doors I made with foam insulation on the inside REALLY hold the heat in the attic!
  7. Don2222

    Don2222 Minister of Fire

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    Hey WoodGeek

    How does this look for air sealing the walls of the shed?

    I got J&M R15 Fiberglass covered by Reflectix Foil and taped with Foil Tape!
    Foil Tape is for Class 1 Flexible Ducts, Fiberglass Ducts and air connectors.
    All Weather Performance, Aggressive Tack, Operating Temperatures -25 to Deg F to 325 Deg F ! ! !
    See pic below.

    The foil is stapled to the wall studs but the foil tape is over the staples and the foil joints!

    Like Mike Holmes would say 100% Thermal Break! ! ! !

    Then the paneling will go on top!

    Attached Files:

  8. woodgeek

    woodgeek Minister of Fire

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    I was never too worried about the shed. Hot tub !!
  9. StihlHead

    StihlHead Guest

    Been there, done that. On both air and water leaks. Many tubes of calk later here (three cases I think) and I have the water leaks sealed. Air leaks are another beast altogether though. Drafts from... everywhere! And the condensation forms seemingly from nowhere. Condensation is a big issue, and this is a great thread, really guys. Good points from all sides, and variety of options and opinions.
  10. semipro

    semipro Minister of Fire

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    Don2222 likes this.
  11. Don2222

    Don2222 Minister of Fire

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    Excellent article. It seems that the spray foam is the way the pros go nowadays for cathedral ceilings. That is the expensive way and not a DIY method for sure!

    Here is a quote from the article

    "The code restrictions on the use of air-permeable insulation between rafters were developed to prevent the roof sheathing from rotting. When fiberglass batts are installed in unvented rafter bays, the batts allow moist indoor air to reach the cold roof sheathing. That leads to condensation or moisture accumulation in the sheathing, followed eventually by sheathing rot"

    Now my system of creating a vented channel with the rafter vents and to make these channels sealed with the polypropylene encased foil allows the use of air permeable fiberglass underneath with no condensation moisture and rot! I know my system is not a currently well know or an accepted method but it sure works great and the article explains why! LOL

    My system is a good DIY method so it costs less!

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