We're working with our architects and engineers on the foundation design for our home. The home is in Menlo Park, California -- so no extreme conditions. We are going with Radiant heating. We are trying to establish what foundation and slab design should be used. First order question is if the hydronics pipes go into the slab or on a topping slab that is insulated. It seems that the topping slab gives higher efficiency (no thermal bridges around edges), but costs more, and looses the thermal mass effect of the full slab. If we go with the whole slab, then how should the slab floor be insulated -- i.e. is it one pour with insulation around the outside of the foundation, or is it two pours -- one for the foudation, then insulation between the floor slab and the foundation? Any thoughts and pointers would be appreciated! Thanks, Richard.

C:\Documents and Settings\o\My Documents\FPSF\EERE Consumer's Guide Slab-On-Grade Foundation Insulation.mht

No layers or lifts. A single slab 4" is standard. Do not bury tube in the foundation. Greener than conventional foundations and more comfortable if reasonable care is taken in designing and controlling radiant floors.

This is a simple heated garage showing a frost-protected-shallow-foundation (FPSF) we designed and installed here in Minnesota using a condensing boiler and radiant floors.

http://www.badgerboilerservice.com/HRF.html

C:\Documents and Settings\o\My Documents\FPSF\The Minnesota Green Affordable Housing Guide.mht

We have designed and specified slab and radiant components for projects all over North America.

Hi Richard,

       It has been my experience that an insulated lightweight topping slab provides the best mass for radiant heating floor systems.  You can use a "bubble wrap" or 2-lb density EPS (Styrofoam) insulating barrier between the foundation slab and the RH tubes and then top the system with a 1.5" Cementitious Underlayment.  This system would provide a very quick heating response and a great thermal mass that distributes heat evenly, unlike a slow responding in-slab radiant heating design or an uneven heating system like the warm-board or grooved plywood radiant heating systems.   Also, these topping slabs resist cracking, unlike standard concrete, are very flat and make flooring installation much easier than flooring installations over regular concrete pours.   I have talked to many contractors and these topping slabs go for somewhere around $2.50 per square foot installed.

   Here are a few companies that manufacture these products:

USG
-  "Level-Rock RH"

www.levelrock.com


Maxxon Corporation
-  "Therma-Floor"

www.maxxon.com

I'm sure they would be happy to provide you with some companies that install these products in your area.  It might be worth it to get some free quotes while the economy is crappy

As for bubble-wrap or insulating bubble foil. It has no place in a properly designed radiant floor heating systems and certainly should not be placed under a slab.

http://www.healthyheating.com/Page%2055/Page_55_o6_bldg_sys.htm

This is common knowledge among professionals in the industry.

Thin applications of cementious materials have their place (my first modern radiant floor installation was a Gycrete over existing slab) they will add little save cost to a slab-on-grade radiant system. References to cracking are beyond the scope and expertise of most in this venue.

Mass is one consideration in radiant floor design and has nothing to do with output.

If only cementious overlayments were that cheap...

Sand is cheap - how about http://www.radiantcompany.com/details/suspended.shtml

No question that lower mass will react quicker to changes in load (ie, less overshoot/undershoot).

Ok, so the choice of going slab on grade or raised floor seems to be in question too. I'd rather get that out in the open to ensure I'm going down the right path.

The pros for full slab are seeming to be thermal mass related, which is likely more useful in the summer months.

The cons for slab seem to be response time, which may be an issue in the bay areas rapidly changing weather on a day-to-day basis?

Our contractor says that since we are preferring wood floor coverings, we should just go regular raised floor with warmboard.

I suspect I'll need to get input with climat/location specific context to help resolve? Any other big items I've missed?

We'll need to choose slab on grade vs. raised floor first.

My inclination is to go with slab on grade with integrated hydronics, since I get some benefit in the summer months.

Does anyone have an experience or lessons that suggest that this might not be a wise choice in our area? I just don't know if the response time issue is something we should be concerned with, as we weigh that against the cooling/thermal help in the summer months.

I was under the impression that the most efficient way to use radiant was to embed the tubing in the center of the insulated slab, but I make obeisance to the more knowledgeable radiant folks on this board.

