radiant heat checking on a few facts

[keeney]

RE The Foil insulation.

It sounds like a good story, but its not going to work well embedded in or under a slab.

Radiant barriers work like a mirror - reflecting the I.R. radiant energy just like visible light. The problem is, embedded in a slab, that "light" isn't going to go anywhere - it is all absorbed by the surrounding material and transmitted via conduction to the surrounding materials, including the aluminum foil. Aluminum is a wonderful transmitter of heat energy, so its just going to hurt the insulating properties in terms of conduction.

The air bubbles will be providing all of the insulation against transmitted heat, and they appear to only be rated at a "U"-value of 0.133 or so.

Heck, dry dirt or concrete has about the same insulating properties on a per-inch basis.

Also, radiant barriers work better for higher temperatures at shorter wavelengths - like the radiation from the sun. The low temperatures of an in-floor heating system just don't put out that much IR radiation.

The name "Radiant" heat is a misnomer for these systems. If you want to transfer serious amounts of heat with radiation, you need larger temperature differences between the objects (think glowing nearly red-hot coals). An in-floor system running at 90 degrees is going to be conducting a lot more heat than it is radiating.

The reason these foil insulation systems are sold is because: A: they look better on the bid - cheaper for the same claimed R-value, and B: Nobody ever measures the actual efficiency of the system after its installed. The floor is warm, so that means its working, so nobody ever complains.

I highly recommend 2" of Polyurethane or Polysterene under the slab and around the edges, or skip the insulation altogether if you want to save on installation costs in exchange for higher heating costs down the road. That might be the right choice for a space that is only heated on an occasional basis like a garage or unfinished basement - it may take decades to recoup the cost of the extra insulation. I'd still put down a vapor barrier under the slab, though.

- Rick

 

[SPIKER]

I want to dissagree with keeny on this slightly for the foil teck stuff.

one the foil does not contact the concrete at all as it would be ate up very quickly... alum & concrete don't mix well! /forums/images/graemlins/wink.gif SO that means the FOIL is actually on the other side from the bubble insulation. as laid it is like this (Poly, bubble, bubble, and final layer is Foil.) now I also intend to have an additional poly layer between the foil & the substrait, 9sand gravel ccrushed rock material.) BUT I also intend to use styro between the outter walls & sides of slab as well as a 4' wide layer under the foil side all the way around the outter edge of the building. insualting the center with the styrofoam boards is not as economic payback as is the outter few feet.


anyhow you may use as you will but with the foil on the under side of the slab and on the other side of the insulation then it only needs to reflect the energy that is peneterating the bubbles. while ALUM FOIL is very heat conductive the stuff used on these type of insulation is more of a poly/foil blend (think of mylar usedin ballons) which it's briliant surface is not a fact that it is only shinny but that it is reflecting nearly 100% of the light that hits it. also see the survival blankets that you can buy for a few bucks that is only a thin sheet of mylar like material they WILL keep you very warm as the keep you're body heat where it is at INSIDE the survival blanket. these are pretty cheap to buy at wally world if you don't belive me get one and try it out. /forums/images/graemlins/smile.gif this is EXACTLY how it works for you're radiant slab.

mark M

 

[keeney]

Looking into it a bit more, I was totally incorrect in my understanding of what the U-value is. Also, I was mis-remembering the R-value of concrete - its about R1 per foot, not per inch.

However, I do stand by my assertion that the bubble-foil stuff is not a good technology to use under an in-slab heating system because regardless of the radiant energy reflected, ultimately conduction will continue through the fairly poor insulating path of only about 5/16" thick bubble wrap.

The quoted R-values for the bubble-foil are obtained only when there is a large non-circulating air-space seperating the radiant barrier from the heat source. Non-circulation is hard to achieve except perhaps in a totally sealed area with the heat source on top such that convection in the airspace does not occur. This might work for the inside of a hot attic roof to save on cooling costs. It might also be OK for the underside of a heated floor above a crawlspace assuming no wind intrusion.

