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Community and Q&A

Best way to fix insulation problem in cathedral ceiling? Please help!

aaron h | Posted in GBA Pro Help on

I built my chalet about 8 yrs ago and was up in the attic and noticed black water stains (probably mold).
after many days and night researching what was wrong, i believe my issue is insulation and air leakage.
i have called all the insulation and roofing businesses (not many in this area) and each says its the others issue.

this site has been the most informative, but i cant seem to find any article or videos on how to add or change insulation in a cathedral ceiling after construction has been done.

i had 3 contractors, plumbing, paint/drywall and insulation. i have a 12/12 pitched roof with about 22 ft long cathedral ceiling and a 11ft wide attic about 4 ft high i can get into. 2ft on center 2×12 rafters with 2 lowes bought rafter vents in each space except for ends(could on fit one). i installed a ribbed metal roof on perlins with 30lb felt and 1/2 inch advantech roofing. the insulation is blown in white fiberglass. the walls were packed with it but the cathedral ceiling and attic is so fluffy combined with the ceilling not being sealed i beleive the r value is way to low.

this is where i need your expert advice. the more information i gather for roofing the more confused i get. i do not use it for storage and need help with making it a conditioned space vs non conditioned, vented vs not vented, and whats the best way to achieve this from only the attic or attic and soffit if i had to(would have to take gutters, aluminum trim, and soffit to be able to block it off).

right now i am thinking a few thing, but dont know enough to decide.
1. remove fiberglass, blocke off soffit and ridge vent, dense pack cathedral ceilingwith cellulose and sprayfoam attic.
2. remove fiberglass seal ceiling and install cellulose in attic and cathedral ceiling, leaving baffles
3. remove fiberglass, seal ceiling, cut strips of some kind of foam boardto slide down rafter bays either with or without rafter baffles.

i am getting a lot of icedaming on roof so i know a big issue is insulation in cathedral ceiling.
and the black stain/mold is lessening since i pulled back insulation and blocke gable vents(since i have full ridge, was told i should block them)

i think my biggest obsticle is trying to fix cathedral ceiling 22ft long from either top or bottom

please help, any and all information would be greatly appreciated!

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  1. aaron h | | #1

    update: i have 20ft rafters and 13 ft is in the cathedral ceiling.

    am now contiplating another option.
    seal off soffit vents, dense pack rafter bays till attic, leave open ridge vents and reopen gable vents.

    any thoughts on this or how to install site built baffles in a 13ft finished cathedral ceiling?

    patiently waiting for anyone to help

  2. Expert Member
    Dana Dorsett | | #2

    Where are you located?

    Going with only dense-packed cellulose or fiberglass in an unvented roof assembly is a risky proposition, but the risk varies with both the climate zone and the color of your roof finish.

    To meet IRC code in an unvented assembly you'd have to use closed cell spray foam on the underside of the roof deck to an R-value per table TABLE R806.5 based on location with the fiber insulation tight to the foam.

    Those are based on IRC code minimums for total R, but it's the RATIO of foam/total that determines the condensation risk at the foam/fiber boundary. In climate zone 8 the exterior-side foam needs to be 70% of the total R, in zone 7 it needs to be 60%, zone 6 would need 50%, zone 5 is fine with 40%, zone 4 would be 30% (though zone 4C can live with 20%). In zones 3 & lower below that it can be substantially lower and still be OK.

    But there's reason to believe that you can get away with less. Download a copy of BA-1001 and read it. All of it. (it's only 13-14 pages.)

    If you went with 1-2" of closed cell foam on the roof deck to protect the roof deck, and used an air tight "smart" vapor retarder like Certainteed MemBrain or Intello Plus between the fiber and the ceiling gypsum to keep the fiber dry enough you're probably going to do just fine. It's WAY safer than violating code based on Applegate's marketing fluff, which counts on the hygric buffering capacity of cellulose alone to save you.

  3. aaron h | | #3

    thx dana, i really appreciate the response and will read the link u gave.

    to answer your questions, i live in northeast pa and have a light tan color ribbed metal roof.

    i have looked into closed cell foam and the 2 contractors that looked at my house cant do anything because of my finished roof and drywall. is there a way to spray closed cell foam in a finished rafter bay?

    if not, i need to figure out how to get site built baffles in place and dense pack it or try smart baffles (which are alot more expensive and no r value).
    if i made 2 ft baffles with foil faced closed cell foam board does it have to be air tight to the sheathing?
    i have 13 ft that i cant get to, if i make a 16 ft long baffle, used 2x2 piece on edge to keep it off the sheething, could i glue the tops of the 2x2 pieces, slid them into place and use a 2x4 or something to push it into place?

    not sure what to do, 3 insulation contractors and 2 said they cant help, the other was a building contractor that did cellulose but never heard of dense packing it and i have a fourth coming on tuesday.

    thx again for the reply

  4. Expert Member
    Dana Dorsett | | #4

    NE PA can be either zone 5A or zone 6A. Find your county in this page and report back:

    It's sometimes possible, but both risky and expensive to use a slow-rise 2lb density closed cell pour. I would NOT recommend that approach. If it gets screwed up during installation or has gaps, it's impossible to fix it without a lot of demolition work.

