Replies

1. GBA Editor
   Brian Pontolilo | Aug 12, 2019 09:13am | #1

   Hey Andrew,

   I'm assuming that this is a low-slope roof, based on your decision to use TPO roofing. Please correct me if I am wrong.

   With that plan you are not hitting the minimum R-value for a roof in your climate zone, which is R-38, but you have plenty of insulation outside of the assembly. Please see this article for more information:

   http://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs

2. 
   Andrew808 | Aug 12, 2019 12:35pm | #2

   Hello Brian,
   Yes, the roof is low slope along the back that used to be a covered lanai many years ago. Otherwise, it is a gentle pitch ~ 12:2 I think.
   I think R-19 is as good as we can do at this point. This is a non-optimal solution by far, but trying to stop the financial bleeding a bit. The current design we were sold is not effective -- 5/8" plywood decking + 2-ply Bitumen. The home is HOT -- 99 deg yesterday. Outdoor high was uper 80's.
   The contractor is going to build up the edges (fascia?) with 2x4 to fit the 3.3" Polyiso.
   This is the maximum I have found anyone will go around here for wind lift/hurricane considerations.

   Any concern about moisture getting trapped in the foam layer between the TPO and 2-ply Bitumen?

   Thank you,
   Andrew

1. Expert Member
   Michael Maines | Aug 12, 2019 01:15pm | #3

   Andrew, as long as the assembly is dry before the TPO goes down, there is no need to worry about anything other than penetrations, seams and edges leaking. They will leak eventually, and it might be hard to tell when they do. But there is no need to worry about condensation or high humidity.

3. Expert Member
   Dana Dorsett | Aug 12, 2019 02:03pm | #4

   In paradise there isn't really much in the way of roof condensation issues that result in moisture build up over time. On clear nights dew can form on the exterior of the roofing, and when the roof is that cool some moisture is taken up into the roof deck as adsorb from the indoor air, but when the sun comes out (does it EVER shine in Hawaii? :-) ) even a low solar reflective index TPO roof will heat up enough to drive that moisture back out.

   With 3" of polyiso between the roof deck and TPO on top of vapor retardent bitumen and R19 below the roof deck, the deck itself doesn't get anywhere near as cool as the TPO, staying warmer than the outdoor or indoor air dew point the vast majority of the time. It's like Michael Maines says- as long as the polyiso wasn't pond-soaked or left out during major storms, there is no way to for moisture to build up between the TPO and polyiso layer either. What few molecules of water get pulled by the bitumen when the TPO is cooler get driven out by the sun in short order. There are no seasons where the temperature at the TPO will stay below the dew point of the indoor air for 8-10 weeks or more at a time. Even if there were (during some freak weather season) the amount of seasonal moisture getting by the bitumen AND the polyiso facers would be miniscule, not enough to affect thermal performance or support mold.

4. 
   Andrew808 | Aug 12, 2019 02:14pm | #5

   Dana,
   Thank you for those thoughts. Yes...it is occasionally sunny in Hawaii :)
   Our days of temperature meeting dewpoint are quite limited, but do occur when our typical tradewinds drop. But, the rapid heating (even of a relatively cool surface) driving moisture out makes sense. I didn't think of it, but the roof deck staying warmer makes sense -- and that is really where we want to avoid moisture.
   Great Stuff!

1. Expert Member
   Dana Dorsett | Aug 12, 2019 04:41pm | #6

   BTW: "...even a low solar reflective index TPO roof will heat up ..." should have been "...even a HIGH solar reflective index TPO roof will heat up ..."

   Even though the outdoor air temps rarely get close to the dew point, overnight radiational cooling into the cold of deep space on cool nights often drops the roofing down to the dew point, with visible wetting of the roof apparent at dawn, lingering for an hour or two. It's more obvious on some roofing types than others, but it happens with all roofs.

5. 
   Andrew808 | Aug 12, 2019 05:20pm | #7

   Thanks again!

6. 
   Andrew808 | Sep 30, 2019 10:42pm | #8

   Revisit ---

   I have spoken with two architectural firms and both will not spec TPO for roofs. They do like PVC roofs, but state that TPO has a high failure rate in Hawaii high-UV environment. The local Weatherbond TPO rep states we can expect ~ 20 years. The architects question that claim, and state more like 8-10.
   Back to water concerns -- -
   I am confortable with moisture buildup, but penetrations could cause a real prolem -- water would enter and sit ** possibly undetected for a long time ** between the TPO, foam, and bitumen.
   A new concern highlighted by the architects -- as the TPO degrades in the UV, it can allow water through tiny holes that may be tough to detect.

   Solution?

   It seems like if I want to pursue this roof, I need an air gap -- maybe purlins ? -- below the foam and basically create a channel for water to escape?

   Sounds expensive.

   At what point am I better off to just rip the whole roof off and start over with a METAL roof?
   Radiant Barrier across the rafters in closed ceiling portion
   Polyiso foam between decking to ~ R19 +
   Metal roof

   Thoughts?

   Thanks!
   Andrew

1. Expert Member
   BILL WICHERS | Oct 01, 2019 12:04am | #9

   You may want to reconsider the TPO roof if two architectural firms in your region are advising against it’s use. Those local firms will likely be advising you based on their experience with the product IN YOUR SPECIFIC REGION, not based on lab tests. In this case, I would generally go with the local experience. If they’ve seen a lot of failures, their advise to you is to help you to avoid a similar failure on your own project. That is something to think about while you’re working on your design.

   Bill

7. Expert Member
   Peter Engle | Oct 01, 2019 08:36am | #10

   I'm not sure I agree with the architects. TPO is a relatively new (1990's) product and there are many manufacturers, so performance results vary. There are certainly reports of the early formulations breaking down, especially with off-brand manufacturers. The big manufacturers (Firestone, Carlisle) still guarantee their TPO membranes for up to 30 years (on commercial applications) in the desert southwest, which is a more difficult climate than yours because of the very high temperatures in addition to the high UV loading. They do seem to be a bit less wind-resistant than some of the other membrane products. If you stay with TPO, I would absolutely make sure that it is from one of the big manufacturers, not an off-brand.

   You could also take a look at PVC roofing. Very similar in look and feel, slightly more expensive, longer track record.