In the case of an in-slab system, there is no air gap (other than inside the bubbles) to prevent conduction. The bubbles are only 5/16" or so thick. 5/16" of bubble wrap is only going to have an R-value of about 1.1 according to the references I could find.

- Rick

 

[rambler]

</font><font color="blue" class="small">( I belive you have mistaken the terms "Open System" and "Closed System". in real terms what it actually is referning to is if the system is pressurized and has no dirrect contact with atmosphear anywhere in the system. a closed system is more of a steam type system and opperats continusly under pressure. boiler/hot water tank, PEX, pumps & Expansion tank and all are "At Pressure of 20~100+ PSI. and is a CLOSED system they DO get makeup water but only when the pressure dropps below the intake fill valves set point. (think of a check valve with city/house water on one side and the closed system on the ohter, the water can flow from city/house to the radiant system but NOT back. the PRESSURE in the PEX system stays constant due to requirement of a Expansion tank, and the AS NEEDED fill from the city water.

in an OPEN SYSTEM there is little or no real pressure in the boiler & piping system other than down stream of the pump where restriction to flow causes SOME pressure but usually only a few lbs. often these are typcial for outside wood boilers (term boiler is not really accurate but is often used.)
the open system requires lower water temps, 150~180 degs F any higher then and boil off of water results and the fill / intake is worked much harder. which means the cold water intake requires morew wood to warm it up. often there are simple float valves which regulat the water height inside the boiler.

there are many brands of tubing one of the better has multi layers, such as "Pex-Al-Pex" (as name implies it is a layer of PEX tubing and a layer of Aluminum tubing then a 2nd layer of PEX. ) these are typical of BARRIER tubing (which lets in/out none of the O2 so steel can be used in the stytem. typically a steel hot water tank and cast pumps.) open systems need to use brass, S.S. and Plastic componets to prevent rust problems.


typical system starts with a good base of rock/gravel, followed up by sand/pea gravel then all compacted. adding a vapor barrier on top of the compacted base. there can be several ways to proceed. you can use Poly Styrene, (styro-foam boards) or Foil-Bubble-Poly insulation all in conjunction with the the vapor barrier, taping seams as you go.

lay the tubing onto a 6x6 mesh grid and keep the runs 300' or less in lenght and runs 1' or closer together. (typically 6~8" for first 4' away from outside walls then 1' for the rest. this is for in concrete systems under floor systems where wood floor is used then the PEX is stapled to the flooring by means of the AL plates typically.

like others stated plastic clips can be used but wire ties to the 6x6' re-mesh is much simpler and faster as well as cheaper.

once the pex is laid & held down pressure test the system! leave pressure ON the system when pouring crete over it. the pressure inside will help it stay un-collapsed if it get stepped on.

also like stated make sure you have detailed drawings of WHERE the PEX is at incase you ever need to drill to mount a vice ect.

one good mail order place for PEX is farmtec I spent almost $2k for mine and I got 2000' of 1/2" and 600 of 1" PEX-AL-PEX as well as 4 rolls of the 8'x100' foil-bubble-bubble-Polly insulation. not bad and I got a free pair of wool gloves /forums/images/graemlins/wink.gif lol


Mark M
/forums/images/graemlins/crazy.gif /forums/images/graemlins/ooo.gif )</font>

This has been a very interesting thread, and the web site with the installation manuals in PDF format is great reading.

When it comes to all other fluid heating systems, 'open' & 'closed' systems mean the fluid is actually 'open (zero pressure) to atmosphere' or 'closed (with need for a pressure tank) to atmosphere'.

I see from reading the pdf's from that web site that 'open' is a way different concept, and does not refer to 'unpressurized' at all! Very different, & perhaps what is confusing the different sides of this.

