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Community and Q&A

Poly-carbonate roofing retrofit, potential issues, and best practice solutions…

Chris Laumer-Giddens | Posted in GBA Pro Help on

PROJECT: Major energy retrofit in Jonesboro, GA (30 min. South of Atlanta).

CONDITIONS: The roof over the foyer is a wood frame structure, with (2) 2×10’s sistered and placed at approximately 4′-0″ o.c. Directly on top of this structure is a 20-year old poly-carbonate material, approximately 15 mm thick. The owner says there have never been any water leaks, but the blower door test results (11k cfm50: Vol – 50k ft3), seems to point to some major air leakage in this assembly. In the middle of the afternoon, the surface temperature on the INSIDE of the roof exceeds 140 degrees F, and the energy model (Manual J) comes up with a cooling load that is ‘through the roof’! (approx. 70k – 80k Btuh for just the foyer).

SOLUTION: We have gone round and round on a good solution, including replacing the material altogether. The cost almost of this approached in dollars what the Manual J came up with in cooling load, so we set that one aside…permanently.

The latest, and the one we all like the best is to air seal the existing assembly, then attach 3/16″ glass coated with Llumar’s “One-Way Mirror” film on the underside of the structure. Each section of glass will have a frame with a flange for attachment to the wood frame. The film is obviously reflective, and has an SHGC of 0.16. We are planning to provide a gap for ventilation around the perimeter of each panel by holding it off the wood frame.

1. Condensation in winter
2. Amount of heat within the space between glass and poly-carbonate (glass and poly can handle the heat, but what other issues?)
3. How much of a gap would be enough for proper ventilation?
4. Is air-sealing the existing assembly a good or bad idea?


If you want to see a picture of the roof, there is one in this article:

Here is a link to the performance specs of the solar film.
Llumar One-Way Mirror Specs

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  1. Keith Gustafson | | #1

    All the smart folk must be on vacation or hiding below ground from the heat

    I don't know if the film you propose would be a 'problem' but I think it would be smarter to keep the heat out of the building first.

    I'm thinking some Christo style sail thingy suspended over the foyer outside, very chic, very artistic

    what about regular old window tint film, will keep the heat out a bit anyway. won't help with cold

    I am wondering if it can be all that air leaky if it really doesn't leak water.

  2. James Morgan | | #2

    External shading and an improved spec on the poly could make marginal improvements but if you're serious about controlling both the horrendous solar gain and the winter heat loss through this thing the answer is pretty obvious. The pyramid of polycarbonate needs to be replaced with a properly insulated solid roof elevated a couple or three feet above the current structure to make room for a continuous clerestory of high SHGC vertical glazing. This can admit plenty of light without the ridiculous energy penalty. The glazing can be electrically operable for highly effective passive ventilation.

  3. Michael Chandler | | #3

    I agree with the concerns about the glazing underneath creating unintended (negative) consequences. Charlies Greenhouse supply sells a low-E twin-wall and triple-/ quad- wall poly carbonate green house glazing that is pretty economical, and attractive and comes in 4' x up to 16' sizes. I'm thinking about the old Kallwall insulated panels that had white fiberglass between two layers of their glazing product (fiberglass with aspirin as a UV inhibitor) so there may be a way to make custom inserts that would fill the ten inch cavity and have transparency as well as R-value. you would have to account for the expansion and contraction of the poly carbonate so I imagine that you would be wrapping the rafters in metal and then using a glazing bar to attache the poly carb to the top and bottom and dense packing the 9" space between with Spider or some other white fiberglass material from above before setting down the upper glazing.

    But the main reason more folks haven't responded is that there is just too liuttle information, photos, size of the glazing assembly etc would be very helpful. James refers to this as a pyramid but i don't see where he got that from your question. (By the way electrical window operators get pricy fast)

  4. John Brooks | | #4

    James followed the link that was posted
    I agree with James
    Clerestory is a better option.