    You can't use foil-faced foam as the baffle, since the foil facer prevents water vapor from escaping into the vent channel. If you want to use rigid foam as the baffled UNFACED 1" (maximum) EPS would be sufficiently vapor-open to work, as would unfaced 1/2" (maximum) XPS. Ideally it would be air tight to the rafters, with a minimum of 1" vented air space between the baffle and the roof deck. As long as it's tight enough that it doesn't allow the blown fiber to get into the vent channel during installation it's tight enough.

    Is the ceiling gypsum board, or is it something else?

  5. Expert Member
    Dana Dorsett | | #5

    double post deleted

  6. aaron h | | #6

    in that link, im in the green "5 and 4 marine" ceiling should be r 38.

    thanks for the info on the foil faced foam board.
    i am getting a sample of the "smart baffle" from one of martins article he thinks are the best baffles.
    i believe these are pretty much all plastic. if that is true then should i be considering them?

    would 1 inch of eps or xps made baffles withstand dense packing cellulose?

    would i be able to fill the rafter with 9 or 10 inches of xps cut 1/4 inch short on each side of rafter width. get a 6ft 1/4 inch hose, attach it to my great stuff pro foam gun and fill that void half way from top and half way from bottom?

    i know these arent ideal solutions but im trying to think of some almost any way to get baffles in without ripping out drywall or metal roof and sheathing. i do have the weak lowes baffles in there.

    what about sucking out loose fill fiber and putting in fiber batts without stapling them?

    i guess ultimately i would want to know what u would do if this was your house since, from what i read, i would call u an expert on this subject.

    thanks! again it is very appreciated!

  7. Expert Member
    Dana Dorsett | | #7

    The green in that map is zone 5. While R38 would have met code per 2009 IRC, the subsequent 2012 and 2015 calls out R49. If you go unvented, whatever your stackup's total R value is (whether greater or less than current code) you need at least 40% of it to be air-impermeable insulation if it's on the exterior of fiber insulation layer.

    In zone 5 as long as 40% or more of the total R is air-impermeable foam on the exterior of the fiber insulation the fiber will remain dry.

    Without knowing the (probably too low) vapor permeance of the Smart Baffle I'm VERY reluctant to recommend them for a cathedralized ceiling. It'll work just fine for the protecting the roof deck, but unless it's at least semi-permeable to water vapor you could end up with wet insulation in cathedralized ceiling application. They are designed for keeping attic-floor insulation from contacting the roof deck or blocking ventilation at the soffits in vented attic applications, where the insulation can easily dry into the ventilated attic.

    If supported at the rafters and at the mid point half-inch XPS should be able to withstand the pressures of 3lb density cellulose, but would probably be compromised by 4lb cellulose. A competent installer would be able to make that call.

    A more rugged baffle could be ripped from 1/2" asphalted fiberboard, which is strong enough to use as structural sheathing on walls. Half inch fiberboard runs about R1.3. The 3/4" stuff is about R2. It's also a fairly vapor-open "smart" vapor retarder, more vapor open than half-inch XPS or 1" EPS even when dry, and over 10 perms at high moisture content.

    Some pictures might be useful in pondering this. If you had to pull down and replace the ceiling gypsum to get it right, how much pain would that incur?

  8. aaron h | | #8

    pic1 and 2 are my great room. i really do not want to rip down the drywall.
    I spent 4 years building this house. only contracted out drywall, paint, insulation, plumbing, and siding

    the rest are from the attic. you can see why i need your help

  9. aaron h | | #9

    is asphalted fiberboard like zip sheathing? both are used for same thing

  10. peaceonearth | | #10


    My son's house looks much like yours, especially the cathedral ceiling and great room. We had similar problems, -ice damming and a few leaks last winter, and did some of the work you seem to need (and about which we got advice from GBA, -thanks again). While agreeing rigid foam above deck would be best, this was not in the cards for a # of reasons, expense not the least of them.

    We had pine t&g boards on the ceiling instead of sheetrock.. Hated to take them down but couldn't see how to do a decent job without. We removed the boards (to be re-used), and took down the 12" fiberglass batts. There was not as much water stain/mold as you have and the sheathing and insulation were in good shape. We installed a baffle of 3/4" xps using cleats on the side of the 13" rafters, as Dana mentions (would definitely be too flimsy in center for dense pack cellulose without support at center bay, but we were putting back the fiberglass batts.

