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November 2, 2009
R Value of Stucco Styrofoam?
We are building a home in Bay Ridge that will have an exterior finish of stucco. My contractor has offered to use 4 inch styrofoam to apply the stucco to because it will act as a better insullator as compared to the more standard 2 inch strofoam boards. Does anyone know the R value per inch of typical styrofoam board?
Comments
The rating should be stamped on the foam boards, as they differ from one type to the next. I have a piece of the pink foam here that states 2" = R10, 4" = R20 . I think that green foam is lower in value and white is way less.
Posted by: southslope at November 2, 2009 9:35 AM
Southslope, I have a full sample of the white one that seems it is called "Expanded Polystyrene Foam", from what I have read it still has an R value of 3.6-4 per sq inch...thats stil damn good i guess! It does not have the rating on it though.
The blue seems to be called "Extruded polystyrene foam (XPS)" and has a rating of 4.5-5.
I googled all this, but I want to be sure im coreect. I spoke with the guy at Air Seal who did our spray insulation and he said the styrofoam has no value at all...from my googling I think he is wrong.
Posted by: dssguy99 at November 2, 2009 9:49 AM
The rigid insulations are usually 3-5 per inch. Your spray insulation guy was probably either knocking a competitor, or referring to the air seal. Some spray foam insulations form an impermeable vapor barrier that contributes greatly to its ability to stop heat penetration through air movement, which rigid foam, fiberglass, or denim, does not.
Having said that, 4 inches of rigid on the exterior is a lot. Usually one would use an inch or two on the outside, in addition to the stuff between the studs. If you're using masonry, a couple inches on the exterior and some on the interior between the furring strips is usually sufficient.
While it's great to superinsulate a home, there is a point of diminishing returns after which added insulation does very little relative to its cost.
Jim Hill, RA, LEED AP
Urban Pioneering Architecture
Posted by: JimHill at November 2, 2009 10:13 AM
But I assume that if there is a 4 inch of rigid it will add to the total insulation R vallue, which is a good thing.
Posted by: dssguy99 at November 2, 2009 12:04 PM
4" will clearly insulate better than 2". BUT...
Be careful here. Make sure you are getting a true stucco finish. Lathe, brown coat, scratch coat, and finish coat. This would have about 7/8" inch of stucco (cement) thickness.
There are other systems where a thin stucco finish is blown onto insulation and troweled. This is called EIFS (exterior insulation finish system) and is of dubious quality particularly when durability is required.
Posted by: HDL at November 2, 2009 1:12 PM
The process we is doing is install the styrofoam, mesh, 2 layers of centnt substance, finally the stucco we picked from Parex. Sounds like EIFS, but the sample wall he did for us looks great. As far as my research has shown, most houses are done this way.
Posted by: dssguy99 at November 2, 2009 2:00 PM
The sample will look great and you're right most new houses are done this way. But traditional stucco done properly is a far more durable choice. I'm sure the product website shows some SoCal subdivision of taco bell like homes with red tiled roofs. Its very popular in temperate climates especially when trying to recreate that "southwest" look.
EIFS's shortcommings are in durability and in accommodating movement in larger scale applications and remaining waterproof. If you are going to use it in the city I would stay away from ground floor applications that are prone to gardeners, snowblowers. garbage cans, vandalism, etc. Also if your neighbor's kid like to throw around a baseball or practice their chip shot you may have some concerns.
Posted by: HDL at November 2, 2009 2:13 PM
HDL-
Yes, we are doing brick for the first layer and stucco on the second and 3rd floor. We only have a few small areas where the stucco comes to the ground (2 bay window columns). I think we should be good with the EIFS...hoping!
Posted by: dssguy99 at November 2, 2009 2:41 PM
sorry...that is brick for the first floor exterior and stucco for the 2nd and 3rd floor exteriors.
Posted by: dssguy99 at November 2, 2009 2:42 PM
There are several things to bear in mind when designing with EIFS. One comment above related to strength of the material.
If you select 4-oz mesh, you will get an EIFS with adequate strength for most applications, at an attractive cost. Use a combination of 20-oz mesh and 4-oz mesh, and EIFS becomes tremendously strong - literally tough enough to withstand sledgehammer impact, or pass the Miami-Dade Large Missile Impact test for severe hurricane conditions! Using high-impact mesh doesn't cost much, and makes a huge difference.
Two other points - for residential use be sure to use a water-drainage system with a secondary water-resistive barrier. And EPS has an R-value of 3.85 - 4.17 per inch depending on temperature.
Posted by: Richard111 at November 3, 2009 4:58 PM
there is no common calculating value on eifs stucco per square inch( rather thickness) as you might think to solve this equation. you must factor in your other substrates as well as facades etc. but the correct formula for a novice to perform this attempt would be for 1" styrofoam would be an r4.... an inch and a half would be an r7, an additional 1/2 inch would create possibly an r8. Styrofoam when applied must be then sanded or planed down to create the illusion that it is flat, thus the purpose of most builders using the facade. since styrofoam must be sanded you will find places in your wall that will not be as thick as other areas. I will end with this... Styrofoam stucco is intented to compliment and coincide with other substrates and its purpose "alone" is not to increase your "R-value". but has been a great way to sell it.
Posted by: bgood at November 3, 2009 5:00 PM
word
Posted by: Return of The What at November 4, 2009 8:37 PM
Miss me Jon?????
The What
Someday this war is gonna end..
Posted by: Return of The What at November 4, 2009 8:38 PM
Please research all the good, bad and ugly about EIFS Systems. I would never recommend a client to use this system. Here are some bad and ugly. Good Luck.
www.dspinspections.com/eifs_facts.htm#The Causes Of Most EIFS Problems
www.stuccolaw.com
www.askthebuilder.com/242_The_Barrier_EIFS_Nightmare_-_It_is_Real_.shtml
Posted by: Aodrafting at November 5, 2009 2:48 PM
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