  5. James Morgan | | #5

    I was a tad hasty suggesting that a a clerestory was 'the' solution to this problem - I confess I was fighting the urge to just write 'demolish'. On reflection there are a number of other approaches which could be applied to this situation to effect a major improvement to energy performance. None of them are likely to be inexpensive however, and some of them would significantly change the character of the space below. The linked article shows a volume which seems to be more atrium than foyer - actually it seems to be an attempt to emulate the impluvium of a Roman villa without the inconvenience of rain falling into the middle of the house. A contractor friend not noted for his diplomacy has been known - when confronted with a deeply foolish idea - to calmly ask a client 'just how stupid can you afford to be?'. For the original client in this case, the president of a bank, the answer seems to have been 'very'. Even very smart people make mistakes and sometimes they're a doozy. Roof glazing seems to a favorite. The mushroom (lilypad?) columns at Frank Llloyd Wright's Johnson Wax building were a resounding success but the Pyrex glazing used to fill the gaps leaked horrendously from the get-go. Eventually they had to place an entire secondary roof over top and install artificial lighting over the Pyrex to halfheartedly simulate the intended daylight effects.

    Michael's suggestions are less radical than mine and not without merit - I do think some external shading would still be desirable - some of the roof framing (the hip rafters) seem to be steel by the way. Keith - don't ever believe that if an assembly doesn't leak water it therefore doesn't leak air. Air is rather indifferent to direct gravity effects and is quite happy to move upwards and sideways as well as downwards. Good idea though about the shade sails.

  6. Chris Laumer-Giddens | | #6

    Thanks for all the great suggestions and discussion.

    We have considered demolition several times, including just removing the poly-carbonate and replacing it with a better performing product. The labor for just removal was $20k...The total cost, including new product and labor was $40k.

    So, we moved on to other options and concluded that the one described above may be the most economical, simple and (we hope) the most effective for the price.

    One of the driving forces is that the client really likes the effect of the roof that is in place, but not the performance. Even at 30' above finish floor, the heat radiating all day is very uncomfortable. Adding the layer of glazing on the bottom will retain the look, lighting effect, and give the increased performance.

    What I'd really like to get feedback on is whether the given solution has serious concerns with regard to condensation and heat. Also, is air sealing the existing roof a good or bad idea. What suggestions would anyone have on how to make this solution the most effective, while keeping the cost to a minimum and overall aesthetic in tact.

    Thanks, again.

  7. Keith Gustafson | | #7


    Oh I have been in buildings where you could see light and they did not appear to leak, but I don't think this roof could get away with being awful without leaking. Maybe at the junction of the plastic assembly to the building it might have good flashing but not air sealing.

    I wonder if the JW building could be done today with modern materials and sealants......boy he did some doozies

  8. Bob Coleman | | #8

    since this project ventures into the area of 'hackology', one thought along that line is covering the exterior of the roof with some sort of long continuos clear film that had a low SHG

    this would seal the air leaks, reject the heat before it enters the structure, and not cost a fortune or require much work. the polycarbonate would hide that its there

    i'm not sure if such a thing exists, or if you have to stitch together smaller pieces. also possible it doesn't hold up but 5 years due to external abuse, but then you just lay another layer over it.

    but like a prior posted said, if you want a legit sun room, you need to tear it off and do it right. your plans sound like a lot of expense and effort for little gain

  9. James Morgan | | #9

    Chris, I think the bottom line is that your plan will help with the comfort conditions inside the space but will offer less than stellar improvement to the overall energy performance of such a poorly-conceived roof assembly. Air sealing the polycarbonate would seem to be a must, venting the perimeter of the new under-layer of glazing will reduce the possibility of condensation inside the assembly but again it will not help the overall energy performance. There's not going to be any major environmental gains here. The rest of the energy upgrades which which you are undertaking around the home should be far more rewarding. Boy, that ductwork is about as bad as any I've seen. Good luck!

  10. John Brooks | | #10

    Edit to say...nevermind

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