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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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    Hello

    I read on the web, that a guy had soffit vents and ridge vent in his shed. He put the foam rafter vents from the soffit to the ridge vent and then put insulaion on top of that. After a while in the cold weather the insulation was soaking wet!
    However I am not sure if I am reading this correctly?
    From >http://boards.hgtvremodels.com/eve/forums/a/tpc/f/9001020091/m/4521068913
    question
    "
    3. I was going to use that thin rigid foam stuff in between the roof trusses that allows air to pass from soffit to ridge, then cover it with 3-1/2 insulation. As far as aesthetics go, I don't mind looking at paper backed insulation in between the roof trusses, but I've heard that this can be a fire hazard and should be covered. What is suggested?

    As soon as I regain access to my pic hosting site, I'll post some pics if that will help."
    Answer
    "I've seen installations where those foam channels were installed all the way from soffit to ridge vent, and the insulation was soaking wet,I believe because they acted as a partial vapor barrier. I don't believe they were ever intended for that use, but only where insulation would be "pinched" at the roof/wall intersection, for maybe the lower two feet of the roof. You need to allow a minimum of 1 1/2 inch continuous clear air space above the roof insulation, and I would caution against the use of the foam channels for the entire distance."
    So how do you properly insulate a shed ceiling or a cathedral ceiling in a house?
    I thought if the Reflectix foil is stapled over the rafter vents and then the fiberglass insulation is put under that, then there will be a nice vapor barrier to prevent the insulation from getting wet?

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  2. StihlHead

    StihlHead Guest

    Good question. I pulled the sheetrock from the ceiling of my garage (remodeling it) and on hot days the insulation has a coating of water on the underside of the paper from condensation. Condensation seems to be a real problem. Roof slope is 2:12, 2x6 rafters 24" o.c., R19 paper backed fiberglass insulation running parallel to the rafters, no venting, gable metal roof on perlins over plywood. The apex of the ceiling has the most condensation, but only on hot days. I would think that is where you need venting. On cold days there is no condensation. Has me puzzled... I would think that after cold days turning hot, the condensation would form on cold surfaces, like the concrete does here after a period of frost. But the condensate forms where the apex of the roof is at the hottest point. And that is where they typically put attic vents on houses.

    I am converting my garage to an accessory building under 10 feet tall to make it local code legal. I am converting the gable roof to a 1:12 shed roof, 2x6 rafters 24" o.c. Question is, how do you insulate it... with R-13 leavng a space at the top and vent the soffits? Or keep the full depth R-19 insulation, covering the ceiling with sheetrock, not venting the soffits and sealing up the insulated space too keep out the moisture?

    HVAC stuff baffles me...
  3. Don2222

    Don2222 Minister of Fire

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    Hello
    Well nice to know I am not alone on this! So here is what I did and I will let you know if it works?

    I put in a strip soffit vent and ridge vent on the peak of the 2x6 roof 16" on center with LVL support beam inside. Then I ran the plastic rafter vents from the soffit all the way up to the peak and stapled them to the roof plywood
    At Home Depot I purchased Reflectix foil that has 2 layers of foil encased in a Polypropylene plastic to stop radiant heat from coming into the shed and hitting the insulation in the summer time.
    See > http://www.homedepot.com/h_d1/N-5yc...0053&langId=-1&keyword=refectix&storeId=10051
    Then I added paper backed fiberglass insulation over the foil to insulate the shed in the winter time.
    In the gable areas I added foam board encased in foil. The walls have R-15 Faced Fiberglass with reflectix foil on the inside stapled over the fiberglass.

    So I will keep an eye on the insulation and let you know if there is any moisture on hot days.
    With the foil and air space for venting I believe it should be fine. What do you think?


    Pic 1 - Ridge Vent on roof with architectural shingles with Grace Ice and Water Shield on entire roof!
    Pic 2 - Strip Soffit Vent
    Pic 3 - Plastic Rafter Vents from Soffit to Peak and Reflectix Foil under rafter vent and paneling on wall
    Pic 4 - Paper faced fiberglass insulation over the foil which is over the rafter vents
    Pic 5 - Roof insulation and 1" R6.5 foam board with foil on both sides on the inside gable area triangle

    Attached Files:

  4. woodgeek

    woodgeek Minister of Fire

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    Answer: One combined vapor and air barrier with finite vapor permeability materials on both sides. To keep this plane temperature above the indoor and outdoor dew points it should be on the warm side of the insulation. In a mixed climate, you may need insulation on both sides of the barrier to achieve this , and even then you will have periods of condensation during certain brief weather conditions. If the airseal is complete, however, the volume of this condensation will be very small, and it will dissipate quickly when the weather conditions change (because of the finite permeability of the materials on both sides). Period.

