I have an old house in New Jersey, (close to 100 years) built with clay block and stucco on the outside. Inside has lath and plaster with only one inch space from wall to lath. Therefore no insulation. The house is tight and the clay block is 8" wide so I guess it provides some R value.

I would like to add rigid foam insulation on the outside, 1.75" or 2" and then either new stucco or furring strips and Hardie board.

Now, can I just glue and screw the rigid foam to the existing surface or do I need something in between? Also, will I need house wrap over the foam before I put the furring strips as there is no wood structure on the outside (nothing until the lath on the inside)?

Per Hardie instructions - foam, furring strips, building wrap then Hardie. Even with that, I would consider using treated wood (or some synthetic) for the furring strips.

Posted By bluebox3000 on 12 May 2010 12:09 AM
I have an old house in New Jersey, (close to 100 years) built with clay block and stucco on the outside. Inside has lath and plaster with only one inch space from wall to lath. Therefore no insulation. The house is tight and the clay block is 8" wide so I guess it provides some R value.

I would like to add rigid foam insulation on the outside, 1.75" or 2" and then either new stucco or furring strips and Hardie board.

Now, can I just glue and screw the rigid foam to the existing surface or do I need something in between? Also, will I need house wrap over the foam before I put the furring strips as there is no wood structure on the outside (nothing until the lath on the inside)?

Is there a cavity/gap between the stucco & clay block, or is it applied directly to the block?

The foam must be semi-permeable or semi-impermeable (meaning unfaced EPS or XPS, no foil or poly faced goods) can be indeed be glued & screwed to the clay block, with the seams & edges foamed/caulked/taped.  A building wrap or felt can then be applied as a secondary air-barrier. 

Whether furring for mounting the siding can then be simply through-screwed into the block with sufficient loading strength to be able to support a stucco or cement fiber facing depends on just what that stuff is.  (Cement block yes, terra cotta, I'm not so sure... maybe depends on how thick and how hard-fired it was etc.)  There's a way to do it, but the particulars of what's truly structural and how one can best attach furring to it will vary. Metal strapping to anchor it a wall header and the top of the foundation might be necessary if the block can't handle the shear-loading stresses at the fasteners using a simple though-screw method.  If the existing stucco has a cavity between the block& stucco, supporting the stucco with metal spacers, it probably CAN hold the load, but you may have to tighten up fastener spacing  to suspend the stucco or siding 2-3" away from the wall if it had be previously only been 0-1" away.

The standard instructions for mounting Hardie to a studwall puts the air-barrier adjacent to the air-barrier wrap, but  there is no such constraint when there's a rainscreen-gap being provided by mounting it on furring (as jonr suggests.)  In fact it's quite the opposite- the manufacturer's technical bulletin on using Hardie over furring clearly depicts the building-wrap next to the sheathing side, not the Hardie side of the furring.  Both the siding and the building will stay drier if the air-barrier is between the furring & foam, leaving a nice back-ventilation to the siding that allows it to dry toward both interior & exterior without loading up the structural wall with moisture (just as with most stucco facings &  masonry cavity-walls.)

As Dana is aware, at least some Hardie installation instructions say wrap outside furring strips over concrete.

Hardiplank siding can also be installed to concrete when the wall is furred out with wood framing or minimum No. 20 gauge steel ... A weather-resistive barrier is recommended between the framing and the siding.

A picture that makes it clear is here:
http://www.amvic-pacific.com/Downloads/hardiplankstraightedgeinstall.pdf

Both the siding and the building will stay drier if the air-barrier is between the furring & foam,

And the subject to rot furring strips will get wetter. Call Hardie for clarification as to whether their instructions have changed or foam changes the recommendation.

