The new shop (pole building) is nearly shelled and the builders will be
packing up their tools this week. Now I get to be busy with preping the
I want to install radiant heat, but getting good information has been
difficult about the floor preperation.
Any wreckers done this recently?
I'm looking at $268 for 1000' of pex 1/2"
2" Extruded poly styryene foam for ground cover (what should this cost)
I was told to use 25psi, anyone know what the stuff at Homedepot is?
2" of gravel UNDER the foam
4" of concrete poured over the pex and the wire mesh which holds down
A Womack wrote:
> The new shop (pole building) is nearly shelled and the builders
> packing up their tools this week. Now I get to be busy with
> floor installation.
> I want to install radiant heat, but getting good information has been
> difficult about the floor preperation.
Time to get a good heating contractor involved.
If you don't want one to do the job, that's OK, then hire them as a
When you end up pouring concrete over your design, you only get one
chance to do it right.
Only 2 contractors in my area
One wants $7000 for materials and install + the concrete!
They bid $1500 for foam, pex, and mesh and I pick it up
Pex is $268 delivered on EBAY with a profit for the seller
Foam I am still priving but I have found the local wholesaler and they
don't recommend buying from the floor guys
Mesh or Rebar, i understand I can get rebar and tie it myself for less
than the mesh.
I have to find out how deep the pex is supposed to be now.
Second issue is contractor number 2 does not return calls.
That leaves me on my own to figure it out.
i sell outdoor wood furnaces here in minnesota
www.centralboiler.com i have had experience with differant
installations. myself i put the pex in the sand under the concrete about
2" deep with foam only around the outside of the slab about 20" deep.
once the sand is heeted up it's a big thermal storage basin. i recommend
not more than 250' of pex on a loop or zone. keep your beginning hot run
on the outside run in about 20" from edge of slab with 12-16 " spacing
on the loops. keep th hottest part of your runs to the outter working
inwards for the return to manifold. i know of many places here set up
like this and it works well. of course it only gets to around 30 below
here at times.
i just use a garden hoe to cu a trench in the sand lay in the pex and
cover. works best with 2 people, 1 to hold the pex in place while the
other packs in the sand as you go.
My research suggests that the need for insulation under the slab is
dependent on the presence of water in the soil under the slab and the level
of any water table. Water will conduct heat rapidly. Dry clay will act as
a thermal mass and while heat will leak out, like you say, heat prefers to
rise. The normal temperature of ground below the atmosphere affected zone
is ~53F. not insulating under the slab will mean that the slab in a covered
building is unlikely to go below ~40F even after an extended period of no
heat, but use caution YMMV. If the heat source is costly and used
constantly then insulation may be indicated, but I would submit that 1" or
1.5" would be sufficient. If the heat source is DIY wood then cost may not
be a major factor and the added thermal mass of the under slab clay may be
more worthwhile, since with wood heat, storage is a bigger consideration
since the fire will not likely be continuous. Not insulating the slab will
also tend to keep the floor cool in the summer.
In any case, heat is far more likely to leak out of the edge of the slab and
I would suggest that is where insulation should/must be placed. As you
suggest 2" vertical rigid board 24" deep and extending to the top of the
slab. Allow the (2x6 frame) wall to hang 2" over the foundation to cover
the top of the foam board. I think that the effectiveness of this board can
be doubled by the addition of another 1" foam board 24" wide buried at 45
degrees which will prevent the frost from penetrating next to the slab.
I have no comment on burying the piping in sand under the slab beyond that
it is an interesting idea. I am inclined to think that the heat conduction
will be better with the tubes in the slab but YMMV.
IMPORTANT, If the tubes are poured in the slab they should be filled with
water before the concrete is poured in order to stop them from floating to
the surface of the concrete.
Good luck, YMMV
While I don't agree with the thought that water will keep the pipes
submerged, it really isn't an issue IF the pipe is stapled adequately
to the foam. Adequately would depend on your concrete guy...is it
going to be poured with a pump truck, slid down a half pipe, skid steer
over the edge etc. My thought is staples are cheep...don't skimp here.
The more the better.
We installed radiant heat in our new house last year. My memory is that
the contractor recommended re-bar rather than wire mesh, not for the
additional strength of the concrete, but because it allowed him to tie
the PEX tubing to the bar at a fixed height, preventing it from floating
anywhere in the concrete mix. He used a thermal reflecting blanket as a
moisture barrier as well as to minimize heat loss.
> Only 2 contractors in my area
> One wants $7000 for materials and install + the concrete!
> They bid $1500 for foam, pex, and mesh and I pick it up
> Pex is $268 delivered on EBAY with a profit for the seller
> Foam I am still priving but I have found the local wholesaler and they
> don't recommend buying from the floor guys
> Mesh or Rebar, i understand I can get rebar and tie it myself for less
> than the mesh.
> I have to find out how deep the pex is supposed to be now.
> Second issue is contractor number 2 does not return calls.
> That leaves me on my own to figure it out.
Looks like they have enough work or very big egos.
we use sand, then foam, then tubes. i think you would be crazy to skip
the foam under the slab as another poster suggested. unless you own
your own hydro plant or oil well or something. the tubes are stapled
to the foam. menards here in mn sells everything you need, including
staples. get 300 foot rolls, as that is the maximum loop length you
are allowed. 1000' foot rolls are not managable without an unreeling
gizmo. do you have a tubing layout plan? most of the time, the tubes
are 12" on center, 6" oc on the perimeter. with a sheet of graph paper,
you could get your own layout figured out. i built a small house this
winter, 24 x 32, and it took 3 300' loops. good luck.
just because you think i'm crazy do'snt mean i'm not. but i still think
i'm right in puttig the pex in the sand.
