Replies

1. GBA Editor
   Martin Holladay | Jun 05, 2011 05:53am | #1

   James,
   It's clear that you're from the U.K. or Ireland. Most U.S. readers don't know what sarking is, and I'm not sure what you are talking about either. I'm guessing that it is asphalt-impregnated fiberboard or something similar.

   Sarking usually refers to asphalt felt or a similar roofing underlayment, but it can also refer to sheet material placed under the roofing to provide reflective insulation or additional waterproofing.

   What kind of material are you talking about?

   As to your questions:
   1. Use battens and screws (or button-cap screws) to attach your foam. Roofers do it all the time. The thermal bridging through the screws is minor and can be ignored.

   2. You still need an air barrier. EPS sheets are not an air barrier, although there are special tapes for rigid foam that you might be able to use at the EPS seams, especially if the EPS has a smooth facing on one side. Plywood or OSB under the foam would make a better air barrier, as long as the seams were taped.

2. 
   user-735498 | Jun 06, 2011 08:32am | #2

   Hi Martin, thanks for your help. I'm from Scotland. By sarking I mean wooden boards that are nailed on top of and perpendicular to rafters. They are like floorboards and are often covered with felt (then battens and tiles). Perhaps I could upgrade my sarking to be an air barrier by covering it with a breather membrane before applying the EPS and roofing on top? There will be an air barrier internally as the rafters will have a suspended ceiling fitted below which will be drywalled. What sort of tape would you use for taping the EPS? How long does it last by the way? What fixings do you use to attach the EPS? I'm having trouble sourcing fixings long enough to handle battens, sarking and 7" of EPS

3. 
   user-659915 | Jun 06, 2011 09:10am | #3

   Couple of thoughts from one James to another:

   If you're in Scotland you're probably getting a fair amount of wind-blown rain. Your corrugated roof will almost certainly continue to leak in those circumstances however tight and well-gasketed the fixings. The solution is straightforward: a layer of draped felt over the battens and below the counterbattens, dressed into the gutter at the bottom. Draped, not stretched tight so there is a well-defined drainage plane below the counterbattens. This is in addition too, not in place of the air barrier Martin calls for. This used to be standard practice in the UK when I worked there under any type of roof finish: any reason you're not using it here?

   Long screws - these are fairly easily obtainable in the US, a mail-order supplier like McFeely's may be able to ship to Scotland. http://www.mcfeelys.com. Or you could use a lesser thickness of EPS and compensate with more fiber below. I agree with Martin that the thermal bridge issues associated with the screws will be minor and not of concern here.

   The air barrier will be more useful if it's on the outside of the fiberglass insulation. Taping the EPS is a good plan. Get a tape that's designed to go with the material you're using, the manufacture should be able to supply an estimated life for your conditions of use.

4. 
   user-659915 | Jun 06, 2011 09:14am | #4

   One more thing: about the skylights. Be sure to use a good-quality roof window like a Velux and use the correct flashing package for your roof finish.

5. GBA Editor
   Martin Holladay | Jun 06, 2011 09:19am | #5

   James,
   You are using the word "sarking" to refer to what we call roof sheathing in the U.S. It's certainly true that board sheathing is not an air barrier -- especially when installed as skip sheathing. Since the EPS may shrink over time, it makes sense to install an air barrier under the EPS.

   Here in the U.S., the most common products for creating an air barrier at this location would be:
   1. Plywood with taped seams;
   2. OSB with taped seams or ZIp System sheathing with Zip tape;
   3. Synthetic roofing underlayment with taped seams.

   If you want your roof underlayment to be vapor-permeable, such products exist. In North America, the best know breathable synthetic roofing underlayment is SlopeShield.

   I wouldn't expect taped EPS to be a dependable air barrier. If you want to tape the seams anyway, I'd suggest that you purchase EPS with a smooth facing. Here in North America, the tapes used for rigid foam seams include 3M All Weather Flashing Tape #8067, Venture 1520, Venture 1521, and Dow Weathermate housewrap tape.

   For more information on tapes for rigid foam, see Air-Sealing Tapes and Gaskets.

   No one really knows how long these tapes will last.

   Your EPS needs to be thick enough to keep your roof sheathing (sarking) above the dew point during the winter. More information on that issue can be found here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

6. 
   user-735498 | Aug 01, 2011 09:01am | #6

   Hi,

   thanks for your help. I have another query: Wouldn't it be better to put an air barrier above the EPS? You could use housewrap with joints and edges taped and held in place by the battens. If you put the air barrier under EPS as you propose, the EPS (most crucially the joints between the sheets) will be exposed to cold winds which will result in bypassing. This external barrier could be installed in addition to an internal air barrier (eg ADA).

7. GBA Editor
   Martin Holladay | Aug 01, 2011 10:32am | #7

   James,
   Housewrap over EPS won't hurt, but experience has shown that housewrap isn't necessarily a durable air barrier. For example, it is often damaged during installation. But your suggestion will work, as long as installers are careful about taping rips.