    So, we assured a good 2" vent channel, put back and improved on the fiberglass batts, flush with bottom of rafters. We then used 2" polyiso fiber faced sheets, and taped seams well. Furring strips went in over polyiso, lined up with rafters, and then t&g boards went back.

    This was a pretty big job. We added R value and air sealing, both much needed, and the improvement is very noticeable. But we have had only part of this winter to see the performance. Very mild winter and so far very little snow, so the jury may still be out. But the room is warmer and no ice dams or leaks so far.

    You probably could install rigid foam below the current sheetrock without removing, but you would need to accommodate the thickness downward into the room, and then install another ceiling of sheetrock or other. Cathedral ceilings have lots of problems, evidenced by all the issues asked about on GBA, especially since there was so little understanding of good practices in years past.

  11. GBA Editor
    Martin Holladay | | #11

    You have signs of mold and ice damming problems. You need to fix this right, once and for all. You don't want halfway measures. Fixing your cathedral ceiling will be expensive, because you didn't get the details right when you built the house.

    If you haven't read them yet, you should read these two articles:

    How to Build an Insulated Cathedral Ceiling

    Site-Built Ventilation Baffles for Roofs

    There is a good chance that your cathedral ceiling has three problems:

    1. Your ceiling leaks air.

    2. Your insulation layer is air-permeable and has insufficient R-value.

    3. You have no air barrier at the top side of your insulation layer.

    You have two choices now. You can fix this from the top (by temporarily removing the metal roofing, installing one or more layers of rigid foam above your roof sheathing -- assuming that you have roof sheathing, and not just purlins -- and then installing more purlins or roof sheathing and reinstalling the roofing) or you can fix this from inside (by removing your ceiling, removing your insulation, and starting from scratch).

    I'm sorry to bring you this bad news. Good luck.

  12. aaron h | | #12

    thx for your response, i believe i will go thqt route if needed in the end. adding foam and drywall would be easier then ripping roof off.

  13. aaron h | | #13

    thx for the reply. i have a few questions before i start planning out what to do next.
    a roofer and myself calculated the intake @ 8.6 sq ft and exhaust @ 4.8 sq ft.
    knowing this, it would help me by lowering intake with a 1 inch gap in rafter bays.

    1. i thought of a way to instal 1 inch xps baffle with 1 inch space below roof decking in rafter bay, which would leave 9.25 inches of dense pack cellulose @ 3.7 per inch would give me r value of 39 with 5 being from 1 inch xps

    2. one of your articles talked about smart baffles, i could cut them down to 1 inch height and install them without ripping anything down per company then dense pack 10.25 inches to r 37.9

    3. i havent read much about roxul insulation on gba, but the batts would give r value of 42 in a 10inch space and leave baffle out? (not sure if its as dense as dense packed cellulose so dont know if its as good?)

    would any of these be an option martin? mold is only on wood, i have not found any on drywall. seems to be 6 inches above and below insulation line.

    or would one of these be an option to start with before i instale foam board and drywall?

    thanks again martin and eveyone!

  14. GBA Editor
    Martin Holladay | | #14

    Do you have a plan on how you can install ventilation baffles in cathedral ceiling bays that are 13 feet long without removing the ceiling? Or do you plan to demolish the ceiling?

    I'm not sure how you would install site-built ventilation baffles (made out of 1-inch rigid foam) unless you are willing to demolish the ceiling.

    If you decide to insulate with Roxul (mineral wool insulation), you can't leave out the ventilation channels. Mineral wool is an air-permeable insulation that requires a ventilation channel between the top of the insulation and the underside of the roof sheathing, just like fiberglass or cellulose.

  15. aaron h | | #15

    thx for the quick response!

    according to dci (makers of smart baffles as u know) they said u can put them together with screws or tape, 1 guy feeds into rafter bay from soffit and another pull from attic with rope (small hole in top with rope tied to it, dci said it will never rip) and push into place with 2x3 or something from top and bottom.
    the ears on the smart baffle will keep in place until dense pack is put in. dci says they are plenty strong for dense pack.

    or another article of yours gave me an idea of getting a site built xps baffle. it was a utube video of using iso and construction adhesive with a pole and block of wood to hold in place until construction adhesive dries. i would build and seal baffles made of 1 inch xps about 14 ft long. not sure on width, 2 things come to mind.