Mounting the furring directly to concrete on furring is not the same as mounting on furring on foam. The no-foam/2x2" furring situation against concrete is similar to a masonry cavity wall, but with a much leaker veneer (Hardie siding), and putting the drain-plane next to the siding keeps more water out of the concrete by making the outer wythe (the siding) less leaky. If up against the concrete, the high permeabability of wrap materials can allow significant amounts of water to enter the concrete if wetted. But when you have foam against the concrete, the humidity would have to somehow penetrate the only slightly permeable foam to be taken up by the concrete. Foam isn't hydrophyllic- it won't wick moisture in, even if the wrap is fully water-saturated. But saturated felt directly against concrete it would give up some of it's water to the wall via both capillary action & vapor diffusion.

I'm not convinced that Hardie has it right in the bare-concrete stackup though, even with the wicking & permeability factors, but with foam between the drain-plane & concrete you'd have plenty of margin.

The average humidity of the furring itself isn't much affected itself whichever side of the wrap is on, since it's fully within the ventilation gap. Furring-gap rainscreens with the furring outside the wrap have a good track record, but rot-resistant furring materials are recommended in very rainy locations. (Pressure-treated, cedar,BluWood, etc.) If it's a 1' story building with 2' roof overhangs it's not much of a concern anywhere.

Look at InSoFast for insulation and furring strip in one step.

Posted By ICFconstruction on 14 May 2010 08:18 AM
Look at InSoFast for insulation and furring strip in one step.

Got any data that that poly furring can handle a stucco or Hardie siding load? (It might, but I couldn't verify that in a quick web search, where exterior apps seemed to be using much lighter stuff like vinyl siding.)

I'm also unconvinced that you wouldn't still need a rainscreen gap between stucco and highly-permeable InSoFast EPS.

Thanks for the interesting discussion.

To answer your question Dana1, then there is no gap between the block and stucco as far as I can see. I haven't tried to make a hole in the stucco to see how it is attached. We currently have aluminum siding over the stucco screwed onto 1x2 furring strips.

My first thought was to take off the aluminum siding and spray foam between the existing furring strips. However using that method I would only get 1" foam and nothing under the furring strips. Not sure if that makes any sense.

Posted By Dana1 on 14 May 2010 11:48 AM

Posted By ICFconstruction on 14 May 2010 08:18 AM
Look at InSoFast for insulation and furring strip in one step.

Got any data that that poly furring can handle a stucco or Hardie siding load? (It might, but I couldn't verify that in a quick web search, where exterior apps seemed to be using much lighter stuff like vinyl siding.)

I'm also unconvinced that you wouldn't still need a rainscreen gap between stucco and highly-permeable InSoFast EPS.

Bumping an old thread. Dana, Insofast has 1/4" x 1/4", 1"OC drainage plane channel. Their new Ex panel http://insofast.com/ex-panel.php has these channels on both sides. Do you think this would be enough for a rainscreen gap?

Posted By timberwolf78 on 20 Oct 2013 05:31 PM

Posted By Dana1 on 14 May 2010 11:48 AM

Posted By ICFconstruction on 14 May 2010 08:18 AM
Look at InSoFast for insulation and furring strip in one step.

Got any data that that poly furring can handle a stucco or Hardie siding load? (It might, but I couldn't verify that in a quick web search, where exterior apps seemed to be using much lighter stuff like vinyl siding.)

I'm also unconvinced that you wouldn't still need a rainscreen gap between stucco and highly-permeable InSoFast EPS.

Bumping an old thread. Dana, Insofast has 1/4" x 1/4", 1"OC drainage plane channel. Their new Ex panel http://insofast.com/ex-panel.php has these channels on both sides. Do you think this would be enough for a rainscreen gap?

It's not exactly a rainscreen, but in conjunction with a housewrap beween the foam and the wood sheathing it's probably a sufficient capillary break & drying path to work, even under stucco.

Posted By Dana1 on 21 Oct 2013 03:38 PM

Posted By timberwolf78 on 20 Oct 2013 05:31 PM

Posted By Dana1 on 14 May 2010 11:48 AM

Posted By ICFconstruction on 14 May 2010 08:18 AM
Look at InSoFast for insulation and furring strip in one step.

Got any data that that poly furring can handle a stucco or Hardie siding load? (It might, but I couldn't verify that in a quick web search, where exterior apps seemed to be using much lighter stuff like vinyl siding.)

