# Old block house R value help needed



## HVAC Girl (Apr 23, 2011)

We have a 1926 block house that I'm trying to figure out the R-value we should use for the exterior walls. The stucco/block/plaster walls on the first floor are 13" think and 10" think on the second floor. Our initial load calcs included standard concrete blocks and we come up with a load of 5 tons when the existing 3.5 ton system is well sufficient which leads me to believe we need to adjust the R value of the walls. I have a pic of the blocks used but are unable to post until I have made 15 posts. Blocks are hollow cell but the concrete seems dencer.
Anyone have any input on these types of walls?
Thanks in advance.


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## HVAC Girl (Apr 23, 2011)

Uploaded a pic of the block as my avatar. This picture is at the garage opening where we're replacing the door framing.


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## greg24k (May 19, 2007)

That looks like Terracotta block which is about 2.0 R.value , Stucco you have about .20 of R.value and plaster will give you about .32 of R.value. multiply this numbers per inch.


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## HVAC Girl (Apr 23, 2011)

greg24k said:


> That looks like Terracotta block which is about 2.0 R.value , Stucco you have about .20 of R.value and plaster will give you about .32 of R.value. multiply this numbers per inch.


Thanks Greg. I looked at some pics of that type of block and they all had a redish tint. This block is grey like concrete and laid in an alternating pattern creating sealed air gaps. This is what leads me to believe we could have a higher R value than standard concrete block.


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## greg24k (May 19, 2007)

HVAC Girl said:


> Thanks Greg. I looked at some pics of that type of block and they all had a redish tint. This block is grey like concrete and laid in an alternating pattern creating sealed air gaps. This is what leads me to believe we could have a higher R value than standard concrete block.


Standard concrete block let's say 12" is about 1.28 R.Value, in most cases for any Air Space 1/2" to 4" the R.value is 1.00

If you take a typical wall construction;
Wall - Outside Air Film 0.17
Siding - Wood 0.80 
Plywood Sheathing - 1/2" 0.63 
3 1/2" Fiberglass Batt 11.00 
3 1/2" Stud 4.38 
1/2" Drywall 0.45 
Inside Air Film 0.68 
Percent for 16" o.c. + Additional studs 15% R.value cavity 85% 
Total Wall R-Values cavity 13.73 
Total Wall Assembly R-Value 12.05

If you remove/change the R.value figures with your wall assembly, you will have pretty much the total needed load for your project.


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