    1. since the flimsy lowes baffles do not extend the entire rafter width, i would make the width 20 1/2 inches wide that way the dense pack could fill the 1 inch space between the baffle and rafter on each side
    2. make them 1/8 inch short on each side, but probably would have an 1/8 void on each side.

    put contruction adhesive on the spacer on each side of the baffle, slide up into rafter bay, slid 2x4x16 under baffle push up into place and secure 2x4x16 to 2x12 rafter until glue dries. could probably do like 4 a day in the spring.

    install 2x? at soffit end and foam seal under baffle for stopper for either idea.

    with your knowledge, what one of these ideas do u like the most?
    do u like dense pack over roxul? (im thinking dense pack is less air permiable and better?)
    do u think xps baffles are better and worth the extra time or do u think smart baffles would be better and save a lot of time plus extend the whole bay?

    again, thanks for your input, it is very appreciated!

  16. aaron h | | #16

    i just reread my post and think i forgot to mention a few things i fixed right after i noticed the issue
    1. both bathroom fans were vented into soffit, which now are both vented out an extjrior wall
    2. the bathroom fan in the attic had holes all over it (i think more was going into attic than into soffit, i enclosed and sealed it with foam board and insulated vent pipe.
    3. closed both gable vents because i have full ridge vent (i think it was an article hear that said gable vents may interfear with ridge vent air flow.

    next on the list is to spray mold i can see, seal 4 receased lights, seal 1 1/2 inch on gable ends which is leaking from below (i think this is the gable end issue for mold) and seal any opening from basement that go to up.

    the bathroom fan i think was a huge one
    just an fyi

    thanks again

  17. aaron h | | #17

    1 last thought for anyone to give there advice.
    i know baffles that extend the rafter bay width are ideal, but to ballance out my exhaust with intake, the baffles would be 16 inches x 1 inch. that would make it 4.8 sq ft for both, but the bug screen in the exhaust would hinder this a little bit which from the article i read here its better to have a little more intake.

    not sure if its better to have a 16 inch baffle to balance or the whole width of rafter bay and be unbalanced?
    or 3/4 inch opening the entire.width would be another option.
    and would foil faced foam board (poly iso?) or xps be better to make the vents out of?

    as far as i know there are no bigger ridge vents for 3/4 inch ribbed metal roof.

    thanks again!

  18. GBA Editor
    Martin Holladay | | #18

    OK, I'm glad to hear that you have a plan for installing the ventilation baffles in your rafter bays. The work you describe can be tricky -- in some houses it is easy, and in others it is impossible. But it will probably work.

    Q. "To balance out my exhaust [area] with intake [area], the baffles would be 16 inches x 1 inch. That would make it 4.8 sq ft for both."

    A. The formulas for soffit vent openings and ridge vent openings are all about the soffit vents and the ridge vents; these formulas are not intended to be applied to the dimensions of the ventilation gap between the top of the insulation and the roof sheathing.

    Here's a summary of the advice for sizing this gap: it must be at least 1 inch deep, but 2 inches deep is better. That's all you need to know about the size of the ventilation gap -- forget the formula. For more information on this issue, see these two articles:

    All About Attic Venting

    Site-Built Ventilation Baffles for Roofs

  19. aaron h | | #19

    thanks martin for setting me straight with that calculation being at the vents. i will use entire width of rafter for vent.

    if you wouldnt mind, if this was your house, how would u do the vent and insulation?
    would you use 2 inch vent or 1 inch for more r value? 1 1/2?
    smart baffles( would be alot less work) or make own out of polyiso?
    densepack cellulose or roxul? (thinking cellulose is denser and would be better?)
    i'm reading cellulose 3.6 to 4 and roxul 4.2 per inch

    and finally, in dr joes video here it sounded like he wanted the baffles to go to the end of the soffit. so on a sunny winter day, the heat coming off the siding wouldnt go directly in the rafter vent. your article sounded like it would be flush with the exterior wall.

    if u would answer those 4 questions, i would happier than a...well i would be very happy!

    as always, thank you, you dont know how much i appreciate questions being answered by someone as knowledgeable as you!

  20. GBA Editor
    Martin Holladay | | #20

    Q. "If this was your house, how would you do the vent and insulation?"

    A. I would either:

    (a) temporarily remove the roofing, add a couple of layers of rigid foam above the existing sheathing, and then add new roof sheathing and new purlins, re-install the roofing, and seal up the soffit vents and ridge vents; or

    (b) demolish the ceiling and start from scratch, using one of the methods described in my article, How to Build an Insulated Cathedral Ceiling.

    Q. "Would you use 2 inch vent or 1 inch for more R-value?"

    A. If this were my house, and I wanted a vented assembly, I would create 2-inch-deep ventilation channels, followed by enough insulation to (at least) meet minimum code requirements. I wouldn't skimp on the R-value.

