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Over roofing question

cliffmarck | Posted in Green Building Techniques on

I have an ice damming issue to resolve – and a roof that needs to be re-done. I reside an hour or two Northwest of Toronto, ON Canada. Zone 6 I believe.

My original plan was to convert my story and a half attic to a hot roof system with a ventilated standing seam steel roof on top (the venting is necessary due to our snow loads – the steel would have an airspace below but the roof still a hot roof I believe). It is currently a vented unconditioned attic so my plan was to strip the roof (or not) – backfill the current ventilation space with dense pack cellulose via the drill and fill method – then over roof with 5″ of polyisio (approx r-30), tape/foam seams – strap, sheath, membrane then steel. Then on the inside batt the roofline and gable ends leaving more than 50% of the insulation on the exterior for the sake of the dew point. I would also remove the current attic floor insulation.

The above method – to me is the gold standard. It does however require a bank account lined with gold – which I do not have. I can appreciate the absolute BEST way to do something – but at the end of the day – it’s a roof, and I’d rather not go into a significant amount of debt for it if I can avoid it.

I have had a couple reputable steel roof contractors recommend simply leaving my attic the way it is then adding 2″ of polyiso (R-10-12.5) on top of the current roof – then strap and roof after taping or foaming the seams of the iso. The polyiso in this method’s only job would be to eliminate the current leakage which is causing the ice dam problem.

I’m a bit skeptical of this method – but these guys have been doing this for 25 and 36 years and say it quite simply solves the ice dam issue – and does so at a MUCH more reasonable cost. I understand the latter method does absolutely nothing for the R value of my attic as it’s outside of the current vent space. It’s also not a ‘green’ solution per se, but if it solves my ice damming issue and is within budget – why should I not do it?

I have this sneaking suspicion I am missing something – that’s where you come in.

Thanks so much for your help.
Cliff

 
 
Brians first question reminded me that I left out some important information in my original post:

The points of air leakage seem to be coming from spots in the sloped portion of my story and a half. A soffit vented bathroom fan – two stink pipes – and a chimney – all appear to have not been air sealed properly. I can’t access them from the attic. I suppose I could pull the sheathing in those areas and air seal from the outside and be on my way – but I still have another source to worry about…

 
….the main cause of my ice dams is likely the flexible ductwork supplying our master bedroom/bath that currently runs through the attic. There is no other way to add ductwork to this area of the house. I considered ripping out those heat runs and adding a ductless mini-split to mitigate the issue but I think encapsulating those heat runs in cc spray might be the most economical option – or going ahead with the conditioned space as described above.

Edited to add information

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Replies

  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Cliff.

    If you are willing to do the work of removing the insulation from your attic floor, why not lift it up, air seal the attic floor, top plates, and other thermal bypasses, and reinstall the insulation and/or add new insulation to an appropriate R-value. Your ice dams are likely being caused by heat loss from air leaks and inadequate insulation on the attic floor. This would be a coast effective, DIY-friendly approach. These articles will help you understand the process: How to Insulate an Attic Floor and Air Sealing an Attic.

    1. cliffmarck | | #5

      Hi Brian

      Thanks for your quick response.

      It reminded me that I left out some important information in my original post. The points of air leakage seem to be coming from spots in the sloped portion of my story and a half. A soffit vented bathroom fan - two stink pipes - and a chimney - all appear to have not been air sealed properly. I can't access them from the attic. I suppose I could pull the sheathing in those areas and air seal from the outside and be on my way - but I still have another source to worry about...

      ....the main cause of my ice dams is likely the flexible ductwork supplying our master bedroom/bath. There is no other way to add ductwork to this area of the house. I considered ripping out those heat runs and adding a ductless mini-split to mitigate the issue but I think encapsulating those heat runs in cc spray might be the most economical option.

      Do you have any insight into whether the 2" polyisio the roofing contractors seem to be recommending? A steel roof manufacturer also recommended this method. I am still skeptical.

      Thanks again
      Cliff

  2. Expert Member
    AKOS TOTH | | #2

    The suggestions from the roofer would solve your ice dam issues but because it is over a vented roof, it will not do anything for comfort or energy loss.