    This is why Tyvek is vapor permeable by design.

    The real problems we hear about occur when failure to airseal causes massive airflows of conditioned air through framing cavities. Then a lot of water can get dumped in the cavity and soak insulation. Careless attempts to fix the problem by adding still more vapor barriers, but still failing to ariseal effectively, will just make the problem worse.

    For a heating climate, high indoor humidity is always going to lead to problems, esp in old homes that have been 'retrofit airsealed' (like mine). While I would be more comfortable at higher humidity, I try to never go above 35% or so in the winter for fear of dumping water in my walls. Before the retrofit airseal, it was effectively impossible to reach such RH levels, but after airseal it is quite easy.

    So Don, you have central A/C in that shed too??
  5. StihlHead

    StihlHead Guest

    Your 'shed' is better built far than my house is. Actually better than my last stick built house too.
  6. Don2222

    Don2222 Minister of Fire

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    Hi Woodgeek

    Hopefully all the foil I am adding will severely cut down the radiant summer heat so I will see. This will be it's first summer!
    If not building a window air conditioner into the wall would be easy! So we will see!
  7. Don2222

    Don2222 Minister of Fire

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  8. nate379

    nate379 Guest

    The key is to have very good venting in the attic space. You want the temp/humidity to be equal to outside.

    Also vapor barrier on the warm side is needed too (or cold side for you guys that have A/C, it's not common up here). Something like a 6mil plastic is commonly used here. Tuck tape any seams or holes.
  9. Don2222

    Don2222 Minister of Fire

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    So Nate

    Are you saying when I look up at the ceiling on the inside, I should have 6mil plastic over the faced fiberglass insulation which is over the reflectix foil which is over the plastic rafter vents going from the soffit to the ridge vent? Is that overkill?
  10. nate379

    nate379 Guest

    No the plastic would be the closest thing to you, in between the OSB/drywall (whatever your inside walls are covered with) and the insulation.

  11. semipro

    semipro Minister of Fire

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  12. woodgeek

    woodgeek Minister of Fire

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    I'm sure you're good in a (leaky) shed situation, but you do have two vapor barriers (paper and foil) and no real air barrier (cuz neither is taped nor caulked).

    Use of poly as a vapor barrier under drywall is not recommended except in the northern-most tier of the lower 48 and AK.
  13. nate379

    nate379 Guest

    Never heard that one before, but I have lived in northern states/canada almost all my life. The couple times I ventured to the south it was just to vacation.

  14. Don2222

    Don2222 Minister of Fire

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    Interesting article but developed primarily for that un-vented roof system used in warmer climates.
    Nothing like that around here!

    Our licensed roofers are required to install vented roof systems to prevent condensation.

    It is just so ludacris for these licensed roofers to put those soffit vents and ridge vents to make the shingles last longer by sucking the heat out of our homes like a vacuum! ! Well they must get extra money from the oil and propane guys because they can sell alot more oil ad propane for heat this way!

    My system continues the rafter vents to the ridge vent and applies foil with plastic under that on the inside of the roof stapled over the rafter vents. This allows the soffit air in under the roof to prevent moisture build up but gives a channel for this cold air to run thru so it will NOT suck the living heat our of our homes!
  15. Don2222

    Don2222 Minister of Fire

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    I would not say the shed is leaky at all! The plastic rafter vents have a stapled flange. The foil is also tight up between the roof rafters and even the paper faced fiberglass is stapled to the side of the rafters. Where is it leaky or not air sealed?
  16. semipro

    semipro Minister of Fire

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    My understanding is that unvented roofs can work in any climate. In fact one of their main advantages is prevention of ice damming, an obvious problem in climates like yours. The Building Science and Green Building Advisor sites both have great articles on the subject.

    I'm not sure about local codes or practices but this issue is quickly evolving, roofing manufacturers are changing their requirements, and inspectors are coming around.
  17. nate379

    nate379 Guest

    An unvented roof is often the CAUSE of ice damning. That and poor insulation allowing the attic to get warm. If the attic is same temp as outside, that means the roof is as well and the snow isn't melted. YES the sunlight will at some point heat it up and then it melts, but that's a bit different.

    It's when it's -30 and you see huge icicles hanging off a person roof, you can tell something isn't right.

    Also if the roof is not vented, it will get VERY hot in the attic in the summer. This will cause quicker failure of the shingles and for sure increase the load on the cooling system in the house. I have been in attics with temps pushing close to 200* when it was about 100* outside.

    Even my very well vented attic on my house will get 85-90* when it's 70ish and bright sun.

    I'm just a "jack of all trades" but I can't see any advatage to a non vented roof.

