Under Slab Insulation, When?

Mark Eatherton1

Here's what my predecessor at Contractors has to say about it.

The question which always arises in the business of radiant heating is: Does the area below the radiant slab have to be insulated?.

The real answer is yes and no. The correct answer depends on two criteria: size of slab and height of the water table below. When our company got involved in the design of the world's largest radiant floor system there was a question of whether insulation should be placed below the slabs. The new National Maintenance Center for United Airlines in Indianapolis, IN, would consist of a total of 20 acres of floor space. The specifying engineers for the project decided to install 1" high density foam board under each slab at a cost of $100,000. In this case it was the right decision. The heavily steel reinforced 14" slabs carry pressure loads of 250,000 pounds plus Airliners. More importantly, after numerous test drillings done in the Spring, it was found that the water table on the construction site would be as close as 3 feet to the bottom of the new slabs. The key factor in deciding to incur the insulation expense was the height of the water table. If the water table in the spring would have been 6 feet or more from the bottom of the slab, the State of Indianapolis could have saved themselves at least $80,000.00 per hangar, because the insulation would have been wasted. In this case, the sulation was necessary and saves money in operating costs.

When it comes to residential and small commercial slabs up to 2000 ft², we always recommend to insulate the whole area with 1" foam board, no matter what. What is the logic behind this thinking?

The BTU's traveling in the downward direction from the radiator slab are equalizing the temperature of the gravel below during the initial cold start up. Once all this gravel mass is heated up, literally all the heat is directed upwards into the heated space. The only remaining heat loss into the ground, however, is through the perimeter foundation of the slab into the solidly frozen ground surrounding it. To prevent this, proper insulation of the perimeter foundation is essential, irregardless of the size of the slab, 2" foam board is sufficient. The amount of insulation necessary below the slab depends on the size and shape of the slab. The basic rule to follow is always insulate at least a 10 foot perimeter strip below the slab with 1" foam board. Research has documented that 76% of the ground loss is within 5 feet of the slab's perimeter. If the remaining uninsulated center of the slab area is 20 feet or less in width, cover the whole area with foam board simply for practical purposes. Anything beyond those dimensions, insulation cost and its practical payback becomes an issue.

Generally speaking, in slab sizes under 2000 ft² total, slab insulation is the right approach. Anything larger, a 10 ft under slab perimeter strip is the right approach. There is, however, one more factor which must be taken into consideration, and that is the groundwater table. If the spring water table is 6 feet or less from the bottom of the slab, total slab insulation becomes necessary, irregardless of the size of the slab. Water this close to the radiator slab will wick energy from it just like a candle wick will suck up wax. We certainly don't want to raise the environments average ground water temperature. That can become costly!

The author is president of Stadler Corp. (781/275-3122), an independent Bedford, Mass.-based supplier of hydronic heating equipment.

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Unknown

Under Slab Insulation. Nescience.

When is it needed and why.

What is the R-value of (dirt) from here to China?

I've got about 3,000 Sq Ft. of under slab Radiant and there is LEDGE present. ( :-o )

It is below aprox. 4-6" of crushed stone. < the ledge.

I have specified 4' of perimiter insulation. The Owner read the Wirsbo installation method. She now has concerns RE: ramp up time & downward heat losses. Smart gal.

What should I tell her?

I believe that FULL insulation is not needed. There is no groundwater present and the ledge is not in contact with the slab.

I've been wrong here before. I may be wrong again. Tell me what you guys would do. I allways will take the advise of my Wetheaded brothers.

See you all at Wetstock :-)

Pipedream

kevin_5

Insulation

Hi Gary, I think anytime a homeowner wants insulation, we'd be crazy to talk them out of it. I've seen people regret not using enough insulation, but never have I seen them wish they'd have used less.

I always recommend it on the jobs I bid, and I price it separately from everything else, because I believe they should have it whether they use radiant heat or not.
Hey, the scorched air guy doesn't include wall insulation in his bid does he? Yet it makes his system perform better although it might work OK without it... Just crank up the thermostat and let it run! I tell them to lay on a cold basement floor all night and tell me which way heat goes.

Our software shows quite a difference in BTU load with and without underslab insulation, and with the Wirsbo manual stapler, the insulation makes a dandy attachment point for the tubing.

