Replies

1. GBA Editor
   Martin Holladay | Jun 15, 2009 03:11pm | #1

   Several issues come to mind:
   1. Two inches of foam board is only R-10 (if it's XPS) or R-12 (if it's polyiso). That's not much. You should be thinking in terms of R-40 to R-60.

   2. Of course, you may have existing insulation under your shingles, if you have a cathedral ceiling or a cathedralized attic. However, in that case, you definitely don't want to include ventilation under your new foam -- a ventilation channel between the two layers of insulation will prevent the two layers of insulation from acting together, and will make the upper level almost useless.

   3. If you have a conventional attic with existing insulation on the attic floor, the new foam won't do much.

   4. If you expect the new foam to provide most of the insulation for your roof, you want at least 8 inches of foam.

   5. It's important to create an air barrier between the roof plane and the wall insulation. That will probably require spray foam above the exterior wall top plates.

2. 
   user-669103 | Jun 15, 2009 11:12pm | #2

   I presume that you are only talking about keeping the heat out in summer. If you are talking about keeping warm in winter we would need to know more, like is it a vented roof or a closed "warm deck". If it is a vented roof, then you might consider also converting to an unvented roof with a "warm deck", in which case you must ensure that it can either breath to the inside or to the outside and than there are no cold wood surfaces that could be condensation risks like at the gable ends.

   Also do consider that XPS does not have the heat tolerance of polyiso.

   Now to answer your question (rather than Martin's reply)...
   Lighter shingles will help reduce heat. However, you should look for Energy Star cool roof rated shingles. Not all light colors are the same, much of the effect is at wavelengths that we cannot see anyway, the only way to be sure is to see the Energy Star cool roof badge.

   Other considerations are how the roof is vented and if it is vented, but that is not the question you asked.

   ** Look for Energy Star cool roof rated shingles. **

3. 
   Riversong | Jun 16, 2009 08:01am | #3

   If, in fact, your goal is to reduce summer heat gain, then you would do better to install a foil radiant barrier and continuous soffit and ridge vents for maximum air movement. A more reflective roofing would also help.

   Where the radiant barrier would be installed depends on where the current insulation is located and on whether and how the roof is ventilated. More information is needed.

4. 
   user-672815 | Jun 16, 2009 08:02am | #4

   I have cellulose insulation (R 52) in the attic floor. The foam boards on the top of the roof are designed to keep out the heat. We don't have air conditioning and don't intend to get it.

5. GBA Editor
   Martin Holladay | Jun 16, 2009 08:54am | #5

   Peter,
   Unless you have attic ductwork -- and I certainly hope you don't -- there is no reason to keep heat out of your attic.

   If you are worried about summer heat flow from the attic to the conditioned space below, the solution is to beef up the insulation on the attic floor. There's no reason to insulate the roof above an unconditioned attic.

   R-52 is pretty good. It should go without saying that you need to have a tight air barrier under your attic ceiling (for example, drywall with all penetrations foamed or weatherstripped). However, if you want an R-70 ceiling, go for it. Just blow in more cellulose.

6. 
   user-672815 | Jun 16, 2009 10:26am | #6

   I'm using Cool-vent composite insulation board that consists of a 4 by 8 panel of Nexgen polyiso, a middle layer of solid wood spacers abd a yop layer of APA?TECO rated OSB or plywood. thuickness 3.8 inches R value 14. Soffit and ridge vents will be included

7. 
   Riversong | Jun 16, 2009 12:39pm | #7

   Peter,
   You're wasting your money on roof insulation. What you need is cool roofing, good attic ventilation, and a radiant barrier underneath the rafters (in that order of priority). Venting the attic will do more to evacuate moisture than venting above the roof membrane. Thermal insulation does little to prevent radiant heat gain. If the polyiso is foil-faced, it will offer a radiant barrier, but that seems an expensive and excessive way to achieve your goals.

8. 
   user-672815 | Jun 16, 2009 01:12pm | #8

   I spoke with experts from two national roofing companies and they "both" said in upstate NY it is not really worth it. They said what i would gain in reduction of heat buikd up I would lose by extra heating costs in the winter. We will have full soffit and ridge vents put in.

9. 
   Riversong | Jun 16, 2009 01:34pm | #9

   Peter,
   Good decision. Caveat: the only ridge vents which function as designed in all wind conditions without allowing reverse flow and moisture intrusion, have external wind baffles. AirVent (ShingleVent II) and Lomanco (OR-4 or LOR9-4) both make such shingle-over ridge vents. I would not use anything else.

10. 
    user-672815 | Jun 16, 2009 02:00pm | #10

    Thanks for all the responses. To be clear the two experts recommended against "cool" roofing shingles because of our location in upstate NY Access to the attic space is extremely limited so adding reflective foil on roof rafters from beneath is problamatic. With a full soffit vent running the entire length of the soffit versus soffit cut ins every three feet (this is what we have now) maybe the ventilated foam board is overkill. It will probably cost us an extra thousand or $1,500.

11. 
    bill jr | Jan 13, 2010 08:00pm | #11

    to peter i live in buffalo i put a cold roof on striped the shakes off then put 2 by 3 stood on edge then 2"foam then 2by3 and so on making sure that you leave the first 2by3 in about 24" so your inside the lower wall then go across the whole roof and do the same thing on other side. i then installed three layers in the rafters and caulked every piece then dry walled no ice not a drop i have a story and a half from 1926 nice in the winter and cold in the summer i used the blue foam some with does not matter same stuff just company. no moisture at all heat it with a small pellet stove about 4 ton not bad for a 3000sq old house