  18. StihlHead

    StihlHead Guest

    I came up with a vent system on my house in CA where it never snowed that worked great. I bought 3 whirly vents and put one above the garage attic near the peak, and two above the gabled roof near the peak of the one story house. I covered them in winter with taped garbage bags, and in summer I removed the bags. Those kept the house at least 10 degrees cooler in summer, and the attic about 20 degrees cooler. W/o venting that atttic was an oven (or rather with the original small vents on either side of the house, one of which had a thermostat activated fan on it). That house had flat layed fiberglass insulation above the house ceiling.

    My house here (where it snows a lot) has what we called a tropical roof on the old Land Rovers used in North Africa. A Landie tropo roof has a solid metal layer a half inch above the roof and there was an air layer between them. Poor man's a/c, but it worked. Here I have a metal roof an inch above he original composition shingle roof on perlins. The air layer in between works well to keep the attic a lot cooler, though radiant heat does get through on sunny hot days. The insulation here is the same as my last house, flat layed fiberglass. I have not had any ice dam problems here, but the metal roof does not have shingles to back fill with ice and melt into teh attic.
  19. Don2222

    Don2222 Minister of Fire

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    Hello

    I agree, venting is needed because without venting, the adhesives used in the roof plywood can deteriorate or Dry Rot can occur because of condensation.

    Anyway, the temperature outside the shed this morning went from 43 Deg F to 71 Deg F and the temperature inside the shed remained constant at 58 Deg F. So I think that means the insulation is holding things steady. Still I am not finished but almost there!
  20. woodgeek

    woodgeek Minister of Fire

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    Airsealed means a continuous film, either taped or caulked on all seams. Stapled or tight does not equal sealed. I'm not being pedantic, when we talk about vapor migration by diffusion through a material like plywood or tyvek, it assumes a continuous seal. In your case the amount of water vapor moving across your materials will be dominated by that carried by the air through those tight or stapled places, rather than diffusion through the Kraft or foil or whatever. The latter can be estimated from material specs (i.e. perms) and designed/evaluated for condensation risk. What you have can't be because it will matter whether your Kraft puckers out by a mm or 0.1 mm (a factor of 10x difference).

    Of course, its a shed and you want some passive climate control....you're fine re condensation risk....until you install the hottub in there.
    StihlHead likes this.
  21. semipro

    semipro Minister of Fire

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    I think you and Don misunderstand.
    Properly insulated, unvented or "cold" roofs prevent ice damming because the heat from the house near reaches the snow on top which is what causes ice dams. Condensation does not occur because the moist air coming past the drywall (or whatever) does not encounter a surface cold enough for condensation to occur.
    Also, roofing material manufacturers have backed off on their requirements for vented roof decks because they've found that temps aren't as hot as they thought and degradation does not occur.

    By your comments I suspect that you didn't read any at the other links I provided. At least read this one:
    http://www.buildingscience.com/documents/reports/rr-0404-roof-design
  22. Don2222

    Don2222 Minister of Fire

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    I did read it and I do understand what the document is saying!

    From the article and the diagram below.
    "•Rigid insulation installed above roof deck
    •Ratio of R-value between rigid insulation and batt insulation is
    climate-dependent"

    The theory behind the document is that Rigid Foam Board placed on top of the plywood roof with shingles on top will keep the cold from the air inside the attic.

    What I said was that Roofers in this New England area just do not do that! I have also not seen how this stands up over the years and how well it works! The foam board will not hold the shingles on the roof when nailing so thick foam board seems to be out of the question?

    Still there is not air circulation!

    I still do very much disagree that the Attic should be a COLD zone! That sucks heat out of the house in the winter time like crazy!

    The cold zone should be contained in a channel directly under the roof and the gable vents should have insulated doors to be closed up in the winter! I have been doing this for 3 years now and saved a bundle on home heting costs!

    Attached Files:

  23. semipro

    semipro Minister of Fire

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    There is 3/8" sheathing attached above the foam and to the original roof deck as a nailing surface for the shingles.
  24. Hogwildz

    Hogwildz Minister of Fire

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    I don't give a chit what that diagram says. If you put batt insulation against the bottom of the roof deck, the deck will rot and turn moldy on the bottom side. Foam or no foam, the decking will rot.
    And ice damns are not exclusively due to warm air leaks in the attic. Melting surface snow refreezing also causes ice dams and leaks. This is one reason the shingle manufacturers used to require 4" exposure on low slope roofs, and now they no longer even accept that.

    Look enough, and you'll find a webpage, article, photo or pdf from someone that will say it how you want to read it. Many years as a roofer is real experience and knowledge.
  25. nate379

    nate379 Guest

    With good insulation it doesn't suck heat out. I have R60 blown in fiberglass on my ceilings. The walls are "only" R21.

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