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mike_duclos 10-08-2007 08:41 AM

Hardiboard siding over Rigid Foam Board

I'm new to the building trades, and I'm working on computer modeling
of residential housing to provide estimates of cost savings
resulting from energy efficiency improvements.

I've modeled my home in REMDesign and the results show
that if I can reduce air infiltration from 2800 CFM50
to about 1800 CFM50 and add R6 of foil faced
polyisocyanurate over the sheathing, I can save
about $325 per year in fuel oil costs.

I'm going to be replacing the siding and the windows,
so this is the only opportunity I have to do this work cost
effectively. I plan to use foil faced polyiso board because
tape adhesion should be very good and I plan to use this
as the drainage plane. On top of the polyiso board
I plan to use 3/4 to 7/8 strapping to align the
windows and nailing flange properly. The Hardiboard
will go on top of the strapping.

Anyone have experince with something like this ?
Any and all advice is appreciated !

Thanks, Mike

AtlanticWBConst. 10-08-2007 09:50 AM

I wouldn't recommend that method.

mike_duclos 10-08-2007 11:32 AM

Thank you for your reply.

Do you have a method you would recommend ?

Thanks, Mike

NateHanson 10-09-2007 09:05 AM


Originally Posted by AtlanticWBConst. (Post 66889)
I wouldn't recommend that method.

Can you elaborate? I've been considering this sort of thing for a workshop that I'll probably build in 2 or 3 years. Two architect friends are proponents of this rigid foam/rain shield type arrangement, with siding spaced off the drainage plane by 1x3 strapping.

AtlanticWBConst. 10-09-2007 09:34 AM

To Clarify:

IMHO: I have concerns regarding the concept of "building-out" additional soft "sub-layers" of materials on a home's exterior. (Rigid Foam board installation under vinyl siding is one thing -Vinyl is never fastened tightly - because this allows it to expand and contract)
Attaching strapping over permeable foam board, and then attaching your solid material siding onto that strapping, creates a layer of non rigid material between your hard material surface and your home's exterior sheathing. This can cause alot of "irregularities" between the surfaces, since the foam is so pliable and strapping is so prone to warpage, bowing & twisting when not attached solidly.

Here is information from the James Hardie site:

"Hardiplank lap siding is installed over braced wood
or steel framing spaced a maximum of 610 mm o.c.
or to minimum 11.1 mm OSB sheathing or an
equivalent thickness of plywood sheathing (Figure 1).
Hardiplank lap siding can also be installed over foam
insulation up to 25 mm thick. Irregularities in framing,
sheathing, and/or foam insulation can mirror through the finished application..."

To me: Adding strapping into the equation (on top of the foam) would compound the possibility of loose attachment points and irregularities - for the trim boards, as well as the siding itself. There is also the issue about roof line drainage and door/window surface-thickenss drainage that can happen when adding that kind of sub-layer thickness onto an exterior surface (foam board with strapping over it).

I am not stating this as rule, but as something that has the high probability to create problems. Even if some architects think it's a good idea, does not mean that it is. We have fixed many, many exterior and interior "architect ideas" that didn't end up working the the way they thought it would, or the way something was designed or listed on their plan details.

mike_duclos 10-13-2007 08:18 PM

Thanks very much for the advice, someone else I asked
mentioned the same sort of issue.

I am now looking at the
CertainTeed fiber cement board product, and their
installation guide addresses the case of installation
over foam. To help address the issue of wavieness,
I'm looking at SuperTuffR rigid board which
is supposed to be very mechanically robust.

FYI, someone mentioned that aluminum should not be
in contact with concrete (this may only be true when
the concrete is in the uncured state), so the
SuperTuffR board is of interest because one side
is plastic faced.

Thanks again for your reply !

AtlanticWBConst. 10-13-2007 08:41 PM


Originally Posted by mike_duclos (Post 67935)
FYI, someone mentioned that aluminum should not be
in contact with concrete (this may only be true when
the concrete is in the uncured state). ...

"Chemically, aluminum will react with the alkalis in concrete (OH) and produce hydrogen bubbles. If you add enough aluminum powder to a full load of concrete, it will spill out of the drum after a little while. Make a cylinder with this concrete and you will notice a mound on top of the cylinder the next day. There is at least one admixture available that uses this principle to produce expansive cement, and this reaction is used in the production of autoclaved aerated concrete.
Significant corrosion of aluminum embedded in concrete will occur due to this reaction. The corrosion of the aluminum will result in expansion and subsequent cracking. If coupled with ferrous metals, galvanic corrosion will also occur. The presence of calcium chloride greatly accelerates the process. Even seemingly small amounts of aluminum can be a problem. In one case, 10-gauge aluminum wire was used to tie rebar together. In less than 9 months there was significant localized distress with spalling immediately over the location of each tie."

Excerpt from:

steve.m 06-24-2011 07:17 PM

I am about to start sheathing my workshop and I saw the JM diagram for adding 1" rigid foam on exterior under 3/8" 4x8 sheets of smart siding, commonsense lead me to ask will the siding warp by not having a solid backing of osb sheathing? After reading your blogs i will put my rigid foam inside under the gypsum. steve.m

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