1) heat tends to go up not down
2) ground holds it's own thermo temp
3) sometimes cement cracks,what happens to the pex imbedded in the
4) the sand helps defuse the heat, giving more even temp on slab.
i did an install in a basement under a new house. the guy put foam under
had one night of freezing temps before he turned the water loose into
the pex, when he did it froze before it made it all the way through.
the only places it did not freeze the slab was where there was pads for
support posts spaced down the cntr with no foam under pads. go figure.
he spent $1000. to rent heat blankets to warm up the floor.
and yes i do have my own hydro plant burning wood. about 12 cord a
winter to heat my house
shop, dry kiln and domestic hot water.
I just did my own 2200 SqFt house, and followed this plan. In addition
I did the garage and covered porch. In all there's 6 zones, and
something like 10 coils.
One think I would suggest is when you have the pipes that come up
though the concrete, feed the pex through 1" electric conduit 90 degree
elbows. This keeps the pipe from kinking or angling up during the
pour. I purchased some of the metal channels used in commercial
electrical applications to support the manifolds during the pour (mine
are out in the concrete field. Drive it into the dirt plum before
pouring and set channel in the concrete. With a little rigging, I was
able to put about 30# of air in the pipes and that way knew if they
were damaged during the pour.
Wirsbo has compression splices that were on hand "just in case" the
pipe was nicked during the pour...thank goodness we didn't need it.
I was told that the bubble sheet wasn't effective as the foam. DAGS and
you'll find pros and cons...I choose to use Owens Corning Foamular250.
They have 2 grades of pink 2" foam, one rated for under slab one is
not. In addition, I put 6 mil plastic under the foam, to keep ground
moisture from migrating up into basement floor. Maybe overkill...but
for a few bucks...I wanted a dry basement, and this is just one more
Best of luck...
Another place you could compare pricing might be here for pex tubing
Thet also have a bubble wrap insulation you could use instead of 2" of
(sixoneeight) = 618
You can buy molded foam panels made to insulate and to hold the pex in
channels. I've seen some that have a foil layer on top to help reflect the
heat. I'm certainly not an expert on them, but they are worth looking into
for the best job.
As for the foam, you can usually buy board stock, expanded foam, cheaper
than extruded. Check out the local foam molders near you and buy direct.
There are companies in every state so you should be able to find one and
save a bundle.
In central Iowa, 2 inch Foamular250 (owens corning at menards can be
had for $19/sheet on sale...around $21 not.
Something you might also consider is putting foam inside your footings
so it meets the foam under the concrete. That will keep the cold from
radiating into the concrete floor from the sides.
With respect, If the outside of the slab and footings are not properly
insulated, then insulating the inside of the footing does not insulate the
edge of the slab from the coldest exposure. If the outside of the slab and
footings are properly insulated (to an effective depth) then the effect of
insulating the inside of the footing will be minimal but will do little
harm, it should certainly be done if there is water under the slab or
By far the most important location to insulate properly is the outside of
the slab. This will also help to prevent frost from penetrating under the
slab in the event that there are periods of no heating in the slab.
Why is this? I have been unable to find any empiracal evidence of a
university conducting tests, or a trade group.
I'm considering embedding a fair number of 1-wire sensors to make some
real-world data available. I might even try one section of wall with
the radiant barrier and the next one with normal insulation to put that
controversy to rest.
I can swap it back next year when it doesn't work as well as they claim
I do not have a cite beyond common sense. IMHO heat transmission/loss rate
is propotional to the delta (difference between the adjoining substances)
and the thermal conductivity of the interface. Heat is easily conducted
laterally through concrete and a concrete > air interface will also conduct
heat readily due to air circulation. Sinilarly heat can readily flow from
the concrete to frozen ground (at a slightly reduced rate) due to the delta
but also at a somewhat further lower rate due to the lack of circulation.
When any insulation or thermal break is placed between the edge of the slab
and the adjoining frozen ground it significantly reduces the effectivness of
the thermal interface and slows the heat transmission. IME the perimeter
insulation, combined with sunlight reflected from the wall, will
significantly reduce the amount of freezing of the soil next to the footing
and this unfrozen ground (depending on density and moisture) also acts as
insulation for the slab. The thin sheet of insulation @ 45 degrees is
intended to maximize this frost free zone next to (and under) the footing.
If a grade beam or minimum slab edge thickening is used instead of a proper
frost wall then reducing the penetration of frost under the slab is vital to
the life of the slab as the frost can cause significant slab stress and
movement in addition to increased heat loss..
This will be a good project and you can do a paper that will impress the
accademics and may even get you cited here. It will be hard to account for
all the variables like soil desities, water content and circulation and sun
exposure but we here can just use those to argue and critisize your
findings. Quantified data for heat loss downward from insulated and
uninsulated slabs would be more interesting to me but are also subject to
the same variables of soil density, conductivity and moisture and
circulation. These measurements will also be affected by the adjoining
areas with different amounts of insulation so we will be able to question
your methodology there as well. I suspect that this work has already been
done by somebody with a significant grant or budget (of OPM) and perhaps it
would be a better use of your interest and efforts to research the existing
knowledge base and report back here with cites, or even better, weblinks.
The bottom line is that this really is not rocket science, and you should
not lose sight of the need to properly do all the other stuff like underslab
gravel, compaction, underslab poly, concrete quality and placing and
finishing as well as site grading and drainage. Slab heat is really nice
but its cost effectiveness also depends on the construction of the building
and the choice of boiler and fuel. A simple small water heater (with
suitable controls) can be used for primary (expensive) or backup but these
systems really shine when combined with a ready source of cheap fuel like
Good luck and have fun.
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