Replies

1. GBA Editor
   Martin Holladay | Aug 12, 2015 03:30pm | #1

   Asphalt felt is a vapor-permeable roofing underlayment (it's a smart vapor retarder) that is quite affordable. It may not be quite as vapor permeable as Solitex Mento, but the price is right -- and it has a century of successful field testing behind it.

2. 
   dirkgently | Aug 12, 2015 04:29pm | #2

   Thanks Martin,
   my project details are based on your advice from last spring.
   I really wanted something that will stand up to the weather for an extended period of time once the shingles are stripped. It may be a while b4 the cellulouse guys can blow the rafter bays full on top of existing fiberglass.
   Then it may take days to install 6" of EPS (type IX), 2x4, sheathing, then roofing.
   small roof area 7.5 square.

   I am hoping the system will dry to the exterior because of interior poly installed in 1980.

3. Expert Member
   Dana Dorsett | Aug 12, 2015 05:06pm | #3

   If you want the thing to be able to dry to the exterior at a reasonable rate you'll want to be using Type-I EPS, and not Type-IX- it's literally twice as vapor permeable.

   Type-IX EPS has a vapor permeance of about 0.3 perms @ 6" (at the vapor-tight end of Class-II vapor retardency ) whereas Type-I would be about 0.6 perms @ 6" (in the mid-Class-II range.)

   At 6" EPS of any density would be quite a bit more vapor-tight than #15 or #30 felt when the moisture content of the felt is elevated by moisture in the roof deck that needs to dry.

4. 
   dankolbert | Aug 12, 2015 06:15pm | #4

   Where is the vapor going after getting thru the underlayment? I can't imagine asphalt shingles that have adhered to each other are very permeable.

5. Expert Member
   MALCOLM TAYLOR | Aug 12, 2015 06:46pm | #5

   Dan,
   That seems to be Uncle Joe's conclusion too:
   http://www.rci-online.org/interface/2011-12-Lstiburek,%20Karagiozis,%20Gassman.pdf

6. GBA Editor
   Martin Holladay | Aug 12, 2015 07:44pm | #6

   Dan,
   Depends on the roofing. Concrete tiles, slate, and cedar shingle roofing are all vapor-permeable.

7. 
   dirkgently | Aug 12, 2015 08:47pm | #7

   Dan, 2x4 spacers on top of EPS creates air gap under new plywood deck. shingles go on that new plywood deck

   Hi Dana, I actually hope it is Type I EPS now....It is re-claimed stuff I got from Insulation Depot and they advertised it as unknown type.....judging from the car tire marks on a few of the sheets I guessed the compressive strength must be great enough to make it type IX.

   If it is possible I will try to R&R a row of plywood and remove some of the poly sheeting below the insulation....That way I do not have to worry about drying to exterior.....but, won't know till the shingles come off.

8. 
   dirkgently | Aug 12, 2015 09:15pm | #8

   One thing I considered trying for temporary waterproofing was Drainable housewrap on the existing roof sheathing....as Joe L said it helps create a tiny air gap which makes foam play nice with sheathing. He felt the tiny thermal penalty was worth the extra drying potential.

   I may try to create my own version of drainable wrap gap using stips of 30# felt as a spacer between the Solites Mento and the EPS foam.

9. Expert Member
   MALCOLM TAYLOR | Aug 12, 2015 09:41pm | #9

   Dirk,
   From my perspective the chief benefit of synthetic underpayments is safety and ease of working on the roof.

10. Expert Member
    Dana Dorsett | Aug 13, 2015 10:51am | #10

    I had assumed from the framing of the question that this was a vapor permeable roofing (steel on purlins or something). If it's asphalt shingles on a sacrificial nailer deck it doesn't really matter- an impermeable peel & stick and foil faced EPS is as good as #15 felt and rigid rock wool, since nothing is getting through the ~0.1 perm asphalt very quickly.

    If the nailer deck is vented (say, mounted on 2x furring or purlins) there is a difference. Since there is poly sheeting on the interior, a vented nailer deck & at least somewhat permeable insulation & underlayments on the structural roof deck are advisable.

    If you're not sure of the density of the EPS it's easy enough to weigh a chunk of known dimensions and run the numbers. It's unlikely to be Type IX if its roofing EPS- Type-I or II is the best bet, but it's easy enough to tell. Type-I is about 1lb per cubic foot, give or take, Type-II, ~1.5lb, and Type-IX, ~2lb. At 6" Type-II would run ~0.45-0.5 perms (~4-5x the drying rate you'd get through asphalt shingles on #30 felt.)

11. 
    dirkgently | Aug 16, 2015 04:44pm | #11

    Just cut a piece of foam to be equivalent of 1 cubic foot.
    It weighs 1# So it is type I.
    Great trick Dana thanks

    I used a new cutting technique. I put a Masonry Blade in my skill saw and it made a perfect cut with almost now dust.
    I hate having all those Styrofoam beads blowing all around the site contaminating the woods.