    Usually most ice dams are caused by interior air leaks, getting those fixed is much cheaper. This also helps with energy use and interior comfort. Because of stack effect, ceiling air leaks make the bottom of the house colder as outside air is drawn in through the many leak paths near the basement and main floor area.

    As for the hot roof, code for your area should be R31. If you bump polyiso on the roof to 2.75" and insulate on the inside with R14 mineral wool batts, you could get there with a slightly more expensive assembly than what the roofer suggested. There is really no need for 5" or more of rigid insulation, the extra cost of that insulation will never pay in terms of energy savings.

    With sealed soffits and insulated gable ends, this is a much more energy efficient solution to 1.5 story construction as proper air sealing around knee walls is next to impossible.

    1. cliffmarck | | #4

      Thank you very much Akos for your response.

      I was under the impression I needed R-60 for my attic solutions. Can you point me in the direction of where you found the R31 indication please.

      1. Expert Member
        AKOS TOTH | | #7

        R31 is for cathedral ceilings. Generally when you do an over-roof, you need an air barrier on the warm side, cheapest is drywall, so it is no longer an attic.

        I also think this should fall under Part 11, which allows for keeping existing insulation values. So if you have 5" to 6" of fluffy in the attic, you don't need to bring it up to full R50.

        1. cliffmarck | | #10

          Ahhhh - thank you. That's exactly what I have in my 'attic'. There actually wasn't a hatch because the space is so tight it was not required by code - but I needed to cut one in to see what was going on up there. Thanks for the clarification.

  3. cliffmarck | | #3

    Hi Brian

    Thanks for your quick response.

    It reminded me that I left out some important information in my original post. The points of air leakage seem to be coming from spots in the sloped portion of my story and a half. A soffit vented bathroom fan - two stink pipes - and a chimney - all appear to have not been air sealed properly. I can't access them from the attic. I suppose I could pull the sheathing in those areas and air seal from the outside and be on my way - but I still have another source to worry about...

    ....the main cause of my ice dams is likely the flexible ductwork supplying our master bedroom/bath. There is no other way to add ductwork to this area of the house. I considered ripping out those heat runs and adding a ductless mini-split to mitigate the issue but I think encapsulating those heat runs in cc spray might be the most economical option.

    Do you have any insight into whether the 2" polyisio the roofing contractors seem to be recommending? A steel roof manufacturer also recommended this method. I am still skeptical.

    Thanks again
    Cliff

    1. GBA Editor
      Brian Pontolilo | | #6

      Hi Cliff.

      Unfortunately I'm not familiar with taking that approach to mitigating ice dams. I have no reason to doubt these contractors if this is their experience. However, as Akos said, you'll be paying only to mitigate ice dams, but missing the opportunity to make the house more comfortable and efficient.

  4. cliffmarck | | #8

    Thanks so much again.

    I am rethinking this considering your first response.

    Maybe the most economical solution would be to air seal my attic space as you indicated and encapsulate my flex-duct with foam as well. Then have the roofing contractor remove sheathing in the effected areas in the sloped areas of the 2nd floor and air seal and re-insulate - then re-sheath and simply put the steel on top of the original decking. My only concern is if we miss a spot - I am back to square one but with my problem covered up with a shiny new roof.

    Would using a thermal imaging camera help ensure the contractor gets all of the leaky spots? Any other advice?

    Thank you so much for your time and consideration.

  5. Jon_R | | #9

    Your first action should be blower directed air sealing.

    Yes on CC spray foam to encapsulate ducts that can't be moved.

    After the above and if you have fiberglass on the attic floor, consider blowing in cellulose for additional R value and some additional air leakage reduction.

    1. cliffmarck | | #11

      This is great direction - and exactly what I needed. I also need to tighten up the main house as some of the structure (log home) has some checking that should be sealed up.

      I will just have to find someone who does this kind of work in my area. Thanks so much.

      Any further advice is greatly appreciated.

  6. maine_tyler | | #12

    Has anyone done a cost comparison of some of these options?

    Of course no one wants to spend more money than necessary, but since you are re-roofing anyways, the exterior insulation cost becomes largely incremental. I don't doubt it's still more expensive, but if it ever were to be done, the time is now.

