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Insulation and HardiePlank siding construction options question.

colmroge's picture

Hopefully in 2013 I'll be redoing the siding on my own home, which is really past its prime.

Currently the construction is 2x4 walls with fiberglass insul, plywood sheathing, tar paper, wood siding.

My plan will be to tearoff siding and tar paper to expose sheathing.  Install housewrap and new windows.  Install rigid foil faced polyiso.  Install Hardie cement board siding.


I'm going back and forth between two constructions:

1. Install wood battens over the studs.  (Maybe 1.5") Infill between the battens with 1.5" polyiso.  Install 1" polyiso over everything with seams taped. Snap lines for the battens. Then install Hardieplank into the battens. 

2. Install two layers of 1.5" foil faced polyiso over the housewrap.  Tape seams of the outside layer of polyiso.  Then install battens.  Then install Harideplank into the battens.  (I'd be left with a gap between siding and insulation.)


Should mention that I can get some of the polyiso for free and any more that I need at cost and Hardieplank at cost ... so any combination of thickness or number of layers of polyiso can be considered.

Labor will be my own time.


What's everyone's thoughts on each of the options I've presented please?

Anything else I should consider?

Thank you in advance.

Windows? (post #207231, reply #1 of 7)

Are you doing anything about the windows?  It could be really wierd to have the windows set back up to four inches from the siding?

Typically foam on the exterior is done in conjuntion with setting windows flush with the whole insulate exterior system.


windows have been accounted (post #207231, reply #2 of 7)

windows have been accounted for.

Option 2 (post #207231, reply #3 of 7)

If you haven't read it yet get the special issue on energy efficient homes.  There is a very good article from one of the principles at Building Science Corp, on lessons learned from the conversion of his barn to the office about a decade back, and what they found when they did a complete tear off to redo the problems they had. 

The only thing I'd change on your option two is to tape both layers of polyiso.  An,d I'd install the windows in plywood boxes. 

colm (post #207231, reply #4 of 7)

Option 2 or a variation.

This does work, foam-batten-felt paper-siding.

The housewrap over the foam?  with proper air sealing of the foam (taping seams etc) is the wrap really necessary ?

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


option#2 (post #207231, reply #5 of 7)

As Calvin said #2 is the way to go.  If you use house wrap under the foam, be sure it is a stucco wrap, not the smooth variety.  It only comes in 6" tall roles if you use the Tyvek brand.  The wrinkled surface is suppose to provide a drainage plane beneath the foam.  If you tape the foam seams as suggested there should not be any moisture getting that far into the wall assembly, but if it doe the stucco wrap gives it a drainage plane to follow down and out instead of wicking to the nearest opening into the sheathing.

Duild out around the openings for the new windows and be sure the buildout is wide enough toleave a nailing surfce for the extreior trim.  Other wise you are giong to be using some really long screws to attach the trim  back to the original framing.

Thank you all (post #207231, reply #6 of 7)

Thank you all for taking the time to read my post and respond.


I have been a reader of Fine Homebuilding for quite a while, and got a subscription back in 2009 or 2010.  I also subscribe to JLC magazine and to Interface as a member of RCI (   Almost every issue of FHB and JLC has some article or piece on energy smart walls, super insulation, energy retrofit, drainage plane ... so much to learn and consider!


I do remember reading the article on the lessons learned from the barn to office conversion.  I will look for the copy.  Does anyone recall what edition it was in?


As I think about taping the outside seams on the foil polyiso and doing away with the housewrap, I wonder what is the need to tape the seams on the inner layer of polyiso if I'm doing a double layer of insulation?  If I run the 4'x8' sheets tall, and the joints are offset, how much fear should I have that water will end up behind that first layer of polyiso against the sheathing - esp if I'm above the R-7 as mentioned frequently in Building Science articles?

There was a "Lessons Learned" (post #207231, reply #7 of 7)

There was a "Lessons Learned" article published this past year that looked at the details they got right and wrong.  One of the things they got wrong was taping the foam insulation.  The problem wasn't the tape, it was the foam board itself that shrank slightly.  They were using XPS (like the blue or pink foam board).  I don't think that polyiso would have the same problem.

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