Replies

1. GBA Editor
   Brian Pontolilo | Jun 17, 2019 08:52am | #1

   Hi Mike,

   Is the shiplapped sheathing going to be your finished ceiling?

2. 
   pakrat1 | Jun 17, 2019 11:20am | #2

   the attic will be conditioned so any fumes / odors would enter the interior of the living space

3. 
   pakrat1 | Jun 17, 2019 11:21am | #3

   Oops forgot your Q / standard attic configuration with ceiling being sheetrock

4. 
   Peter Yost | Jun 18, 2019 10:29am | #4

   Hi Mike -

   what function(s) does the I & W (assumes this stands for Ice and Water shield?) serve?

   If this is your concealed bulk water management control layer, why not put it topside of the rigid insulation and your plywood? Then you can tape the rigid insulation as your continuous air control layer and this will keep any off-gassing from communicating with the interior of your home.

   If your I &W is topside, be very sure it is the hi-temp type because if on top of your rigid insulation, it will be seeing high temperatures in Houston.

   Peter

5. GBA Editor
   Brian Pontolilo | Jun 18, 2019 10:48am | #5

   Mike,

   I have heard of the off-gassing odor of peel-and-stick roofing membranes infiltrating homes. In fact, we just had a question about this in the last couple of weeks. I like Peter's idea of installing the membrane over the foam. Also, make sure that your drywall ceiling is airtight and that the room has good airflow.

6. 
   pakrat1 | Jul 26, 2019 07:06pm | #6

   Well I just created a new post showing my options. It is essentially the same as this one. I couldn't find this post till I posted the new one

   Answer to peters response, I am concerned that I&W on top deck would not stop leaks from faulty roofing or leaky ridge vent install getting in the lower layers of foam then ship lap gaps

   also what about air seeping past the 7+ thousand holes (which are the size of the screw threads which are larger than the screw shafts ) then under the foam and into the ship lap gaps, I would bet these add up to a square foot of more over a 70 sq roof Kind of tough on the ACH50 numbers

1. Expert Member
   Akos | Jul 26, 2019 07:49pm | #7

   Your screws should go into the rafters not the shiplap. If you are worried about roof leaks under the foam, put a layer of underlayment, much cheaper than peel and stick.

   Ship lap leaks in two ways. From inside through the gap to the outside and from the inside along the gap sideways to the outside. Peel and stick only fixes the first, taping the foam does the same thing for way less money. About the only way to fix the 2nd is to rip through the ship lap about half way along the top plate and caulk/spray foam.

   If you are finishing the inside with drywall, any reasonable attempt at air tightness will keep most smells from the roof out. Many houses here with no insulation with torch down directly on lumber decks with drywall ceiling. Might smell a bit for a new install, but goes away.

7. 
   pakrat1 | Jul 26, 2019 08:57pm | #8

   Akos

   Most of the vented nailboard MFGs say to follow their nail patterns which do not match the rafters, besides, the problem is the 7 thousand holes that end up larger than the screw shanks after a spinning screw shreds the sides of the hole. Because the ship lap is not perfectly flat air can move from the holes to the gaps and into the attic or go from ship lap gaps to holes to vent channel. Remember we are talking about at least 7 thousand screws - sort of like 7 thousand staples without caps in the underlayment. This attic will be conditioned and coupled directly to the house so any odor could be a problem
   I am planning to put a strip of peal and stick from the eve to just inside of the plate all the way around the entire perimeter of the roof and use spray foam to fill the gap between the plate and underside of the ship lap

   I debated about trying to mark the rafters except they are not very evenly spaced, 70 year old house, but instead am going to put a lot of ringshank nails into the planks and go with the MFGs screw spacing. I will probably attempt to hit some rafters but 10" screws can be tricky to hit a target near the center 7" away

1. Expert Member
   Akos | Jul 27, 2019 07:43am | #9

   I didn't realize you were working with nail base. Out of curiosity why did you go with this instead of regular rigid insulation if you are strapping it out and putting a layer of plywood on top?

   If you really want to make sure this is air tight, your best bet is to put a layer of either open or closed cell on the underside of the deck. You can recoup some of the cost of as it can replace some of the insulation on the outside. 4" of open cell between studs is about the same performance as 2" of rigid.

   Otherwise, go with the peel and stick and air tight ceiling. Even if you are careful, this will not be as tight as the SPF approach.

8. 
   pakrat1 | Jul 27, 2019 02:15pm | #10

   having forgotten this thread I started the thread below which shows my concerns and stackup options

   Just to clarify the plan was either pre built nailbase/1.5" vent channel/2" foam on top of staggered 3" layer of foam or 3" and 2" foam layers and site built vent channel using 2x4s and plywood top deck

   using nailbase, the only way to create a proper air barrier is on top of bottom deck or top of bottom layer of foam problem is peal and stick on bottom deck would protect against leaks because screws would not shread the sealing material BUT I am concerned about the smell from the bitumen through the ship lap

   peal and stick on bottom foam layer -- no smell -- but as stated above, lots of leaky screw holes

   site built ( lots of hips and valleys make for tricky furring strip install ) gets labor intensive ( know anybody in Houston TX with this kind of experience ?? ) but the furring strips would inprove the sealing ability of the peal and stick on the top layer of foam

   Thanks for your advice
   Mike

   Link to the new thread
   https://www.greenbuildingadvisor.com/question/options-for-air-and-water-barrier-and-local-qualified-builder-for-this-roof-system?discussion=new