    I have a similar 1.5 story re-roof project and am grappling with a similar price vs final product dilemma. Does anyone have ballpark figures on what sort of cost differences you're likely to see (2x? 3x? etc) for over-roof insulating vs keeping it a vented attic? Of course the final products aren't the same... but

    1. cliffmarck | | #14

      Hi Tyler

      I am in the midst of pricing out everything right now. It's a bit slow to get quotes in due to our global health issue (covid-19) - but they are coming nonetheless.

      Obviously, every area is different in terms of labor/material costs but I will post some sort of comparison here once I have it.

    2. Expert Member
      AKOS TOTH | | #17

      Every house is different, putting even a ballpark cost at any of these options is too hard.

      If you have a 1.5 story with some R11 batts here and there and ducts/hvac in the attic, over-roof would probably be simpler than trying to re-locate ducts/hvac and air seal plus insulate properly.

      I own a 2.5 story with not too many uglies and it was simpler to air seal the ceiling the knee wall area with SPF and blow in insulation elsewhere. Over-roof in that case would have been probably been an order of magnitude more in cost.

      1. maine_tyler | | #19

        Thanks Akos. Wow an order of magnitude. That's surprising.

        Definitely highly dependent on the details...

        I did come up with a self generated quote for exterior rigid. The tough part with the interior sealing work is it seems more variable (don't know what your going to get into labor-wise).

        So you crawled in the attics to spray the kneewall and ceiling? Did that take care of most the leakage where they met with the sloping section?

        1. Expert Member
          AKOS TOTH | | #21

          Luckily the house didn't have knee walls, the 1/2 story had full height walls that were supporting a flat roof.

          This was standard 1/2 story air sealing. SPF over top plates, blocking+SPF at floor joist underneath walls. Vapor barrier on ceiling and walls with batt insulation, blown in for attic floor. Vented attic with edge vented flat roof.

          The over-roof would have meant re-doing the whole roof + rigid +new deck+new fascia/trim, about the same amount of batts and way more SPF to seal things up. Inside I can do myself, outside you are paying for labor which gets expensive very quick.

  7. maine_tyler | | #13

    >"the main cause of my ice dams is likely the flexible ductwork supplying our master bedroom/bath. There is no other way to add ductwork to this area of the house."

    Are they in the main attic? No way they could run in one of the side attics is there? If so, you could consider changing those to conditioned space.

    1. cliffmarck | | #15

      They are in the main attic. There are no side attics. It's a square log home with half of the 2nd story (the roof) stick framed on top of the top course of logs. Hard to explain the layout without pictures. There is one spot I could run them inside the building envelope near the slanted ceiling, but it would look awful and not really be practical. I would have to move lighting fixtures, and build bulkheads etc. I figure that encapsulating them in place would be a lot easier and less unsightly.

      Thanks for your thoughts.

  8. cliffmarck | | #16

    After Akos' comment above I am thinking it might not be as cost prohibitive to go ahead with an over-roof scenario considering I only need R-31 as opposed to the R-60 I originally thought I required. This also might mean I can get away with not doing as much or any trim work on the exterior as it might not look as silly as I originally thought it might.

    The only issue I see is that I have 12" deep rafter bays (ON 12" CENTERS!). Those bays would have to be backfilled to eliminate the vent space creating a hot roof. I would then have 12" of batts/cellulose on the interior of the sheathing - leaving only about a third of my insulation r value on the exterior of the sheathing. Would I be setting myself up for a moisture problem?

    There is also currently the standard 6 mil poly on the interior side of the insulation.

    1. Expert Member
      AKOS TOTH | | #18

      Check with your local building inspector of what you need.

      In Zone 5/6 there is very little energy savings by going much more than R35. More than insulation, they key is air sealing.

      For your case with 12" rafters, if you fully fill the rafters, you would need much more exterior insulation. You want to keep the ratio at 50% for zone 6. Use thinner high density batts, either mineral wool or high density fiberglass, as these can be friction fit and stay tight against the roof sheathing. There are also insulation support wires you can get to keep things nice and tight. 12" OC will be a pain. Might be simpler to spray the underside of the deck with 4" of open cell foam instead.

