Under Slab Insulation

[ehjorten]

Let me start by saying that I tried several searches, but couldn't find the information I was looking for.

My questions are:

What PSI rating should I use for the 2" XPS blue board under my slab? It seems the concrete only is about 0.5 - 0.6 psi, but driving vehicle on it and the like could add some pressure, but how much? It will obviously be distributed by the concrete...What is the stuff at HD/Lowe's?

What is the best (opinions given) vapor barrier to buy and where can I buy it? Same with the foam. I am in Washington State. Seems someone here found a good deal online for a good thick, tear resistant one.

What about those roll vapor barrier/thermal insulation things? Are they any good? Seems they don't ever give an actual R-value on them. I was looking at "The Barrier" version.

Thank you all for any help!

-Erik

 

[Steve in Mi]

Dow rigid blue foam board will work, afterall, it is used under highways and aircraft landing strips. I didn't presume to have a more demanding requirement so I used it under my floor.

4 or 6 mil polyethylene film is a suitable moisture barrier.

 

[ehjorten]

Okay, but Dow rigid blue foam board comes in everything from 15 psi to 100 psi. The stuff they use under highways and landing strips is the Geotechnical stuff that is either 40, 60, or 100. Should be chosen based on the live and dead loads of the design. I am just not sure what those are for under slab in a home shop.

 

[Ord]

I was told to use the 25 psi, and that 40 psi would be overkill.

 

[larry4406]

People get confused on units of measure. Typical design soil bearing pressures range from a low of 1500 psf (lbs per square foot) to 2500 psf. These are equivalent to 10.4 and 17.4 psi, respectively.

At the end of the day, all loads (live and dead) are reacted by the soil.

The quoted values of 25 psi and 40 psi for insulation equate to 3600 psf and 5760 psf, respectively. These bearing strengths greatly exceed soil capacity (i.e, the soil is still the weak link). You should be fine with either of these.

 

[tdkkart]

Most people use the pink stuff, and there's a reason for it but I don't remember what it was. Menards carries Formular 250 which is what is typically used around here.

Do a search for "Barrier" and I think you will find that it and similar products are not very well regarded. They have there place, but under concrete ain't it.

 

[ehjorten]

Thanks guys! The lowest Dow lists is 15 psi which is a little lower than the 2500 psf rating...so I think 25 psi it is! I wasn't really impressed with what I found about "The Barrier", so it sounds like it is confirmed here.

Any good places online to get this stuff for a deal? I'm gonna go look at Lowe's and HD today and check their prices...16.27 a sheet for Formular 250 at HD. Lowe's didn't even have anything.

 

[gatchel]

I too have been trying to decide what is the best psi rating to use under the slab. I want to use a foam that will be able to support a lift in the future. There seems to be a lot of "i used this and it works fine" but I am concerned that murphy's lay will be waiting for me as it usually is. Rather than going to 100 psi which may be expensive and overkill I 'd rather be able to figure out the proper load rating. Is it really as simple as taking the lift mount surface area and dividing it by the lift ratingor vise versa? That would be the psi load on the concrete...is there also a way to determine the way the concrete actually distributes the load on to the foam...

I have seen some formulas but get a bit cautious as I don't do this every day and don't want to miscalculate. I plan on using a 9000lb two post lift of some sort.

 

[gatchel]

Thanks guys! The lowest Dow lists is 15 psi which is a little lower than the 2500 psf rating...so I think 25 psi it is! I wasn't really impressed with what I found about "The Barrier", so it sounds like it is confirmed here.

Any good places online to get this stuff for a deal? I'm gonna go look at Lowe's and HD today and check their prices...16.27 a sheet for Formular 250 at HD. Lowe's didn't even have anything.

just bought Foamular 250 for the foundation walls. Mention to them that you are a contractor and that you are close to needing the insulation and are checking prices. When they tell you the price tell them that they are close to getting the sale and see if they can do any better. I got another 10% off so that they would "get the sale". Doesn't hurt to try.

 

[ScaldedDog]

I used Dow 40psi stuff, and would do it again. I did the math, and my Excursion is 40psi at the contact patch, though the concrete and rebar dramatically reduce that at the bottom of the slab.

The key, I found, is to make sure the underlying substrate is perfectly flat. If it isn't, the resulting air pockets will cause the concrete to crack.

Mark

 

[rburke65]

If you Google the Dow board you'll find lots of info.

 

[gatchel]

This is from the Foamular application guide from Owens Corning.

If you look at the chart it is pretty impressive based on their examples.

If I read it right, you can have 7200 lbs in a 10" diameter circle on the concrete and it puts less than 4 psi load on the foam (example 1) based on the concrete specs provided. Am I reading this right?

So a lift will be slightly mor weight but over a larger area. Even still, if the weight is doubled Fomaular 250 is still plenty to support the lift??????? Right? I need some reassurance here....

 

[R_Willis]

Is this something that is required? ..or just something people do?

Purpose being? ...just a slight advantage in warmth?

 

[Graham08]

Not required, but it does make a big difference in both winter and summer. In winter, the extra insulation makes it easier to heat the building, and keeps the floor warmer.

In the summer, it (along with vapor barrier) goes a long way toward keeping the floor from sweating.

I had foam under the floor of my last building, and I'm planning to do it again when I build my new shop.

 

[ehjorten]

The insulation reduces your expense in heating the building and in milder climates like where I am located, the inside temperature stays pretty nice in the winter without even heating! Also, if you use hydronic heat you will waste a lot of energy heating the ground below grade without insulation.

 

[gatchel]

Any engineers that can chime in on my questions in post 12 above?