    Q. "Would you use Smart baffles (would be a lot less work) or make your own out of polyiso?"

    A. I would probably make site-built ventilation baffles.

    Q. "Dense-packed cellulose or Roxul?"

    A. Cellulose.

    Q. "In Dr. Joe's video here it sounded like he wanted the baffles to go to the end of the soffit, so on a sunny winter day, the heat coming off the siding wouldn't go directly in the rafter vent. Your article sounded like it would be flush with the exterior wall."

    A. Either way will work.

  21. aaron h | | #21

    thanks sooo much!

    i feel a weight has been lifted now that i have a plan.

    i will go with site built baffles as u suggested.

    last 4 quick and easy questions (i promise)
    1. foil faced poly iso or xps?
    2. how would u seal the butt joints? (guessing foil faced tape on poly? not sure about xps)
    3. 1x2 or 2x2 of same material to keep air space?
    4. how to attack that to the foam and to the shearhing? (guessing foamboard construction adhesive for xps will do both, not sure on the foil)

    thanks again for all your help!

  22. GBA Editor
    Martin Holladay | | #22

    I strongly suggest that you read the articles I linked to. Then you will have to make some decisions based on your preferred plan of attack.

    Although I have recommended that you either add rigid foam above the roof sheathing or demolish your ceiling, it sounds as if you don't want to take my advice. Instead you want to try to snake some ventilation baffles into your rafter bays without opening the bays up from the interior.

    This may work; it may also be too difficult to accomplish. It's worth a try, but even if you succeed, you may still have air leaks at the ceiling plane and a roof assembly with an R-value that is below minimum code requirements.

  23. aaron h | | #23

    i have read them, but i dont remember reading the pros and cons of each material. i will reread them again.

    the plan is to do mold remediation now
    air seal everything i can from attic
    once it gets warm, remove all insulation and soffits
    airseal everything i missed and double check what i did
    try the site built rafters installing them from soffit and attic
    with a 2x12 rafter, i believe i can achive the r38.
    2 inch airspace, 2 inch polyiso baffles, 6.25 inches dense pack cellulose
    baffle r 12 + 6.25 inches of dense pack @ 3.6 is 38.1

    if the vents dont work, then most likly remove the metal roof and do as you suggest with foam.

    i hope u dont feel as if i am getting around the problem, i feel the biggest problem is airsealing attic, top plate on wall, and getting an insulation that is alot less air permiable than loose fill fiberglass which i feel i need to try first without tearing roof or ceiling off.

    i hope i havent affended by not doing as u suggested, because i truely value your opinion.

    very interested in how you feel about my above plan.

    again thanks for your input!

  24. GBA Editor
    Martin Holladay | | #24

    I wish you the best, and if you can get the existing insulation out of the rafter bays, and can insert new ventilation baffles from the attic, your plan will work. I certainly understand why you don't want to incur the expense and hassle of demolishing your ceiling.

    My article on site-built ventilation baffles discusses the question of whether ventilation baffles need to be vapor-permeable. Once you read that section of the article, you'll have to decide whether you want to use foil-faced polyiso (which is a vapor barrier) or a material that is more vapor-permeable.

  25. aaron h | | #25

    thanks martin for all your help
    i reread your article and was still not sure on what to use. i am in agreement with you that it shouldnt be air permiable. so i am doing more research on polyiso and xps and found this article.

    its a good read and if its true, i made my choice, but another article i read after this one said that polyiso is better r value, but didnt talk about cold at all.

    its the dreaded egg conundrum. one day they are good for you, the next they are not.

    wonder what you and anyone else thinks about that article.

    nevermind, i see you already know and have written an article about it.

  26. GBA Editor
    Martin Holladay | | #26

    Your insulation is air-permeable. Your ventilation baffles should not be air-permeable. (However, it's going to be hard to get a perfect air barrier above your insulation if you are sliding the baffles in from the attic.)

    I don't think it matters very much whether your baffles are vapor-permeable or vapor-impermeable, but not everyone agrees with me. It's your choice.

  27. aaron h | | #27

    just had my third and final cellulose contractor at the house today. the only one that says he dense packs.
    $5000 just to do cathedral ceilings and i have to remove the fiberglass loose fill. and hes telling me that the cheap lowes baffles i have will not crush when he dense packs it!?!
    those cant hold dense pack, correct?

    the other 2 do not know what dense packing is.

    guess i will have to calculate out how many bags per rafter bay put a half bag in at a time, then pack it with a 2x6 on the end of a stick?
    i can build a box same dimentions as my rafter bay to hold 1 cubic ft, then weigh out 3.5 lbs and pack it in with that 2x6 on a stick to get a feel for how tight it should be packed.
    then cap top of rafter bay with a mesh i have so the cellulose doesnt expand out of the bay.
    (from what i have read 3.5 lbpcf packs it to 150 to 200%, so i would assume it would try to expand if not meshed off at top)

    anyone have any thoughts on this?

