[Greenbuilding] Polyiso strength on roof

Mon Dec 8 15:17:24 CST 2014

I favor EPS because it is vapor permeable and way less expensive, although slightly less R value.  I have a supplier that can form it to specified thickness and length, avoiding waste.  That lower cost is somewhat reflective of lower embodied energy to build it.  I would have some concern that the standing seam would be a vapor barrier.  If rafters are already on 2’ oc or less, I would install sheathing.  23/32 osb or plywood meet the 15 minite requirement of Section 2603.4 2012 IBC according to Engineer wood association.  See attachment.  Another benefit to using the thicker sheathing, you can install long screws that can get the grip they need from sheathing.  At 10” thick or larger foam, it becomes challenging to hit the rafter.  On top of foam, put 2x3 furring strips  horizontally  fastened by long screw through both foam and furring strip to nail metal roofing. The polyiso can be a vapor barrier, with a perfect installation, perhaps the ventilation space needed above the insulation would not be needed.  Perfection is sometimes hard to come by. I prefer vapor permeable and 2x3 to provide ventilation with corrugated metal roof above that can span 2’ in our area.  Check the span max relative to local load requirement.  I like the prefinished corrugated metal roofing because it installs quickly and the ribs running vertical gives a vertical ventilation path to a ridge vent.  Vapor wants to rise out of the assembly.  If you use standing seam, the span may be less and you need to be careful about how it is vented.  

My thoughts for what it is worth.  

Eli 

From: John Salmen 
Sent: Monday, December 08, 2014 10:01 AM
To: 'Green Building' 
Subject: Re: [Greenbuilding] Polyiso strength on roof

Its funny as its thought that the expanding agent causes the decrease and as the blowing agent outgases over time the performance drops as well. EPS improves in cold weather. Here is a link to the building science realer world tests summary http://www.buildingscience.com/documents/special/thermal-metric-documents/thermal-metric-summary-report

From: Greenbuilding [mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of Alan Abrams
Sent: December-08-14 6:14 AM
To: Green Building
Subject: Re: [Greenbuilding] Polyiso strength on roof

and now comes Building Green, to out the news on polyiso: that its performance plummets as the temperature drops...

http://www2.buildinggreen.com/article/polyiso-manufacturers-turn-blind-eye-problems-cold-temperatures?utm_source=BuildingGreen.com%20Mailing%20List&utm_campaign=c7088d50b1-BGB_2014_12_08_members&utm_medium=email&utm_term=0_d558b0594a-c7088d50b1-157062105&mc_cid=c7088d50b1&mc_eid=d0e88c5192

-AA

Alan Abrams
certified professional building designer, AIBD
certified passive house consultant, PHIUS

certified passive house builder, PHIUS
cell     202-437-8583
alan at abramsdesignbuild.com
HELICON WORKS Achitecture and Education

On Sun, Dec 7, 2014 at 5:53 PM, Sacie Lambertson <sacie.lambertson at gmail.com> wrote:

Recommend highly staggering the EPS on the roof, using the foam sealant/glue John mentions AND covering all with thick layer (s) of felt.  We have had a condensation problem with a similar combination (metal roof attached to straps etc).  I am pretty sure it is because the foam was not staggered and or sufficient foam/glue was not used in all the nooks and crannies.  Needs some real hands on supervision and frankly better if the details are handled by you rather than the construction people.

Plywood not necessary.

We're in NE Kansas.  Lots of wind.

Sacie

On Sun, Dec 7, 2014 at 9:03 AM, John Salmen <terrain at shaw.ca> wrote:

  I would strongly recommend eps rather than polyiso for the roof insul. There are a number of benefits you can research. 2” foam can span the 5.5 centres but I would bond the staggered foam layers together using some foam adhesive (spray poly foam works well). Depending on the type of metal roofing (fastening) it could be laid directly on the foam (with a wrb) eliminating the ply layer. Metal roofs are very prone to condensation inevitably wetting the sheathing so the ply is a weak component. If ply is used a a very well applied wrb is crucial (membrane). The ply is really just there for fastening so if a z strip or other fastening system could be used it would minimize material in the assembly.

  John

  From: Greenbuilding [mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of Ron Cascio
  Sent: December-07-14 9:45 AM
  To: 'Green Building'
  Subject: Re: [Greenbuilding] Polyiso strength on roof

  Sounds good to me.

  Ron

  From: Greenbuilding [mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of Bill Allen
  Sent: Sunday, December 07, 2014 10:52 AM
  To: Green Building
  Subject: [Greenbuilding] Polyiso strength on roof

  Hi all

  I am about to reroof my 1926 colonial and am looking to also insulate per the BSC "deep energy retrofit" plans consisting of plywood sheathing, 2 layers of 2" polyiso, another layer of plywood, then roofing (in my case, standing seam metal roofing).

  The original roof was wooden shingles that are still in place with 2 layers of asphalt on top. Also still in place are the nailers for the wood shingles...roughly 1x2s, 5.5" on center across the rafters.

  My question is: if I leave the 1x2 nailers in place during the demo can i exclude the first layer of plywood and install the polyiso directly on the nailers? It would then have plywood on top, secured with long screws into the rafters. Less material and less cost.

  Thanks for any thoughts/comments.

  Sent from my cell phone

  < dy>

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