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EPS on flat roof

walnut23 | Posted in Energy Efficiency and Durability on

Hello, we are building a new home and we are going to have a large open space or Great Room with 16′ ceilings.  The walls are going to be 10″ exterior concrete with 2″ CC spray foam and batt insulation in between 2 x 4 studs.  The ceiling is going to be stick frame with 2 x 12 joists at 24″ OC.  Floor to Floor height is going to be 16′-0″.  The exterior concrete wall is going to be a parapet wall that stands roughly 1′-6″ above the flat roof.  Finally to the question part…flat roof assembly.  I am in Maryland, climate zone 4 and planning on roof assembly as follows: 2 x 12’s with fluffy batt insulation at R-30, 2″ CC spray foam on 3/4″ sheathing on top of joists.  2″ Rigid insulation on top of sheathing, and TPO flat roof.  Does this sound like a good assembly?  any recommendations?  Roofing felt over the plywood prior to rigid foam?  Do you foresee any problems with not having another layer of sheathing over the rigid?  Do I need to have some kind of protection board over the rigid – We are planning on installing solar panels in the future.  We will have radiant heating in the floors to help with high ceilings in the winter time.  

Also would there be any advantage or even possible to place the rigid on top of the joists and then the plywood sheathing and then the TPO?

Thanks and look forward to your replies

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Replies

  1. onslow | | #1

    walnut23,

    There are a great many questions that arise when looking at your drawing. Have you had any input from a structural engineer at this point? Also, have you checked with local code or found a builder that is on board with your ideas?

    At the risk of sounding harsh, the drawing suggests many fail points and engineering challenges. First off is the wall height which will be just under 20' at the parapet top if you really intend to have 16' clear under the roof rafters. If also concrete, the two walls parallel to the roof need to be provided stabilizing force. This need could have profound effects on your general design if indeed you can cast a wall that tall without hiring commercial level concrete specialists. The notch/ledge as shown in your diagram makes no sense.

    Additionally, you do not specify any form of cladding. If plain concrete, your wall will become a massive moisture sink. The same wall if not isolated from your interior slab will suck the heat out of the slab at the perimeter. Hopefully you have read here on GBA about sub-slab insulation.

    The roof details are not defined enough to say whether 2x12 material is up to the task. The span if undivided will be limited. 24" centers seem untenable. You will also find that good 2x12 stock may be very hard to obtain. TJI stock is much more manageable. Trusses offer many advantages over either. Only an engineer can say and only and engineer can certify plans.

    The flash and batt proposed is not a good idea. Thanks to the membrane roof, you will be exclusively drying to the interior, a task confounded by the air stacking effects you will encounter with such a high ceiling. You will need to have some way to pull air off the ceiling for both heat and A/C purposes. Despite being in CZ4 you still need to bias the insulation to the side above the first roof sheathing. I do not know how Maryland has faired in the recent arctic bulge, but CZ4 often uses 14-15F as the 99% heating design point. Best to design a bit beyond as additional R above will still be useful against summer heat load.

    You haven't indicated any lighting plans, so be mindful of penetrations in the ceiling plane become entrance points for humidity to gain access to the underside of your sheathing. I would suggest R-19 batts under and R-30 or higher on top. You will definitely need something better than tar paper as your first roof barrier. I would urge a self adhering and fastener sealing grade of material. You will be make a lot of penetrations from above.

    Nail-base panels could be a good choice for above sheathing insulation though you still need to provide a pitch to drain the roof. Commercial roofing insulation can be had in tapered form, but simply having a sloped truss is much easier for most trades to deal with. You will definitely need to have your engineer factor in future solar panel weight, especially since you will need to go with a weighted anchoring system on a membrane roof. Plan now for roof to utility room access for the electrical feeds. Even with nail base panels it may be necessary to add another layer of sheathing to provide a smooth foundation for the membrane.

    I would look into TPO, EPDM and bonded PVC membrane materials carefully. I went with IBS brand of PVC membrane and the warranty is something to check on other materials. Again, expect to find qualified installers by looking at commercial roofers. The details matter a great deal.

    Even if you aren't subject to local codes or inspectors, I urge you to put forth more details and seek out a good structural engineer.