  28. Expert Member
    Dana Dorsett | | #28

    The baffles need to both be air-tight and at least semi-permeable to water vapor, otherwise the fiber insulation itself can take on moisture from interior moisture drives, especially if your ceiling is difficult or impossible to air seal. Cellulose can buffer a lot of moisture, but I'm not of the school of thought that thinks it's going to be fine forever to put a true vapor barrier (like hard plastics or foil facers) on the exterior side of an assembly with a hard-to-air seal interior.

    Half-inch plywood or OSB would work- they're both semi-permeable when the moisture content hits mold-growth levels, but half-inch or 3/4" asphalted fiberboard would be better. Fiberboard is not like ZIP (which an OSB product)- it's an order of magnitude more vapor permeable. When ashpalted on both sides it's extremely tolerant of condensation or liquid water (you can leave it out in the rain). If you can't find it elsewhere through distributors, the mid-western box store Menards carries it- don't know if there are any in eastern PA yet (?). You may have to order online for pick-up at the store if your local store doesn't normally stock it. This particular one-side asphalted fully structural version is rated 28 perms & R1.22 :

    If it's asphalted on one side only, the asphalt should be on the top side (the side facing the roof deck) to make it more resilient against roof leaks.

    You are correct that you can't dense-pack against a open framing bay. It needs to be blocked (mesh is fine.) If you're pulling down the ceiling you could damp-spray it in place, which is cheaper & easier than dense packing. With a permeable or semi-permeable vent baffles damp spraying would be fine too. The water activated adhesives keep it in place despite the fact that it's not under spring-loaded pressure after installation the way dense pack is. It's somewhat more air-permeable, but still far more air-retardent than batts.

  29. aaron h | | #29

    hi dana,

    thanks for the respnse!
    sounds like a good product, but i have not seen the fiberboard in any building supply places around here.
    for this reason i was going with 2 inch of xps (for the extra r value and strength) and i believe its semi permiable, even at 2inch, and because i have read that polyiso looses its r value in cold.

    i believe insulation and air sealing are 2 issues i have and will solve when it gets warmer and i can remove all insulation. but i also believe ventilation is a big issue and i can fix that now.

    if you wouldnt mind, please take a look at the attacked pic of my shed dormers.
    my roof is a 12/12 pitch and the shed dormers ( one on each side in the middle of the house) are 4/12.
    its a continuous ridge beam. the dormers have soffits, baffles and a ridge vent.

    i am thinking that the dormer ventilation is short circuiting the cathedral air flow.
    i have no mold issues in the dormer part and is the worst on the gable ends.

    should i have soffit vents on these dormers because they are so much higher than the cathedral soffits?
    i would still have more intake than exhaust even if i blocked these off.

    if need be i can take a video of the inside of the attic, post it on utube, then post the link here

    thanks for the help!

  30. GBA Editor
    Martin Holladay | | #30

    You wrote, "I am thinking that the dormer ventilation is short circuiting the cathedral air flow."

    What does that mean? I assume that the dormer has a cathedral ceiling -- so what is the difference between "dormer ventilation" and "cathedral air flow"? Aren't these the same thing?

  31. Expert Member
    Dana Dorsett | | #31

    At 2" XPS is semi-IM-permeable, at about 0.5-0.6 perms, which is why it isn't a good idea in zone 5 unless fully 40% of the total R is foam. ( If you're gutting & rebuilding the ceiling you'd probably get away with it if you installed an air tight smart vapor retarder on the interior though.)

    For perspective: It takes a full year for 2" XPS to relieve the amount of moisture 28 perm asphalted fiberboard would pass in just one week. It takes 2 months for what the fiberboard would pass in one day.

    Semi-impermeable is just that: Nearly impermeable. At 2" XPS would qualify as a vapor barrier under code definitions in Canada.

    At the same R-value, R10 (2.5") UNFACED Type-II EPS would semi-permeable, at a bit over 1 perm, more than 1.5x the drying rate of R10 XPS. At 2"/R8.4 Type-II EPS would be rigid enough for dense packing, and more than 2x the drying rate of XPS of the same thickness. It's still at the vapor-tight end of Class-III vapor retardency, but it's at least semi-permeable.

