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Cool roof on a rigid foam retrofit

David Mosijchuk | Posted in Green Building Techniques on

Hello,

I have been doing extensive research prior to begining my remodel on my new home. I would like to get some advice on how to aproach my remodel.

The home is a 2600 sq ft contemporary style house. The majoroty of the roof is an unvented catherdral roof assembly. It is a simpole gable roof, although the North face is much steeper. It currently has an aging asphalt roof that will need replacing soon. I am planning a PV array instalation and need advice on what sort of assembly would be the best combination.

Based on research I know that a metal roof is most likely my best option for longevity and performance with the PV array. I saw a few rigid foam retrofit roof details on GBA. These were hot roof assemblies. However, I was curious if anyone had run across a cool roof assembly with a rigid foam retrofit? See the attached image for specifics. The rigid foam has 1×3 battens installed over the top with a pre-galvinized continuous vent installed at the eave.Over this goes the new sheathing, then the ice and water shield. Finally the standing metal seam would go on. There would be a ridge vent installed along the top.

I would like to know if there are any benifits to this type of assembly. I realize that the venting would certainly aid in cooling the metal, however, in seaching for a cost effective solution, I am unsure if any benifits would be marginal and not worth the extra expense. I look foward to any insite on this.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    David,
    The approach you describe is the best way, by far, to insulate a cathedral ceiling.

    Make the rigid foam layer as thick as you can afford -- R-49 is not too much insulation, although it's common to install less in hot climates -- and you'll have an excellent roof assembly.

  2. David Mosijchuk | | #2

    Martin, I appreciate the immediate response.

    As far as foam thickness goes, I am seeing recomendations for a zone 5 house to have an r value of 20. Using XPS, this would translate to 4 inches of foam. Would a 2.5" layer taped and sealed, toped by a 1.5" layed with stagered seams, taped and sealed; be an effective aproach? I am assuming that the 2x10 rafters have fiberglass batts, just based on the insulation I see in the small attic space and 2x6 walls in the house.

  3. GBA Editor
    Martin Holladay | | #3

    David,
    If you are fairly sure that the existing rafter bays are insulated with fiberglass batts, you can count the R-value of these batts as contributing to your R-49 goal. (Of course, the quality of the installation makes a difference -- many batts in cathedral ceilings are close to R-0 in performance.) It's important to verify that the rafter bays aren't vented; if there are soffit vents and ridge vents, these must be carefully sealed.

    You're right that R-20 rigid foam above the roof sheathing is enough in Climate Zone 5. (For more information on this topic, see How to Build an Insulated Cathedral Ceiling.)

    Most green builders avoid the use of XPS because it is manufactured with blowing agents that have a high global warming potential. Either polyiso or EPS would be a better choice; in cold climates, EPS is preferred because it performs better at low temperatures than polyiso.

  4. Dirk Gently | | #4

    David and Martin,
    since this is a retrofit would it be wise to check if there is interior plastic sheeting as a vapor barrier before proceeding with this type of rigid foam roof insulation?
    Until a true breatheable rigid foam insulation comes along to install on roof decks.....many of us are doomed to have cold leaky ceilings (unless interior drywall is removed to rip out poly barrier).

  5. David Mosijchuk | | #5

    Dirk, I had wondered the same thing. At this point, I won't know exact details of the roof assembly until I begin pulling it apart to replace the asphalt shingles. I may be able to gain access possibly in the small attic that is besides the cathedral roof to examine wether there is plastic sheeting underneath the sheet rock.

    Martin, to address several of the points you made. There are no soffit vents nor a ridge vent on the majority of the roof, a 1/4 of it on the west side of the house has an attic space that does have a ridge vent. I plan to convert this to a condition attic as I currently have duct work in there. In regards to the foam choice, I was drawn to the XPS due to the fact that EPS and Polyisio both have features I find to be a disadvantage in my situation. EPS has a much lower r value and therefore I would need much more to attain the R values I am looking for. As far as Polyisio, I have read that the r value performance degrades in cold weather due to the blowing agent gases beginning to condense. This is inconsistency is not something I would like as I would much rather the house perform consistently in the winter rather than the summer.
    Finally, I am purchasing the foam second hand, in order to recycle and reduce the impact my choice of insulation has on the environment. I have found a local individual that has factory seconds, surplus, and used sheets of foam.

  6. KEVIN ZORSKI | | #6

    David - Installing a first layer of polyiso followed by a layer of EPS, will keep the polyiso warmer and consequently won't degrade as much in cold weather. You may be able to get these foams second hand as well. Recycled XPS is certainly greener than new, but it could be argued that recycled EPS and polyiso are greener still.

  7. Dan Burgoyne | | #7

    Under metal roofing, you would want to use Grace's Ultra HT Ice and Water shield product, as it performs better under high summer temperatures.

  8. David Mosijchuk | | #8

    Kevin,

    EPS does have a muh lower r value and therfore I would need a thicker layer of foam to achieve the desired r value. In the end wouldnt the outer layers of polyisio achieve the same end result as a combination of polyisio and EPS?
    Also, how much of an impact do cold temperatures have on the performance of polyisio?

  9. Brad Hardie | | #9

    David,

    The best combination and bang for your dollar is EPS over Poly-Iso, just make sure to stagger the seams horizontally and vertically. There are many places to buy used insulation, and many types of "Gently Used" insulation are available. Try http://www.insulationdepot.com, they had great prices for me, and large quantities were available or could be sourced with advance notice.

    Don't forget that because XPS is a blown insulation (like Poly-iso) and will lose R-value over time, as the blowing agent off-gases, although it won't get Thermal Drift from cold temps like Ploy-Iso.

    EPS, because it is expanded does not degrade over time.

    Another aspect of EPS vs. XPS - EPS is semi-permeable, and if it does get water in it, it will shed it. XPS, although more resistant to water and vapor, once it is wet internally can takes months to rid itself of water. Just place a sheet of XPS in a puddle of water for a few weeks, and then take it out after being there for a while and the XPS will still hold the water like a sponge. This further degrades the R-value and long-term studies have been done on XPS that was placed subgrade and then taken out and tested.

    EPS is also partially recycled, and can be in the future.

    Don't be surprised to see the R-values of exterior insulation go up in future code books to prevent condensation issues mid-wall that can continue if you use the current code as a guide.

  10. David Mosijchuk | | #11

    Brad,
    A very thourough and informative response. That does change some of the details for the roof assembly. I appreciate ypou taking the time to offer some "seasoned" advice.

  11. Kevin Dickson, MSME | | #12

    I would paint that white stuff as shown in the photo. Get some latex mis-tints.

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