-Rosalinda

Menlo Park is nice, I lived there back when there were Chevy's and Kepler's was just radical books, ah how time changes things.
So your problem, yes don't know how big your building, do know that the bay is not that cold, or hot for that matter, do know that Menlo is high rent, people buy and sell regularly, remodel, change walls, toilets, sinks and on.
You will never be able to relocate much if cast in stone. Maybe not a problem for you, but the next fellow, just might want a different plan.
Platform permits future changes with minor corrections.
Topping pours are easy, effective mass and responsive.
We distribute RHT floor panel system,another straight forward install, friendly to many finish surfaces, responsive, cost effective, efficient alternative to a topping pour.
Another pro to platform is that even with a topping pour your floor will have some diaphragm give. an entire slab home is a very hard floor to live with.
If you do go slab, 2 inch rigid foam for insulation.
Never use sand for heat mass, it works like insulation.

My 2 cents
Dan

Raised floors have benefits and drawbacks. I personally would go raised floor. It allows you to run plumbing and electrical under the floor allowing for repairs and future changes. Warmboard is a great product or you can pour Therma-Floor over the radiant which will give you fairly quick response and some thermal mass.Therma-Floor is a proven product which has been around for quite sometime. If the tubing is placed in the slab, it should be in the top 1/3 and perimeter insulation is a must.

Raised floors are a vestige of root cellars and there are few convincing arguments for the waste of resources. In Albuquerque they would laugh. Where the tube should be, depends on the structure. Most modern residential structures do not suffer significant swings in temperature (response time the only noticeable improvement in suspended radiant tubing). Dan has a point with the springy step of a suspended floor, but you can put nearly any floor covering on a slab. As for resale, why would you build a house for someone else? The days of building a house on spec. ar over for the foreseeable future.

Of course, if you use more than 4" of concrete and do not control design water temperature with outdoor reset, placing the tubing 2" below the surface may be beneficial.

I have sold several million feet of radiant floor tubing at wholesale and retail for residential and commercial slab-on-grade construction without the marginal benefits of raised tube, since it adds considerably to the cost of installation and little to the performance. This is an advanced design tool best left to the experienced, as it can present a real hazard if one lets the concrete guys "pull" the wire with attached tube to the "top third" of the slab. Think of the same guy making relief cuts with a diamond saw (my most common experience with PEX tube "failures").

If you must raise PEX tube in a slab, it is properly accomplished with chairs.

Thermal mass and response time are not directly linked. Response time is dictated by the structure more than climate. Here in Minneapolis and down in Albuquerque temperature swings are a bit more dramatic than along the cost. If the house is well built, it will not know when the sun goes down or the wind picks up.  Response time is not my first design consideration.

A certified, experienced designer will use a dedicated radiant floor software package which will include Manual 'J' and incorporate weather data for every major city in the US (see ASHRAE Fundamentals).

I have designed snow melting systems in the Sierras and sub-floor radiant systems in Oakland. The answers are in the Fundamentals.

PS

Sand is for sand boxes. This is one of the silliest - make up your own - science projects I can remember (right up there with "install bubble foil below the slab).

Except that "Radiant Floor Company" is an established company in the business and actually uses sand. On the other hand, why not add some cement to it.

This is not an argument. Bubble foil is an established business providing a nearly useless product. Why use any filler?

There are many accepted methods that are functional and efficient (sand is not one of them). This is a gimmick, attractive to those who do not value their time and are easily confused.

Morgan is right, as am I
Sand will Not work if you bury pipes in the sand. It works like an insulator. I did this on a 500 square foot addition on my home 15+ years back, always have to run that area hotter 135 deg, and response is real slow. Any filler needs to be solid. I prefer the RHT floor panel type system rather than sleepers. A sandwich type system will provide diaphragm to the building and the support and or nail surface for a large selection of finished floors, it responds quick, and works well with low delivered water temp so condensing boiler, geo thermal, solar can be applied efficiently.

Slab over platform, like em both but a proper slab (not monolithic pour) will have a perimeter foundation, fill and 2 inch rigid insulation isolating the slab perimeter and underside.
At this point you are already so close to platform design that the difference in cost is minimal. Menlo Park Ca is probably building out between $300-400 per square foot, average home sell price this month $1,200,000. (average)
Spend the few extra dollars to have future options that a slab does not provide. And if you want to be real nice to your trades men pour a rat slab nice and smooth under the plat form so your workers can use those little mechanic dolly's to wheel around in while working under.
Now I am at .04 cents
Dan