Most outdoor wood & corn furnaces are 'open' design, and run with the heat transfer fluid actually vented to air & exposed to oxygen. Whereas most indoor wood & corn boilers are built to tougher standards & are 'closed' systems with a pressure tank & no exposure to air and only small amounts of replacement water.

Same terms for totally different concepts.

--->Paul

 

[Farwell]

I have dark brown 10x12 roll up door on my pole barn that faces South and is insulated with 3/8" foil covered bubble insulation. When ever the sun hits the roll up door a large amount of heat is conducted through the insulation. The bubble insulation does some good because with out the insulation the inside of the door is so hot that I can't touch it.
Don't know if this applies to any other application, just passing on my personal experience with it.
Farwell

 

[dmccarty]

Farwell,

I don't like crawlsapces since they can be such a pain to maintain. They can be mold magnets for a variety of reasons. Our old house was on a slab. On one very hectic morning the toilet overflowed and ran for minutes. ALOT of water flowed out of the toilet. I was lucky in that downhill was the utility room which like the bathroom had vinyl flooring. The water never got to the carpet. With a crawlspace I would have had water falling through the floor and into the insulation. That would have been a huge mess. This house was almost 30 years old. Given the leaking toilets and 30 years I know the bathroom flooring would have rotted out during the time I owned the house. WIth a slab it was not an issue.

Slabs dont allow hot or cold air under your living space. Slabs dont allow moist air under the structure of your house. Mold is not an issue.

Our utilities are in the attic and fairly easy to access. One reason we put in a slab was to have radiant heat but we took it off the list since it did not make money sense for our climate. Kinda wish I had put in the PEX since the tax breaks are going to be better next year but I'm still not sure it makes money sense. /forums/images/graemlins/grin.gif

Now if we could have had a basement thats a new conversation. /forums/images/graemlins/grin.gif

Later,
Dan

 

[5030tinkerer]

Very interesting thread. I will be building a new home (with basement, no crawl space /forums/images/graemlins/grin.gif) along with an outbuilding. I like the radiant heat option but am scared of doing something materially wrong for lack of knowledge. Everyone seems to have a different opinion. Argh! /forums/images/graemlins/confused.gif

Does a consensus exist on site preparation, etc.? Do I dig a little deeper to accomodate the installation? I am looking for radiant heat in the basement slab and on both the first and second floors. Does anyone have experience with the warmboard product (warmboard.com) for the first and second floors in lieu of using the more conventional gyp method?

 

[dmccarty]

3830tinkerer.

I would get Joseph Lstiburek's book if you are building a new home. Also see if you state provides information on solar heating. NC has a state solar center which has publications and classes as well as information on tax incentives. The Feds now have incentives for solar usage in 2006. We wanted to use solar to heat the water for the radiant floor but it was not economical for us to do so. The new credits may have changed that calculation. Building passive solar into you house is really a non cost issue if you have a south facing wall that can be filled with windows. To fit our house site as well as other design consideration our house does not meet the states standard for solar design for a tax incentive perspective. BUT, on a clear day with temps in the 50s the house does not loose heat on the south side of the house. Monday, I left the house and the thermastat was at 73. I got home at 10:00pm and it was still at 73. The heat pump was set to turn on at 68. We had warm temps maybe touched the high 50s. The house had to have gotten over 73 to have been at that temperaturs 5 hours after sunset.

Also check out the Journal of Light Construction website. They have discussion forums on topics that could be interesting for you. They certainly are a great resource if you have a question regarding construction since they have most likely had an article on the subject.

For your site you will have issues with the frost line. I think in my area of NC the frost line is about 6 inches. For that reason there are few basements here. In a colder climate, the deeper the frost line and the deeper the footers have to be. An with deep footers you might as well build a basement. Slabs can be built in northern climates but the foundation has to have some details to prevent problems. I'm sure Joseph Lstiburek's book on Cold climates would show those details. The local building inspector should know as well.

Later,
Dan

 

[5030tinkerer]

Thanks for the tip! I'll check it out.