  32. aaron h | | #32


    thx for the quick response. the dormers are just like a regular 1 or 2 story house.

    it so hard without seeing it, so tbis might help:

    after the video, i was pulling out the insulation where it was bad and it seems just like u and all the articles i read, need to air seal all pipes and wall.
    im pretty confident that the air leaks coupled with the insulation being so air permiable and baffles not being air tight is my issue. the gable ends are th worst because of where the 2x12 rafter attackes to the 2x6 wall which is where the drywall is fastened to. all these small gaps are adding up to alot of air movement.

    i was going to seal what i can, but feel its a waste of time. once the weather breaks, i will remove all insulation treat any mold, air seal everything, install airtight baffles, reinsulate with cellulose.

    2 final questions
    1. will a 1/2 inch piece of polyiso cut tight and sprayfoamed for an air wash stopper in the soffit be strong enough to hold dense pack back?
    2. out of 4 insulation guys, the only one i would consider is one guy said he could fill the cathedral ceiling with wet spray cellulose.
    a. can this be done and fill all voids in the cavity?
    b. would there be much of a difference in air permiability?
    c. he suggested to leave the baffles as is (cheap lowes baffles) and just wet spray the cellulose, over
    top of them. good or bad idea?

    thanks again for all your help!

  33. GBA Editor
    Martin Holladay | | #33

    The link to the video doesn't work.

    Can you answer my question? What is the difference between "dormer ventilation" and "cathedral air flow"? Aren't these the same thing?

  34. aaron h | | #34

    weird, the link works when i click it.
    you can google "u tube cathedral ceiling mold issue" and its the first one.

    i should have explained it differently. i was suggesting that the cathedral sofit were pulling less air than the dormer soffit. the dormer soffit is 12 ft away on a 15 deg angle from ridge vent. the cathedral soffit is 16 17 ft away on a 45 deg angle. probably 10ft in hieght different.

    i dont have a smoke pencil so i used some baby poweder and puffed some in front of most of the baffles and ridge vent. it was windy and sometimes it would suck and sometimes it would blow.
    so it didnt seem like my assumption was correct.

  35. GBA Editor
    Martin Holladay | | #35

    Here is a better link to your YouTube video:

  36. GBA Editor
    Martin Holladay | | #36

    From the appearance of the mold in your attic, it looks as if conditioned indoor air is leaking into your attic through cracks and holes in your exterior walls and your sloped ceilings. These types of air leaks can be tricky to locate after a house is built -- they are much easier to catch during construction.

    This type of air leak can occur through electrical boxes. Cracks that allow this type of leak include unsealed seams where drywall is attached to a top plate. It's also possible for indoor air to travel in joist bays and through interior partitions. The best way to tackle these problems is with blower-door-assisted air sealing -- but even with good tools and an experienced weatherization crew, these types of leaks can be tough to address.

  37. Expert Member
    Dana Dorsett | | #37

    Damp sprayed cellulose is pretty good stuff, and would be far more air-retardent than mid-density fiberglass. As long as the interior is reasonably air tight it's a good way to go. It won't help with air-sealing the way dense pack would, since it's not being applied under pressure, where the moving air at the leak points drag cellulose into the gaps, but it's still a good way to go.

    I have no experience with damp-spraying cellulose on cheap vent chutes- an experienced & competent damp spray installer would have a better handle on that than me.

  38. aaron h | | #38


    as always thanks for the advice. a picture (or in this case a video) is worth a thousand words.
    thank you for confirming my latest idea of the issue.

    when i first asked about my problem here, i originally thought ventilation issue.
    i feel good from all the advise, questions and answer that my ventilation is good ( and will only get better when i install full airtight baffles) and the main issue is air leakage.
    and with the loose fill fiberglass, it just compounded the issue.

    i agree with the airdoor test and was just researching to see if there was an energy audit type place in the area that does the door test.

    the plan (hopefull the finaly plan) now is to:
    1 start in the basement and work my way up airsealing everything i can.
    2 once warm enough, will suck out all insulation in attic and cathedral ceiling.
    3 seal everything i can up there.cathedral part will be tricky but with things like this
    it might be achievable. one site siad it was 46 inches another said 40 plus a 3ft reach, maybe possible?
    4 do door blower test and go from there. if it doesnt pass, i will have to remove drywall (crossing my fingers)

    does this sound like a good plan?

    1. will a 1/2 inch piece of polyiso cut tight and sprayfoamed for an air wash stopper in the soffit be strong enough to hold dense pack back?
    2. out of 4 insulation guys, the only one i would consider is one guy said he could fill the cathedral ceiling with wet spray cellulose. can this be done from attic and have no voids?

    thanks all the help!

  39. aaron h | | #39


    i cant believe i hadnt thought about it, but i finished both gable ends in t&g hemlock.
    which makes even more sense why the gable ends are so bad!

    how do i seal that off? or do i just need to worry about sealing it off in the attic?

    thanks for everything!

