Are slab & walls on styrofoam too heavy?
Streamline
| Posted in Energy, Heating & Insulation on
I am placing 2″ rigid styrofoam on visqueen over pea gravel in preparation for hydronic heating. Once PEX tubes are attached, I will have 4″ concrete with fiber mesh poured. My concern is when I fur out the basement with 2X6″ walls, would it be too heavy for the loading on styrofoam below? The interior walls are not load bearing as the foundation walls are carrying the load. I am just afraid that the styrofoam may be compressed over time and crack the sheetrock, etc. Can someone with experience give me some feedback? thanks.
Replies
You'll be fine. There's virtually no load from those walls.
Mike Hennessy
Pittsburgh, PA
XPS has a compressive strength of about 25 psi (see the Dow website for info). Concrete has a density of nominally 145 lb/cu.ft. That makes a foot thick concrete mass over a square foot of foam average out to about 1 psi. For a 4" thick slab, that is about 0.33 psi, vs. the 25 psi it will carry. The small load of the wall above the slab will be distributed by the slab over a huge area. Even if the wall were to bear directly on the XPS, I doubt that the stress would be great, but run your own estimate of that.
Dick,
Thanks for the numbers. It is comforting to see the compressive strength of XPS. I am also having a stairway going to upstairs from this floor. Without knowing the loading put on by the stairs where it sits on the concrete, do you think it is a reasonable assumption that there isn't an issue on the XPS below either?
thx.
I don't see how it could. The area of concrete at the foot of the stairs will spread the weight over quite an area. Run some numbers. Suppose the stairs were 3 feet wide, covering 12 feet laterally, loaded to 40 psf. That would be 3x12x40 = 1440 lb of load, half of which is supported at the bottom. Suppose the weight is spread over the 3 ft width and only a foot wide (it would be more). That gives a load of 1440/3/144= 3.3 psi, still well within what the XPS will support.
Dick,I really like your approach to making worse case assumptions and proving that they are still in limits. Thanks for the insight.
You can go to the Dow Website and look up the compressive strength .
It varies depending on the product. Foamular is the same.
http://www.dow.com/styrofoam/na/pro-us/products/
So are you useing styrafoam per first posting or XPS as now under discussion?
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
"So are you useing styrafoam per first posting or XPS as now under discussion?"Styrofoam is a Dow tradename for polystyrene foam.If it is EPS or XPS and made by Dow it is Styrofoam.If it is made by anyone else it is not Styrofoam, even if regardless of whether it is EPS or XPS.At one time I think that they even stuck the Stryofoam lable on their polyiso foam..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
bill... piffen's question to the OP was the same as mine..
is he talking about the brand Styrofoam.... (Dow )?
or the generic , commonly used term for all foam... as in
"hand me that styrofoam cooler with all the beer in it"
u no... like "kleenex" is the generic for facial tissues
and formica is for all laminates..
or shakes, when they mean "shingles"Mike Smith Rhode Island : Design / Build / Repair / Restore
but would would you do
if that cooler was blue?
Assume we all knewit was extruded too?
me......
say it right....
say it now...
pass the beer ....
don't be shy ....Mike Smith Rhode Island : Design / Build / Repair / Restore
Guys, excellent question. I had assumed all 2" styrofoams exhibit the same properties. The one I am planning to use is R-Tech brand from Homedepot. You are saying I need to go to XPS to get the 25 lb compressive strength?
What I was getting at is that you need to know the compressive strength and properties for the brand yuou are using if that is aconcern.From all I recall, the ratings can vary from about 12# to 32# PSI that the various ones can handle. Any of those should be enough for just a basic floor slab. you know best if your walls will be loaded. From your description, they will not be loaded, so any of these foams is fine.Whether to use EPS or XPS depends on how wet or well drained your subsoil is there. This will not effect the strength so much as it will make a difference whether the insulation does you any good or not. water running through the soil immediately in contact with a slab will rob it of heat energy a whole lot faster than dry soil or air containing stone will. To prove that to yourself, stnad in a 20MPH wind at fifty degrees.
Then lay down on solid material at fifty degrees.
Then let water at fifty degrees run over yousee which will make you cold fastest?
The water will.IN addition, water will cause you other problems with a foundation.So make sure first that you have good dry drained mineral fill compacted, then learn the properties of the insulation you are choosing.I thought that Styrafoam was a much weaker form of EPS and more able to absorb water than XPS. That is why I asked. Apparantly - technically, there is some variation in interpreting all that.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
I think that stuff at HD is more than adequate for the loading.
You don't need 25 lbs unless you plan to park the space shuttle on it.
Joe H
stream... i know you said "styrofoam".... but is that ,in fact, what you will be using..?
all of the foam products have different compressive strengths
If the foam is only EPS, the compressive strength apparently is only 15 psi. That still is strong enough. Normally XPS would be used sub-slab. Some recent info claims that EPS is adequately moisture-resistant as well, but that's another subject.
dick.. when we do insulated slabs , we special order our EPS at 2.0 lb/cf density instead to the less expensive 1.0 or 1.5
we also order it treated with borates ( PerformGuard )
a lot of scandinavian studies say that the old theory of eps absorbing a lot of moisture is wrong
Mike Smith Rhode Island : Design / Build / Repair / Restore
Mike, do you know offhand what the R/inch of the higher density EPS is? The normal EPS is about 4.0, vs. 5.0 for XPS. And how about cost of the 2.0 #/cuft EPS vs. XPS?