      With a hybrid exterior rigid/interior fluffy roof you generally don't want interior poly. Either use a smart vapor retarder or drywall with a coat of latex paint. This will allow your roof dry towards the inside.

      If you have just one duct to worry about, I would air seal first. If your roof can make it through one more winter, you could see if doing the over-roof is really needed after the air sealing.

      1. cliffmarck | | #23

        I'm not overly interested in replacing the drywall on the second floor - mainly because I don't enjoy sleeping in a dog house. But thank you.

        I think you are right on air sealing. If I can effectively air seal and encapsulate that duct then wait another Winter and watch - I may have it beat without going over the top.

    2. maine_tyler | | #20

      12" rafters is a good bit of space. Part of the issue with many older _.5 stories, besides air sealing, is that the sloped section is 6" deep minus venting.

      What is the slope currently insulated with? And what's the finish on the interior (drywall?)
      Is venting currently soffit to ridge?

      If you DO go ahead with the complete over-roof, you may find this interesting in regards to the advantage of stripping the roof: https://www.finehomebuilding.com/2018/06/21/retrofitting-foam-insulation-roof

  9. cliffmarck | | #22

    It's been a year since I've actually been in the attic. I crawled up today during the kids 'quiet time' and found I've got about 7" of batt insulation - estimate R-22. If I were to backfill the remaining airspace 4" with loose fill fiberglass at r-2.5" I'd be at R-32 beneath the sheathing. To over-roof properly I'd need R-32+ which would equate to about 5.5" of polyiso. My rafters are actually OC 16" - my mistake.

    I've only gotten quotes for 2" of polyiso so far - installed it is about $1 per inch/square foot Canadian. It adds up quickly. It is about the same cost per square foot to install spray foam.

    Venting is currently soffit to gable end vents which will be upgraded to ridge vents - I can imagine this is also a contributing factor.

  10. cliffmarck | | #24

    It might also be worth mentioning that we replaced our propane furnace this year with a high efficiency ducted heat pump. We went with a Daikin Fit Heat Pump and are extremely impressed. We used to pay around $400-450 a month in propane in the dead of Winter - this year on a very cold month we paid about $100 in extra electricity. Your costs would obviously vary due to geographical location but for us - it's been huge.

    More importantly for this issue - it means the savings for adding more and more insulation are subject to diminishing returns.

  11. cliffmarck | | #25

    I've gotten all of my quotes back and have decided I will be going forward with an exterior rigid foam solution with 4" of polyiso. I will then add dense pack to the inside and insulate my gable ends making it a conditioned space.

    I did mention I would add some pricing information (obviously your layout and local contractor rates will differ - regardless here it is):

    1)I priced out removing all of the existing insulation and then adding 5" of cc 2 lb spray foam. For 1800 square feet of roof this would cost me $7800 CDN + Tax. R-31 assembly.

    2)Adding 4" of polyisio to the exterior for 1800 square feet of roof is $7200 CDN + Tax. R-25 plus I will add another R-25-30 on the interior. Our HDD are about 4,000 so I'm comfortable being slightly below 50% of the recommendations for zone 6 on my 'exterior' rigid foam. I'm really at the extreme bottom of zone 6 or top of zone 5.

    3) Pull areas I know are issues and have local contractor spot foam leakage spots - after I pull the shingles and sheathing in those areas - approx $2500 in roofing materials/spray foam contractor.I could not and can not for the life of me get anyone to do an air blower door text in my area. Everyone on cresnet and locally who used to offer this service no longer offers it due to government grants closing down. Even though this is the cheapest option my roof is on it's last legs and waiting another Winter is a gamble. It also might not work as I may miss spots and I'll be back to square one again.

  12. cliffmarck | | #26

    I have 2 more questions since posting this and feel it's appropriate to ask them here for continuity's sake. If I don't receive a response - I guess I will start a new thread.

    1) The roofer is going to apply a membrane over the existing shingles (overpriced tear off so even though I should do it - I won't $6.5k) - then 4" of polyiso with the seams taped. On top of that they will attach battens and 4" z-bars then a 24 gauge galvalume ss roof. They will roll form it on site. Does this seem like an appropriate assembly?

