Masonry Veneer Question and Nominal dimensions

[Guastavino]

Hi All,

So this isn't exactly a structural engineering question but I'm not sure what forum to use, and since it has structural implications, I figure I'll ask them here:

1. An architect I work for told me that the "air space" in the veneer code (TMS 402 chapter 6) includes the insulation thickness. For example, say you have a 1'-2" nominal masonry wall, 8" CMU, 2" air/insulation, and 4" brick. Let's say you need 2" of insulation for Energy code, so you have a 2.75" air space (But in reality it's close to 3/4") if you assume the mason keeps the 1'-2" wall thickness and you add back the 3/8" from the CMU/brick. Masonry code (TMS402) has a 1" minimum air space per chapter 6. This wall is built ALL the time where I practice. I'm just curious the intent of the TMS code.

2. When you spec a nominal 1'-2" wall as mentioned above, do you detail it (in sections etc.) assuming it's 1'-1 5/8", 1'-2", or something else? I know they can't build it that tight, but I'm curious how others do it. I typically hold the 1'-2" for the purpose of determining working points etc.

3. When you dimension interior walls such as 8" CMU, do the masons you work with know that it's a centerline nominal dimension. I see this only as a problem when you have a string of dimensions crossing 8" walls. If say, a mason built using face to face dimensions, he would lose 3/8" each time and you could have a problem after a dimension string is added up.

With that said, I've only had one issue dimensioning with masonry dimensioning where something like this added up and it was in Revit where revit used ACTUAL dimensions and the precision on the dimension string was set at 1/8th". Then you can image it cut 7.625" in to a 3-7/8" every time and you can imagine that that 1/16th roundup over a string of dimensions had the steel fabricator going nuts. This was the first revit project the company I used to work for did, thus the learning curve. Everything worked out in the end, but was a short nightmare.

Thanks to all.

 

[concretemasonry]

You are right about it not being a structural issue and more of a common sense to achieve what is desired. - A gap behind the veneer.

Here, for a nominal 14" wall that is common on many schools, the wall is built with an nominal 8" CMU and use a joint reinforcement 24" o.c. that is the "eye and pintel" type. The eye projects out beyond the face of the CMU about 2". The 2" XPS is placed after the CMU wall is laid. - It may be the same day or later on depending on the schedule.

Once the insulation (cut into 2' wide strips is placed) the veneer can be laid up using the "pintel" reinforcement link, that allows for veneers of any variable height to insure the desired appearance. The pintel piece is "L" shaped and can placed when the veneer is placed.

This allows any waterproofing of the surface of the exterior CMU/insulation portion be applied. This allows a small (the size gap)is not important because the air is not needed for any insulation, but to provide a way for any moisture to drain downward. The result is a insulated CMU/XPS wall with a break to allow the veneer to protect the insulation and the system of vents low and high on the wall to ventilate as needed and avoid the wind-driven rain problems.

I built my 1650 sf lake home using 14" (13-5/8") CMU below grade and the switched to 8" lwt CMUs above grade. When the wall was up, I applied Thoroseal to the CMUs. I then placed/adhered the 2" XPS the next summer. The brick veneer was laid a year later.

It worked so well that I was able to eliminate any interior insulation after year of monitored interior temperatures because of the thermal mass inside the XPS and 4 - 6' sliding doors facing south in northern MN when the outside temps were frequently down to -10F to -40F.

The key to the effectiveness of a cavity wall is just to have an air gap that allows the air to circulate in some way even though there may be some incidental contacts between the veneer and insulation.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[XR250]

@CM;

Never heard of 14" CMU.

 

[Guastavino]

I believe he is referring to 8" CMU + 2" Air space + 4" brick =14" wall. Not 14" CMU block but a multiwythe wall with total thickness

 

[concretemasonry]

njlutzwe -

I was referring to a 14x8x16 (nominal dimensions)CMU for a foundation wall that is available in many parts of the U.S. and internationally. - In Russia, they use a similar metric unit.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Guastavino]

Gotcha, my misunderstanding. Any thoughts on my original questions?

 

[EngineeringEric]

I dimension using true CMU dimensions. For your wall i would dimension from outside of CMU to outside of CMU. That's what i do at least. The only problem is when we have to dimension to both sides of a block and then we dimension to both and its thickness for reference.

I also call out "FO CMU" on some plans

 

[Guastavino]

Even with true CMU dimensions I see potential issues. For example let's say you have a 4' long wall. That would be (3) courses at 15.625" plus (2) mortar joints at 3/8" and you have 3'-11 5/8". Point being is that true CMU dimensions aren't much better and masons start to get upset when you use oddball dimensions. And they can't build it that close anyway. It just seems to me that it's anyone's guess as to how it's interpreted and that there isn't truly an industry standard for dimensioning masonry. Or at least, that's my experience. That's why I raised the questions, to see if there is any industry standard that is maybe lost, or one that I'm not aware of.

 

[dhengr]

Njlutzwe:
Look up (Google) modular dimensioning of masonry. Your 4' long wall is a modular 4' long. It consists of one half a head joint (3/8")/2; plus three blks. and 2 joints , that is 3' - 11&5/8"; plus one half of a head joint, that is 3/16" which equals 4'. That’s three conc. blks long, not three courses, which is a vertical measure for conc. blk. or brick. Conc. blks. are 16" long when you add the half head joint to each end, and they are 8" high when you add a half bed joint to the top and the bottom of the blk.

 

[Guastavino]

dhengr,

I didn't see that in my google search. Also, 3'-11 5/8" plus 3/16"=3'11 13/16", not 4'-0". Again, my point being that dimensioning actual seems to me an exercise in futility. And I'm not sure where you get that you have to add half of a head joint?

I guess where I am going with this is that there seems to be a lot of differing opinions on dimensioning masonry, and all of them boil down to it's going to vary, so why not use nominal to make it easy, and in reality, just about as accurate when you work out the details and actually draw the blocks/joints. I'm open to being convinced otherwise, but I haven't been convinced yet.

And my original question remains about the air space for anyone that ones to try to take it on.

Thanks