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Old 10-25-2014, 10:30 PM   #1
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Best Practices Detail for Exterior Foam Board Insulation


I searched the forums but couldn't find a comprehensive thread on adding R-Value to a home using exterior foam board. I have done an exhaustive amount of research and yet still have question. Hopefully this thread can turn in to a Best Practices Detail so please only reply if you are experienced in your answers.

I recently bought a old farm house in northern NJ and the exterior clapboard is beyond repair and must be removed. The current exterior walls have NO insulation and no sheathing. The clapboard is nailed directly to the true 2"x4"s. I plan on removing the clapboard and then installing fiberglass roll insulation (Kraft paper facing in). Then install 1/2" OSB. Now this is where a ton of questions pop up and I can choose different options.

Option A: (Innie Windows and doors)
1)Install all windows and doors as customary right up against the OSB (with all proper flashing.
2)Then install Tyvek DRAINWRAP (not their standard house wrap) creating a drain plain between the Tyvek and the next layer which will be
3) Fabricate 4" exterior extention jams for all windows and doors
4) 2" XPS Foam Board (R-10) The XPS will be installed with 4" nails and washers all the way to the studs.
5) 1"x 4" vertical furring strips as nailers
6) Vinyl Siding or Hardiplank (haven't decided)

Option B: (Outtie Windows and Doors)
1) Install 2" XPS Foam Board directly over the OSB (I could even skip the OSB and use diagonal metal bracing for the framing but I would rather have a ridged backing.)
2) Install windows and doors (all proper flashing) over the XPS and secure with long screws directly to framing.
3) Install Tyvek Housewrap (no need for Drainwrap)
4) 1"x 4" vertical furring strips as nailers
5) Vinyl Siding or Hardiplank (haven't decided)

Outtie windows will look more conventional while I feel like Innie windows will be safer as far as drainage plane. With outtie windows the Tyvek will be installed over the foam board which Im not so sure about either. I really want to used the 2" XPS to increase the Exterior R-Value and reduce the thermal bridging from all of the studs.

Last question is am I correct in using XPS in New Jersey where temperature ranges from 0 degrees F to 100 degrees F? Polyiso Rigid Foam Insulation Board has a higher R-Value (extra 3.5) and is its own vapor barrier but from the research I have done so far I think that is suited for warmer dryer climates.

Hopefully we can get a quality scientific conversation going. buildingscience dot com says that both methods are acceptable but I'm still not sure.
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Old 10-26-2014, 10:08 AM   #2
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Roxul. then 1" xps over the studs. then 3/4" sheathing. then housewrap.
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Old 10-26-2014, 10:23 AM   #3
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I agree except there's no need I can think of for 3/4' sheathing on a wall.
Even better would be spray foam of you can afford it.
A house that old is likely balloon framed.
Your going to need to add fire blocking at the top and bottom of those walls.
Has the wiring been up graded in those walls?
If not now's the time to do it while the walls open.
You can not cover up knob and tube with insulation if that's what you have.
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Old 10-26-2014, 11:08 AM   #4
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Roxel instead of fiberglass? 3/4" sheathing over XPS?


Quote:
Originally Posted by Fix'n it View Post
Roxul. then 1" xps over the studs. then 3/4" sheathing. then housewrap.
Thanks so much for your reply. I had not even considered Roxul. The home is in Northern NJ (climate zone 5) so I can get quite cold in the winter. The main reason I want to at 2" XPS is for a higher total wall R-Value. To my understanding Roxel has 0 value is this correct? Would you still recommend roxel knowing this and if so why?

Sheathing over XPS? Interesting. Ive read a ton and never has that been suggested. I mean it makes sense i guess but Im not sure if it provides the structural support for cross bracing since nails or screws will have unsupported space between the sheathing and the studs. I have attached a pic from http://www.buildingscience.com/docum...ing-sheathing/ showing their recommended methods.