I'm also unconvinced that you wouldn't still need a rainscreen gap between stucco and highly-permeable InSoFast EPS.

Bumping an old thread. Dana, Insofast has 1/4" x 1/4", 1"OC drainage plane channel. Their new Ex panel http://insofast.com/ex-panel.php has these channels on both sides. Do you think this would be enough for a rainscreen gap?

It's not exactly a rainscreen, but in conjunction with a housewrap beween the foam and the wood sheathing it's probably a sufficient capillary break & drying path to work, even under stucco.

Thanks! How about the fact that this particular panel is r-10. I am in Mpls(zone 6 I believe) and r-10 exterior insulation is less than recommended by GBA for 2x6 walls? Please let me know what you think of this wall assembly for this zone? 1/2" drywall 2x6 + cellulose insulation zip sheathing insofast 2.5 inch panels(r-10) stucco with acrylic topcaot

It's the IRC that has established R11.25 as the required minimum exterior R for wood sheathed 2x6 construction in climate zone 6 to be able to go with latex paint as your only interior side vapor retarder. ( GBA has several blogs explaining the "whys" behind the code.)  Minneapolis is toward the cool edge of zone 6.

http://publicecodes.cyberregs.com/i...par025.htm

Mind you, at outdoor temps where the foam/ZIP interface hits 40F (the dew point of 35% RH 70F air) , the R-value of 2.5" of Type-II EPS is slightly more than R11.25, but you'd be skating a pretty fine line there. It's better if you can build in some margin.  The buffering capacity of the cellulose buys you a bit, as does the drying channels but it would be much safer with 3" of exterior EPS.

Note, vented cladding over wood sheathing is NOT sufficient in climate zone 6, the way it is in zones 5 & lower.  If you used something like GP DensGlass sheathing instead of ZIP (as is often done in commercial construction with reservoir claddings like brick or stucco) you'd be in pretty good shape though.

Alternatively you could keep the ZIP and use a smart vapor retarder such as Intello Plus or Certainteed MemBrain under the drywall and have plenty of margin.

Thanks, it's extremely informative. Could having an inch of CC foam in the cavity + cellulose be an effective alternative?

Sure, a flash-inch would put more than R15 outside of the condensing plane, and the sheathing can dry more quickly to the exterior than toward the interior as long as there is at least SOME ventilation between the stucco & EPS. But MemBrain or Intello is a cheaper solution, with much higher drying capacity toward the interior.

Dana,

I have another exterior foam and stucco question.

Let's assume the wall assembly is drywall - 2x6 16"oc with cellulose or roxul - zip sheathing - 2" polyiso - furring strips

The recommended furring strips would be 1x3 or1x4 16"oc.

I am running into issues finding stucco contractors who are comfortable with the furring strips with this much gap in between. The other issue is nailing the lathe at 6"oc both vertically and horizontally as best practice.

What do you think about having 1x4 furring strips at 8"oc to overcome these concerns? Is that a lot of thermal bridging? How about the screw penetration through the sheathing which are not centered on the studs? Wouldn't that cause condensation?

I would appreciate your help.

Ever considered stucco style fiber cement siding? (Every stucco guy seems to be some prima-donna artist, they all hate foam of ANY thickness!)

Ha! You are right about stucco guys and foam thickness.
I have been looking at couple of LP products. One is the stucco panel, but the seems will have to be covered with trim and can work if I wanted a half-timber tudor look, which at this point I don't.
They also have a relatively newer product called carrarastucco, which is basically an acrylic top coat on top of their precision panels. The panels go up like drywall with exterior grade joint compound, tape etc. Its an interesting product but I have called LP to inquire about it and left messages but haven't heard back.

The other option is to go with a double stud wall assembly, but the decrease in square footage and potential moisture issues are steering me towards exterior foamed walls.

You can use a self rising foam that is sprayed. It is liquid first in form and then expands to fill up the void. This type of foam can be used by any contractor such as remodel contractors gta or any construction specialist.