Mike,Scandinavia may be drier than North America. Back in the 80's I attended a seminar given by Joe Lstiburek. The question of EPS below grade came up. He proceeded to show us a bunch of slides from when they had to replace (in Canada) an entire foundation's worth of waterlogged EPS with new XEPS, on their nickle. They were using a backhoe to lift the old panels which were far too heavy with water for men to handle. His conclusion was: "Never again. Only XEPS below grade."Bill
Amen to that .
Don't believe it?
Get samples of the various foams.
Do an empirical test, fill a 5 gallom bucket to the top, put the foam pieces in the water and cover so they are forced below the waters surface, Let sit for two weeks in a warm place.
I did it .. the results were very clear... only XEPS was still floating, without ANY absorbtion.
This in the late 70's when I did that experiment.
sometimes i agree with Joe Lsti.. sometimes i don't
i get my EPS from Branch River..
here's their statement... "PerformGuard EPS is stable and has very low moisture gain when used in prperly drained designs. PerformGuard EPS retains 95% of it's R-value even at 4% moisture gain."
it's my understanding that MOST of the ICF's are made of EPS
(Polysteel, for instance )....
from 1978 on we've been using foam under slabs... for the first 15 years or so we'd only use Styro-SM or Foam-ul-R, or Amoco.. thinking they had the crush strength we needed and they didn't absorb water..
but eventually i did more research and found that i could get everything i needed from EPS and more.. i could get treated foam.. as far as i know ..only EPS is available treated
here's the spec :
type IX ( 2 lb/cf density, compressive strength 25.0 psi,
Water absorption by total immersion, max., volume % 2%, R-value (2" ) 9.52 )
for a 4' x 8' x 2" sheet i pay $ 26.82 for PerformGuardMike Smith Rhode Island : Design / Build / Repair / Restore
The good news is, what you are doing works just fine and produces great results.
The bad news is, your concrete guys need to be familar with the challenges of pouring directly on the foam.
There are two issues that scare me everytime we do this (just two weeks ago the floor of our current project was poured). Voids from water, and voids from slab curl.
If the joints in the foam aren't sealed with tyvek tape water from the concrete can pool under the foam creating voids. How can this be since water weighs less than concrete...
A styrofoam pannel is quite rigid and can support a surprising amount of weight over a short distance. That combined with the challenge of getting the gravel level can produce areas prone to pool. These same areas are much less likely to cause problems if water can be kept out from under them since the boyancy of the foam then won't be a factor in the foam bridging.
Get the surface dead flat and tape all the edges of the foam and you won't have problems with this. Tyvek tape sticks just fine to foam.
An option that is a bit more work, but much less likely to cause problems, and something we are going to start doing as standard procedure, is placing the foam and vapor barrier under 6" of pea gravel and sand. This provides an excape for the excess water in the concrete, the extra weight on the foam helps cut down on the likelihood of pooling, increases the thermal mass that's within the insulated shell, and is easier for the concrete guys to finish because less water is coming to the surface.
The other problem, slab curl, happens more often with insulation directly under the slab because it can cause one side of the slab to cure at a different rate than the other (concrete shrinks as it cures) and the slab either bows up in the middle as the edges curl down or the edges curl up. Either is very bad.
We are 80% convinced this happend a slight amount on a past project and the weight of some framing caused cracks in areas that normally should be bullet proof.
Now we pour a 5" or 6" slab where we used to use 4". The extra weight helps resist curling and the extra depth helps protect the pex when contraction joints are cut. In the past, the contractors I've worked with that do top notch building never cut contraction joints when pex was in the slab, but I'm now convinced it's worth it, if not to prevent random cracks, to make it harder for the slab to curl.
We also now use a guy that works around foam all the time and is much more concerned about conditions during and after the pour, as well as the consistancy of the mud itself.
Good luck.
Beer was created so carpenters wouldn't rule the world.
Don,
FWIW I agree about the sand /gravel above the foam.
I started pouring slabs (and footings) on Styrofoam XEPS in the late 70's. We quickly discovered the advantages of placing a layer of sand or 3/4 minus over the foam.
Easier to level the final grade, allows the water absorption and helps even out the cure just as you stated.
We also place 6 mil plastic beneath the foam just to help prevent water infiltration.
Thanks for the conformation that we're on the right track. Even though we currently have the best concrete sub I've ever worked with, it would be nice to make his life easier and we'd sleep better.
Did you have any problems with foam under the footings? The various types of foam are rated for more than enough weight for our projects, but it's something that very few have any experience actually doing. We'd like to do it with our ICF foundations to both keep the concrete in the wall a bit drier and reduce heat loss through the footer.
Cheers.
Beer was created so carpenters wouldn't rule the world.
Don,
To my knowledge their has been no problem with the foam under footings.
We used it in 3 projects back then that were solar homes and the concept was complete isolation from the gound.
I never heard of any problems and I have been around all 3 owners for yrs. since we built for them.
Dovetail,
A layer of sand should never be placed between poly and a concrete slab, nor between foam and a concrete slab. Here's the link that explains why:
http://www.buildingscience.com/bsc/resources/foundations/sand_layer_under_slab.htm
Martin, Good link.
I am not someone who believes everything that BS has to offer however.