Have you ever done a job in the manner you are suggesting? If so what climate zone?
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Best Practices Detail for Exterior Foam Board Insulation-insul1.jpg  
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Old 10-26-2014, 11:21 AM   #5
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I was considering spray foam


Quote:
Originally Posted by joecaption View Post
I agree except there's no need I can think of for 3/4' sheathing on a wall.
Even better would be spray foam of you can afford it.
A house that old is likely balloon framed.
Your going to need to add fire blocking at the top and bottom of those walls.
Has the wiring been up graded in those walls?
If not now's the time to do it while the walls open.
You can not cover up knob and tube with insulation if that's what you have.
Thanks so much for your reply.
1) if you don't see a need for sheathing then you are suggesting metal strap cross bracing? Also code requires sheathing at the corners for support. I guess I could use 3/4" sheathing on corners, metal cross brace straps on walls and then use 3/4" foam to match the wood sheathing before Tyvek and 2" XPS.
2) I was considering spray foam instead of fiberglass in the cavities but that can get expensive quick. I calculate that I have 2,100 sq/ft of cavity space. Spray foam is by the "board foot" which is 1 sq/ft at 1" thick. The framing is true 2"x4" so at 4" thick i calculate open cell at $.50 per board foot or $4,400! with a R-Value of about 14. I find it hard to justify when fiberglass R-13 will cost about $1,100. Closed cell would be about 30% more with a higher R Value but It can get very hot AND very cold in NJ so I think the insulation must be semi-permeable. Your thoughts?
3) Yes house is ballon framed and I will be installing fire stops
4) Wire was upgraded but long ago. Its old greenfield so I will be upgrading all wiring at that time (and plumbing vents)

So many things to consider my head is spinning.
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Old 10-26-2014, 08:43 PM   #6
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Roxul is excellent insulation ! i was convinced , on this forum, to use it. and i LOVE it !

i did not do my job as i described. but i didn't remove my sheathing, i removed the old plaster and lathe inside. and i completely rewired. mine goes like this = drywall, roxul, 1" xps, 3/4 sheathing, house wrap, 1" xps, vinyl siding.

zone 5 also (iirc)
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Old 10-26-2014, 09:41 PM   #7
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I would think that you have some let-in bracing in the corners.

I would also recommend using metal bracing. Put your insulation of choice in the wall, then simply sheath with 1" xps or poly iso, or thicker if you can afford it. Poly iso will give you 7 plus 13 in the fiberglass or whatever gives you 20. XPS will give you 5 plus 13 equals 18.

I wasn't aware that it is required to use osb or plywood in the corners with metal bracing.
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Old 10-27-2014, 09:04 AM   #8
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You can get ridged board insulation with OSB attached from the factory.
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Old 10-29-2014, 11:25 AM   #9
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Hi tduballstar,
Sounds like a fun project, best of luck with the renovation.

You are correct. It’s a good idea to use a weather barrier with exterior foam. As for which goes on first, that choice is yours. We recommend using DuPont™ Tyvek® HomeWrap® on top of the foam or Tyvek® DrainWrap™ for enhanced drainage when installed under the foam. Also, DuPont does not recommend taping the foam seams regardless of whether the Tyvek® is installed under or over the foam. Not taping the seams will have very little effect on the foam’s R-value, and because of the low permeability of the foam, the open seams will help the wall breathe.

You can find more information about using a weather barrier with exterior foam sheathing, here: http://www2.dupont.com/Tyvek_Weather...h_Bulletin.pdf

Another option is to use DuPont™ Tyvek® ThermaWrap™ R5.0. It provides the air and water management benefits of DuPont™ Tyvek® weather barriers with an R-value of 5.0. This breathable alternative to exterior foam insulation can be installed behind various types of cladding and allows moisture that may get inside the wall to dry and escape to the outside, helping reduce the chance for water damage and mold. Learn more here: www.ThermaWrapR5.Tyvek.com
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