  2. walnut23 | | #2

    Thanks so much for the detailed and frank response! We are just finishing up the design development phase and trying to move into construction documents. We have a structural engineer on board who has already approved height and thickness and broad details for rebar, etc. but are not going to move forward with any CD drawings until the budget is approved. Great point about penetrations in the ceiling and sub surface insulation. Ultimately, my sketch is more directed toward the insulation aspect of the roof assembly. To be clear, you would recommend R-19 batt below the sheathing, between the ceiling joists, some kind of GRACE or similar self adhearing membrane over the sheathing and then roughly 6” of XPS rigid insulation with membrane roof on top? Thanks for your time and look forward to your response.

  3. Expert Member
    Michael Maines | | #3

    In climate zone 4 for an unvented roof, you need at least 30% of the total R-value to be closed-cell foam, and at least R-15. You have over 40% in the R-21 foam layers so you're safe from moisture accumulation ("condensation.")

    Whether it's safe to enclose the structural sheathing between two layers of closed-cell foam is debatable. It's safer to allow drying in at least one direction, but if the sheathing is dry when the foam and roofing are installed, it should remain dry and free from problems, as long as there are no mistakes during construction or accidents after construction.

    If you focus on air sealing, the stack effect won't have much of an impact. Do multiple rounds of blower door testing and aim for 1.0 ACH50.

    You can install EPDM roofing directly over polyiso foam; that is how the vast majority of commercial buildings are roofed. The foam compressive strength for that application is usually higher than what you can buy at a big-box store; talk with your engineer about what to specify--they will probably recommend 25 psi polyiso.

    I agree with onslow about the unfinished concrete. Unless you take special precautions, advised by an expert, you will end up with a giant sponge with rampant mold.

    It's probably too late but in case it isn't, I have to make a pitch for not building a house with these materials. We are in a climate crisis and the concrete and foam you are planning to use have very high levels of up-front (embodied) carbon emissions. There are many ways to build with lower levels of embodied carbon emissions, while still having a durable, attractive, comfortable home.

  4. onslow | | #4

    walnut23,

    I would second Michael's plea for alternative building materials now that that cat is out of the bag. I suspect it will be a lot less costly what with steel and other materials still up above a few years ago. If a modernist look is important, you could look into commercial building panels. That aside, I am relieved to hear you are working with a structural engineer. I will put a few links below to some bedtime reading you might want to try. Many of the things that bothered me after seeing the drawing you attached may be being addressed by the engineer already, but if you don't know what questions need answering you can't ask them.

    While EPDM may be adhered directly to foam I would look forward to the time when replacement or repairs arrive. Will it be possible to do either without damaging the foam? I have used fiberglass faced polyiso foam, often used for roofing insulation, and I have used recycled XPS from a building roof tear off. The XPS is at least 25 psi grade and would survive a great deal more foot traffic than the polyiso. I mention this as my experience with EPS on my own roof is a bit mixed. It is not very tough in the grade used for my nail base panels, so it is only the OSB surface that made it workable or screw-able. My PVC membrane is not adhered but mechanically fastened and heat sealed at the laps. I am trusting to be off planet before it fails.

    You mentioned the possibility of solar panels on the roof in the future. The need for weighting down the panel mounts combined with possible high underside temperatures suggests to me that a final uppermost layer of polyiso insulation on the roof would help protect against heat pushing either EPS or XPS to its limit. EPS can soften at 165-180F, a temperature my metal roof can hit. I have not had the opportunity to measure solar panels on a roof and may be wrong about underside heat. Still it would bear investigation.

    You haven't indicated what kind of drainage you have planned for the roof. I have two sections of three sides of parapet with the fourth, down slope end completely open. Scuppers can work, but if leaves collect on the roof it can lead to problems. Mostly the scupper flow capacity must match the worst rainfall expected. Parts of Maryland receive as much or more snow than I do here in Colorado, so plan for icicle issues and possible plugging if going with scuppers. I would strongly recommend against the drain style often seen on big box stores and the like. Very prone to problems. Remember, no one will see behind the parapet, so go for as much pitch as you can.

    Here are some links to Building Science papers that might better outline the concerns I had when viewing your preliminary sketch. It may seem like a lot but the presentations are pretty well done.

    https://www.buildingscience.com/documents/digests/bsd-controlling-cold-weather-condensation-using-insulation

    https://www.buildingscience.com/documents/digests/bsd-012-moisture-control-for-new-residential-buildings

    https://www.buildingscience.com/documents/digests/bsd-138-moisture-and-materials

    https://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion

    https://www.buildingscience.com/documents/confpapers/cp-1201-masonry-wall-insulation-interior-embedded-beam-simulations/view

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