    2) On the interior - instead of drywall I am considering using Certainteed Membrain or more likely Intello's smart barrier product. Would this work okay? Drywalling in this tight TIGHT spot wouldn't be realistic.

    Thanks
    Cliff

    1. Expert Member
      AKOS TOTH | | #27

      1) Membrane over the existing shingles should be fine. Shingles are already a good vapor barrier, so adding on top doesn't hurt.
      If I understand correctly the Z bars are part of the standing seam, just make sure they stay above the foam. You don't want Z bars to go through the foam as this creates a huge thermal bridge. Z bars on top of strapping is fine.

      2. Any variable permanence barrier works on the warm side, Membrain is fine. This should be detailed as an air barrier. You can't dense pack against Membrain though without a mesh, might be better to use one of the fancier beefy permeable products that can handle dense packing pressure.

      Make sure the space in the mini attic area becomes part of the conditioned space. You don't want a semi conditioned space that is isolated from the house by a layer of insulation.

      P.S. Lot of flat site formed metal roof is prone to oil canning. Talk to your roofer if this would bother you.

  13. cliffmarck | | #28

    Thanks Akos

    This is great information - I'm not sure what they meant about Z bars but will definitely clarify with the contractor prior to installation to ensure it doesn't create a thermal bridge.

    I will be moving the attic insulation to the roofline and removing the current poly vapour barrier from the floor (out of curiousity - would it do harm to leave the poly in place?).

    I am aware of oil canning - and it would bother me. I'm planning on using a raw galvalume roof with a 24 gauge steel and a few small striations to hopefully keep it in check - it's my understanding that all three of those things should help my case.

    If all that doesn't do the trick and it still oil cans - c'est la vie.

  14. cliffmarck | | #29

    So it turns out their plan IS to attach the Z bar to the existing roof - it then would run between the foam panels 4" then jaunt outwards where the battens and ultimately roof would be attached to. They would tape the seams over the z bar.

    They kind of scoffed at the idea of using long screws instead but are talking about it.

    You say huge thermal bridge? Would it make a massive difference in my insulation level? Any other potential problems from this?

    1. Expert Member
      AKOS TOTH | | #30

      Some good info about Z grits and thermal bridging:

      https://www.rdh.com/wp-content/uploads/2017/07/2018-27-03-RDH-Cladding-Attachment-Solutions-Brochure-V4-Web.pdf

      You do loose a fair bit.

      If the Z grits are parallel to your rafters, the combination of the rafter+metal will be a pretty large thermal bridge through all your insulation, enough to cause local snow melting and ice dams.

      An in between option could be to cross strap with 2x4 on edge. You still loose a bit of R value, but nowhere near as much as with Z grits. I've done this with 2x3s and it works quite well. You still need long screws though, so maybe it is just simpler to go with the strapping.

  15. cliffmarck | | #31

    Thank you - if you didn't mention this I may have had a very expensive roof with the same old ice dam headaches. I appreciate your input.

  16. cliffmarck | | #32

    Just to keep things updated here. After speaking with my roofing contractor - they have agreed to fasten the polyiso with long screws to hit the rafters through battens instead of using the Z-Girts. He did mention that it would be a substantially weaker assembly than the Z-Girts but I think he was just talking out loud. He assured me they would make sure they hit the rafters with the screws.

    The steel will then be form rolled on site and fastened to the strapping.

    I'm pretty sure I'm ready to roll on this project but one last question - should I be worried about the strength of the assembly using the battens as the middle man?

    I'd like to avoid bothering the roofing contractor anymore if at all possible - hopeful I am good to go.

  17. Expert Member
    AKOS TOTH | | #33

    You are not in a windy area, so the strength difference, if any, is moot. I wouldn't worry about it. As long as they get most of the screws into the rafters, the metal roof is not going anywhere. Typical roofing polyiso is 20PSI, so a 4' length of 1x4 battens can support around 3000lb, it will take a LOT of snow to dent the foam.

    Make sure they protect the edges of the foam around the perimeter well. This is prime spot for critters, it has to be completely closed off.

  18. cliffmarck | | #34

    Perfect. Thanks again for all of the direction.

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