EPS under slab requirements?

[Affinity Fab]

Hope I am not being too technical but the insulation under the slab needs to meet the ATSM specification C578 for "Rigid, Cellular Polystyrene Insulation"

There a two categories of material under that spec,
Expanded Polystyrene, aka "EPS"
Extruded Polystyrene, aka "XPS"

and within both those categories are seven different classification types,

http://docserver.nrca.net/technical/9997.pdf

See the attached link above for the specified compressive strength of each of those materials. This shows you the 10,15,25,etc. psi materials



See Table 2 on page 5 of the link above provided by Franz1.0c,

They don't come out and say it but it looks like they do not recommend C578 EPS Type I (10 psi) since it isn't even shown in the table. It also looks like they do not recommend EPS Type II (15 psi) either since the allowable bearing capacity column is left blank for that one.

So looking at table 2 it would seem at a minimum you should be using EPS type IX (25 psi material) or XPS Type IV (25 psi material),

So say you use 25 psi material, how much load should it really be subjected too? Well 25 psi x 144 sq in/sq ft = 3600 psf but that it is for short term loading. Should you have long term loading such as the dead weight of the slab, structure, pallet storage, etc, then you should have a factor of safety of 3.0 to cut down on the permanent long term deflection/compression of the insulation,

So, 3600/3.0 = 1200 psf (8.3 psi) which conveniently comes out to the same allowable bearing capacity shown in table 2!

Now don't know your loading on the slab so can't comment on whether on not the 4" slab is strong /stiff enough to limit the bearing stress on the insulation too 1150 psf (8.3 psi) but for normal garage loadings I doubt it is a problem.

After all that I wouldn't be putting anything in less than 25 psi material.
Also table 2 shows you the Max Effective R-values for different types based upon vertical or horizontal installation. This may help you determine thicknesses and type to be used to also satisfy any thermal requirements.

Thank you this is exactly the type of info I've been looking for!
I understand how to roughly calculate the load being exerted by the slab & what's on top of it which seems to be a pretty low PSI BUT none of that answered whether 10 PSI foam was acceptable. Just for the sake of round numbers, lets say you do all the calculations correctly & come up with 5 PSI that your slab & its contents will exert, this doesn't automatically mean 10 PSI foam is ok. What is the factor of safety required? Is there a specification out there that addresses this? That's what I was trying find out.

Based on this if I do decide to insulate I will stay away from the 10 PSI stuff & go with 25 or greater. I suspected this would be the case but I just couldn't find anything to back it up. Thanks all for your input!

 

[GMCGarage]

Usually Live load is a 1.6 factor of safety, 1.75 when it comes to dealing with vehicles.

Go with the XPS

http://ww3.owenscorning.com/sciencedoesntlie/

 

[Franz1.0©]

Can we get back to the table that matters?
https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwj5qNOt4ovSAhUGQyYKHcbzDUgQjRwIBw&url=http%3A%2F%2Fwww.masterfile.com%2Fimage%2Fen%2F700-00748564%2FBack-View-of-Business-People-Standing-on-Boardroom-Table&psig=AFQjCNGVNGErx1_dSC3Jj3TEW5dA7-769w&ust=1487030406760301

NOTE: the males stand on more surface while females primarily maximize shoe contact to a single foot thereby exerting higher ground pressure than males.

 

[wssix99]

Hope I am not being too technical but

Excellent summary!

Wondering how a 25 psi foam could only be rated for 8.3 psi of "bearing pressure" (1200 psf); I did some more digging. Owens Corning mentions (in their fine print) that a 3:1 factor is used to handle long-term creep of the material: http://msdssearch.dow.com/Published...foam/pdfs/noreg/179-02548.pdf&fromPage=GetDoc

So, all of these foams would be expected to compress under load over time. I would think the 10 psi would definitely be a sketch proposition.

 

[wssix99]

Can we get back to the table that matters?

NOTE: the males stand on more surface while females primarily maximize shoe contact to a single foot thereby exerting higher ground pressure than males.

This is sexist. Men also wear high heels now.

Hey - maybe we have a cheap subtitute for lakeroadster's proctor tests? ^ Test your compaction and get a pair of killer shoes, free!

 

[WNYflyer]

Thank you this is exactly the type of info I've been looking for!
Just for the sake of round numbers, lets say you do all the calculations correctly & come up with 5 PSI that your slab & its contents will exert, this doesn't automatically mean 10 PSI foam is ok. What is the factor of safety required? Is there a specification out there that addresses this? That's what I was trying find out.

Well I don't have a copy of the complete ASTM specification but from my understanding the published compressive strength is the short term pressure that would cause the insulation to compress 10%. The ultimate compressive stress the insulation can take I have no idea As far as 10 psi insulation is concerned for long term loadings a safety factor of 3.0 should be utilized which gives you 3.3 psi which of course is less 5.0 psi. I am not familiar with any all encompassing design specifications and associated safety factors, but then again I am not an expert. It seems like it is currently a gray area and you need to piece information together wisely.

Excellent summary!

Wondering how a 25 psi foam could only be rated for 8.3 psi of "bearing pressure" (1200 psf); I did some more digging. Owens Corning mentions (in their fine print) that a 3:1 factor is used to handle long-term creep of the material

Thanks, The devil is in the details and those pesky notes/footnotes!

Very useful content, thanks.

No problem.

 

[Affinity Fab]

Well I don't have a copy of the complete ASTM specification but from my understanding the published compressive strength is the short term pressure that would cause the insulation to compress 10%. The ultimate compressive stress the insulation can take I have no idea As far as 10 psi insulation is concerned for long term loadings a safety factor of 3.0 should be utilized which gives you 3.3 psi which of course is less 5.0 psi. I am not familiar with any all encompassing design specifications and associated safety factors, but then again I am not an expert. It seems like it is currently a gray area and you need to piece information together wisely.

No problem, I was more or less summarizing what I had initially asked, that you answered very well. Thanks again!