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Re: [datacenter] raised floor cooling the sub-floor slab?



On Thu, 8 Sep 2005, Josh Richards wrote:


> He's claiming that the slab sub-floor is not only heated by the earth
> and "soaking" in the cooling that we provide, it is also getting hit

[ first, I'm no expert in any of this. mostly idle speculation based on
information I've picked up over the years.. ]

This is the first time I've ever heard anyone complain about a concrete
slab /heating/ air... The usual complaints are about it stealing the heat
out of air..;)

Concrete, and the earth beneath it, has an extremely high thermal mass.
This means it's hard to change it's temperature.

Additionally, there's a good chance your slab is insulated from below.
It's best practice, and I'd hope anyone placing a slab for a commercial
structure didn't skip this!   There's probably no easy way to find out,
unless you know the contractor or crews who did the work.

The most common insulation are roughly 1" or 1.5" sheets of blue
foamboard.  There will be a vapor barrier placed below this, as well. (if
they skipped THAT, you'd know.  You'd never keep your air dry!)


If you're able, you might want to dig a small hole along the edge of your
building, down to just below the slabs bottom depth.  Measure the
temperature of the soil at high noon.  I bet it'll be pretty cool if it
hasn't seen direct sun exposure.

Is there an area of floor that's not under the raised floor?  Perhaps a
dead-air spot where the stairs/ramp is?  Stick a temp. probe there and get
another data point.

> on the back and side by the sun where it is exposed at the parameter
> and creating heat which then has to be cooled (since the slab extends to

I suppose that to /some/ extent you might get some heat in along the
edges.  Add a few temperature probes in these areas, inside and out, and
see how hot the surface gets?

If you find that there is a fair amount of heat transfered to the interior
surface, you can insulate the inside and/or outside of this wall.


> The figures PG&E estimated add up to 15% to 20% of our bill.  While I

I suppose it's possible...but my gut feeling says there's no small amount
of bull in that.

There's a horribly hard to read article here (down past the banners) that
appears to discuss the "mass enhanced R-value" of brick and the like.

http://www.environmental.builderspot.com/page/page/800503.htm

Compare this to whatever insulation you have in your walls (wood, steel,
stucco, plus studs or girt, plus insulation, plus interior drywall..), and
I bet that your walls are a more constant source of heat.

>     here.  While I don't particularly recall the slab being warm before
>     we built out I really didn't make a habit of walking back there
>     barefoot much. :)  Though, admittedly, it was likely warmer than
>     49F and therefore I'm sure has SOME impact.

I'd be almost willing to bet that the ground under the slab is closer to
50-55F most of the year.  Just a gut feeling, and your localized
measurements would give you a more accurate idea of what the difference
is.


> Then there's the question of, if they are correct, how to insulate the
> slab sub-floor properly.  I don't want to have a mess to clean up in a

In normal construction, the most common method is to stud the floor, put
fiberglass bats down, and put a wood subfloor over that.  This could
probably be done under a raised floor, though it'd be a pain in the ear.

You could layer blueboard, gluing it down with construction adhesive.
Since you're not walking on this, you could possibly just leave it
uncovered...I've never see it deteriorate. :)

You could also get a spray foam contractor in and get them foam your
floor.

There's also bubblewrap.  The silver-backed stuff designed for the
purpose, usually.

Of course, you can mix and match all of these as well..

> might cause).   I presume there are insulators designed for use _in_ a
> plenum that is constantly in use, in particular for use in a datacenter
> or clean room type environment.  Any idea what some of them are?

Most of the insulation I've seen inside air plenums (normal HVAC
environments, not clean ones..) is just fiberglass board.   The fiber side
is very often left exposed to the plenum air.  Similar stuff is used to
fabricate insulated plenums.  A few sheets, some cuts, some duct tape (not
duck tape..;), and you have yourself an insulated plenum.


Of course, the expense of insulating under the floor after it's been
installed (and in a live datacenter?) might well not be worth the
difference it would make.  Measure measure measure.. :)

...david



---
david raistrick        http://www.netmeister.org/news/learn2quote.html
[email protected]             http://www.expita.com/nomime.html


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