  40. aaron h | | #40

    thx dana

    i guess that answers one of my other questions above.
    could i fill cathedral cavity with a ft or 2 of loose cellulose with rental blower and pack it myself untill its at 3.5 pcf?
    it sounds like the high air pressure is what you need to dense pack, at the preferable way

    no one around here knows whag dense packing is! uugghh

  41. peaceonearth | | #41


    I just looked at the foam gun you linked. If you don't mind the cost I think its a no brainer as you will have a lot of important places to use it, and it will be an important part of air sealing your house. I'm on my 2nd gun and wouldn't use foam without one (unless I bought a two part pack). Mine were the standard length barrel, and there are places that I cannot get to in a reasonable way (rim joists are tough; the can of foam needs to be kept upside down), so I think the 40+ inch barrel will be a godsend. Pay careful attention to how to keep it and store it after use, since if the foam hardens within the gun you can throw it away.

  42. aaron h | | #42

    thx howard
    if it keeps from rippingout drywall, ill buy 2 of them.
    and i agree, once u use the pro fun, youll never go back!
    i too have the standard and am already seeing its to short for certain places.

  43. aaron h | | #43

    researching some more...

    would open cell spray foam be a better option than dense pack cellulose in this situation?
    can open cell be sprayed over the cheap baffles i have installed now?
    i know a full bacfle is better, but just want to look at all options.

    no one around here knowledgable in dense pack, knowing i have attic leaks and open cell being an air barrier, im thinkin it might be the way to go?

    i would have the proper thickness according to martins article for open cell to be considered an air barrier

    thx for the help

  44. GBA Editor
    Martin Holladay | | #44

    Your original question implied that this is an insulation problem in your cathedral ceiling. It may be that.

    However, after looking at your video, I wouldn't call this an insulation problem. My gut tells me that your problem is basically one of air leakage from the interior of your home into your attic. Many of these leakage paths may be in your walls, although some appear to be in your ceiling assemblies.

    This type of air leakage issue really can't be diagnosed over the internet. I urge you again to find a home performance contractor who offers blower-door-directed air sealing.

  45. GBA Editor
    Martin Holladay | | #45

    This screen shot from your video seems to show mold on your wall sheathing -- perhaps evidence of air leakage paths in your walls.


  46. GBA Editor
    Martin Holladay | | #46

    This screen shot is not as clear -- it's on the other side of your dormer -- but it also shows mold on your wall sheathing.


  47. GBA Editor
    Martin Holladay | | #47

    And this screen shot shows mold on your wall sheathing at the gable end of your attic.


  48. GBA Editor
    Martin Holladay | | #48

    I stand by my previous advice. Stop obsessing about your insulation choice, and start focusing on figuring out your air leakage paths.

  49. aaron h | | #49


    i agree 100%, i believe my biggest issue is air leakage. espically on gable ends because of the t and g on the gable ends.

    i am still looking for a company in my area that does the door blower test.

    my question is, now that i know from one of your articles that open cell is an air impermiable insulation, do you feel it might be a better option than cellulose, since no one knows about dense packingaround here. i would still have a full baffle.

    one foam installer said he could put a hose down my rafter bay and pull hose as it fills.
    not sure if this can be done this way, but without finding cellulose insulators that know about dense packing, im thinking i should look into it more.

    even put open cell under my attic decking (2x10) after rafter bays are filled to make sure there are no air leaks.


    thanks again! and thanks for quick response!

  50. aaron h | | #50

    please excuse my ignorance, but im feeling, from what little i know, that an open cell insulation would seal those air leakages?

  51. GBA Editor
    Martin Holladay | | #51

    Sealing methods differ depending on their location. Air leaks at electrical outlets can sometimes be addressed by caulk. Large air leaks are often addressed with a rigid product (often a rectangle of foil-faced polyiso sealed in place with canned spray foam).

    These air leaks might originate in your basement. They may travel through your partition walls. There may be leakage paths in joist bays inside your home. These air paths are quite tricky. You need an experienced weatherization contractor to help you track them down.

  52. aaron h | | #52

    sorry for asking so many questions, as always its very appreciated!

    i think i understand your logic now.
    even though i know i have air leakage in the attic and i could plug those leaks from the attic, they could stem from anywhere and i should know where they originate before completely sealing the attic with say an open cell foam.



  53. GBA Editor
    Martin Holladay | | #53

    Yes, that's right.

    The two most important places to address these leaks are the basement and the attic. I wouldn't start by pulling down your ceiling or changing your ceiling insulation.

    Here are links to two articles to get you started:

    Air Sealing an Attic

    Air-Sealing a Basement

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