Posted By Dana1 on 19 Jun 2013 05:27 PM

The masonry ties become a problem for stone veneer on 2" foam though- a lot of masonry contractors will all but refuse the project with foam more than 1" thick.  I'm not sure where the foam thickness limits are before you have to go to full-custom ties, but it's a cost adder even at 2". If you're building for 100+ year sustainabilty, $tainle$$ ma$onry tie$ would be in order. How committed are you to the stone?

Lately I go back and forth on the stone because of the extra labor I'll have to put into it, but being a stone mason by trade makes the veneer economical for us. Aesthetically I prefer it.

My interest and concern though is what effect the stone has on the r-value and overall performance of the wall: will the stone veneer require that the primary route for the wall to dry is the interior? Or will I have to provide an airspace between the veneer and the exterior foam? I've always been highly suspect of this method for a variety of reasons.

Does the stone veneer offer any positive or negative performance for the wall?

The stone and any mortars use are a moisture reservoir. Most stone is fairly vapor permeable, but some stone will wick & store more moisture than others. And therein lies the problem:

When the sun heats up stone that soaked up dew or rain, it releases the moisture at a high rate, rapidly brining the RH of the air film on the interior side to nearly 100%, which is why you need some level of vapor retarder between the stone and any susceptible wood. I view moisture-reservoir siding of any type (stone, stucco, brick) as a definite NEGATIVE performance feature from a moisture handling point of view- they only add a problem to the stackup that then needs to be solved, and lower the resilience of the structure to moisture. This is a bit of an exaggeration, but it's sort of like using a sponge for a raincoat.

From a thermal point of view the stone brings nothing positive, and a even slight negative if it's thick enough to have much thermal mass. Mass walls can be a benefit to net performance when the thermal mass is inside (or mostly inside) of the insulation, but becomes a slight net-negative when on the exterior in most climates. The IRC even accounts for that in it's code prescriptive Rs:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

If you look at the Mass Wall column in TABLE N1102.1.1 you see two numbers. The smaller number is the R-value for continuous insulation when at least half of the R is on the exterior of the thermal mass, the larger number is for when there is more R on the interior. In cold climates they're not much different, but in mixed or cooling dominated climate it's pretty big. The larger number is comparable to the "whole-wall" R of the numbers used in the WOOD FRAME WALL column at a 25% framing fraction, indicating that the mass is basically useless when mostly outside the thermal boundary of the house. The effects of thermal mass on energy use performance of different mass-wall stackups has been modeled and simulated to death by the US DOE and others, some of the results of which can be found on the Oak Ridge National Lab website.

BTW: I just became aware of a product specifically designed for masonry ties on thicker foam:

http://www.youtube.com/watch?v=_VX_Hap1n7c&feature=player_embedded#at=269

The masonry-tie version starts at 3:50 into the video.

indicating that the mass is basically useless when mostly outside the thermal boundary of the house

A much better answer without extrapolation is "it depends".

More detailed sources show external thermal mass to be clearly advantageous in almost all cases. See table 4, wall 4 here:

http://www.ornl.gov/sci/roofs+walls/research/detailed_papers/dyn_perf/index.html

If I could ask in conclusion and for a final clarification,

a full 4" (horizontal studs are 24" centered) of dense pack cellulose with air barrier to the interior and 2) 1" layers of xps as sheathing will allow drying to the exterior and interior, but the xps has a high global warming potential.

Substituting foil faced polyiso for the xps will lower the global warming footprint and offers comparable r-value performance but will push the drying direction to the interior since polyiso is somewhat vapor impermeable...

but if I'm applying a stone veneer (most likely granite) the polyiso will also serve as an effective vapor barrier controlling moisture infiltration from the veneer into the wall.

Do I have it right?

I will use polyiso instead of xps, and need to give the veneer more thought. The project is taking shape in stages so I have time to figure siding out.
If there's any suggestions for a better approach towards insulating I'd appreciate hearing them.
The timbers are a full 8" and all insulation is on the exterior of the framing.
Thanks very much for all the input. This site has been invaluable and I greatly appreciate it. Daniel

Will the stone veneer be self supporting and have an air gap between it and the wall or will the veneer be stuck directly to the foam on wall?

If thin veneer will be directly attached, I would use Type IX (2# nominal density) EPS foam on the exterior as the mortar used to attach the veneer will stick to the EPS with good strong adhesion. You don't want foil faced foam in this application.

(2) 1" layers of staggered seam Type IX EPS will give you about R8.5 to R9 on the exterior and the R value won't degrade over time like XPS or Polyiso. Plus the EPS should cost less then either of the other options. Plus, the R value of EPS goes UP as the temperature drops.

If you will be installing thick self-supporting veneer with an air gap, then any of the rigid foams with or without foil facing should be OK.

Posted By arkie6 on 23 Jun 2013 11:53 AM
Will the stone veneer be self supporting and have an air gap between it and the wall or will the veneer be stuck directly to the foam on wall?

If thin veneer will be directly attached, I would use Type IX (2# nominal density) EPS foam on the exterior as the mortar used to attach the veneer will stick to the EPS with good strong adhesion. You don't want foil faced foam in this application.

(2) 1" layers of staggered seam Type IX EPS will give you about R8.5 to R9 on the exterior and the R value won't degrade over time like XPS or Polyiso. Plus the EPS should cost less then either of the other options. Plus, the R value of EPS goes UP as the temperature drops.

If you will be installing thick self-supporting veneer with an air gap, then any of the rigid foams with or without foil facing should be OK.

I'd prefer not to provide an airspace between the foam and the stone if I don't have to.

Will the (2) 1" layers of type IX EPS alone offer the vapor barrier necessary?

Type -IX EPS runs about 1.0 perms @ 2" thickness (R8.7) which is about 1.5x as permeable as XPS at the same R-value (1.75")

Even 1.5lb density Type-II EPS, which would still be about 1.5 perms @ 2" (R8.4), which is about 2x that of XPS at that R-value, but still sufficiently vapor-tight to be protective against peak moisture drives from masonry, and you could even go to 3" (R12.6) and still have some exterior drying capacity if you had any sort of air-gap.

Typical specs for EPS of different densities look like this:

http://www.buildwithplymouth.com/eps_properties_spec_sheet.pdf

In my neighborhood Type-IX EPS is slightly more expensive per unit-R than polyiso, but it's not a huge upcharge. Type-II EPS is usually comparable to or sometimes cheaper than iso at any R value. YMMV.

Without ANY sort of vented air gap behind the stone you're sort of asking for trouble- freeze-thaw expansion issues can separate the stone from the EPS in short years if applied directly. It's pretty common to apply stucco or concrete directly to EPS in warmer climates, but you're in zone-6. The only bonding mortars for EPS I've seen used in my area (zone 5) were used for bonding the EPS to the exterior of CMU or poured concrete walls where mortar is on the warmer side of the insulation, protected from deep freeze/thaw cycles. I've yet to see it used for bonding masonry cladding to the EPS, and would be very wary about it's long term prospects in a climate that cold.

I have some stucco applied to foam (no mesh) in a cold climate. No issues with bonding or freezing.

Moisture problems caused by external masonry claddings are mostly the result of air infiltration. Stop the air movement with good air barriers and you stop 98% of any moisture problems. Roof overhangs also make a significant difference (they keep the cladding dryer).