I'm told that without underslab insulation, the ground will warm up 8 feet down or so, and that could take a week or more. If a warm front comes through and they want to shut the system down, they've got a lot of heat stored up and it just keeps on comin'.

I know there are differing opinions on this, but when a customer is trying to do things right and not cut corners,how can you go wrong giving them what they want?
Kevin

hr

Check out www.

beaverplastics.com They have a colorful graph showing slab loss, in their library. It takes a while to load, be patient.

ME check around I have only been able to locate hi-load foam in 2" thickness. Very pricey, and only in 2' X 8' sheets. Fairly heavy also, for foam

hot rod

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Steve Miller

Words of wisdom

"Insulation doesn't cost, it pays"

Duncan_2

All right, you got my curiosity up, hot rod!

What exactly is that picture we're looking at in your post?

It looks like foam routered out for an aluminum extrusion, but I don't recognize the extrusion. If that's two inch foam, the extrusion looks to be only 1-3/4 inches wide.

Is it a manufactured product?

Sidenote: I have uninsulated four inch thick radiantly heated concrete slabs in my home, floors measure 12' x 21'. One is on grade, one is below grade. Three sides are exposed walls, one 21' side contacts the interior of the building. They take more like a day or so to come up to heat from ground temperature. Soil here is decomposed granite, water table is way down, not even close to the slabs.

Canuck

I know I'm anal - but

there is no such word as 'irregardless'. Use 'regardless' in its place.

Earthfire

Canuck is right and wrong

From the MIRRIAM- Webster Collegiate dictionary on line "One entry found for irregardless.


Main Entry: ir·re·gard·less
Pronunciation: "ir-i-'gärd-l&s
Function: adverb
Etymology: probably blend of irrespective and regardless
Date: circa 1912
nonstandard : REGARDLESS
usage Irregardless originated in dialectal American speech in the early 20th century. Its fairly widespread use in speech called it to the attention of usage commentators as early as 1927. The most frequently repeated remark about it is that "there is no such word." There is such a word, however. It is still used primarily in speech, although it can be found from time to time in edited prose. Its reputation has not risen over the years, and it is still a long way from general acceptance. Use regardless instead. "

Unknown

Earth fire,

DAMN. Your pretty SHARP. :-)

It's usace is, in my opinion as well as the opinion of my good friend standing here besice me is yes and no.

Vocabulary is and has never been one of my favorate subjects. ;_(

Thanks all the same.

pipedream

Unknown

Under-Slab Ins.

Regardless, Mark Etherton states clearly in his post that he did not write the specific document.

I found the post to be so helpfull that I printed it.
Showed my prospective customer.
She now wants complete coverage.

Fine. Like Taco says "do it once - do it right"

It's not included in our price. I hope I can get the G.C. to handle this big blue blanket.

Pipeedream:-)

Chris Maderia

The simply reason

The earth would have to be satisfied before that slab would be meaning wasted oil consumption and a lot longer response time. Basically you are wasting your customers fuel dollars.

Janet

Reflective Insulation??

I'm looking for input on using reflective insulation over the slab for a retro-fitted radiant system for an existing slab-on-grade home. The water table is estimated at 20+ feet; the house is 1,950 sq feet.

A few details: the existing slab is not waterproofed (1960's construction). Plastic that was previously laid down under the carpet had trapped some moisture. The lack of proper drainage could have contributed/been the cause of the moisture?? (Makes me wonder about the water table, though). Ceilings are 7 1/2' so we're trying to keep the thickness of the radiant system down, without affecting efficiency.

Questions: Should we install insulation? Do reflective barriers work well in this application? If not, should we skip the radiant system altogether, or are there other solutions?

I work for a GC outside of Sacramento, CA, and we are looking for a knowledgeable radiant installer good at problem-solving unique issues, for this and other homes. Pls. email me directly if you are/have a referral - jm@directcon.net. Thanks.

Mike Kraft

RE:slab on slab

Janet,
You are going to need edge insulation protection and typically a 1" extruded polystyrene foam board.Then your pour will need to be at least an inch above your tubing.So if your'e working with 1/2" pex you will lose around2.5" of your room height.

You dont need to flush the radiant though........you can still install it in the walls and the ceilings.

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cheese