radiant heat with no insulation under slab

realolman

What do you think is the possibility that  a house bult on a slab in the 50-60's has any insulation under the slab?

What do you think  is the difference in efficiency between radiant heat installed in a slab back then like that,  and radiant installed in a slab with insulation underneath?



thanks

Unknown

realolman

Good to see you posting! As radiant was so new to contractors then, I

would hazard a guess and say "likely not",,, remember, the

learning process takes time.With the knowledge we have now, the difference between having & not-having insulation should be like night and day to us all.P.S. How have you been old friend? 

Jean-David Beyer

That is an interesting question.

My house was built in the early 1950s and the downstairs is heated by radiant heat in the slab (1/2 inch copper tubing in the concrete). And I have no idea if it is insulated or not. The snow does not always melt away from the house on very cold days here in New Jersey, so it might be insulated, if by no other mechanism than the tubing not getting too close to the outer walls. And the water table is about 8 feet down in May and June, so perhaps the earth beneath is not washing the heat away by convection.



Now it may be that the problem is not as bad as I originally feared. If you look at the diagrams in Chapter 2 of John Siegenthaler's book, Modern Hydronic Heating..., page 24, you will note that most of the heat loss occurs near the outside walls near the ground-air interface. I infer that, while insulation is desirable everywhere, provided there is no flowing water or something like that, the losses from the middle of the floor are not very important, just around the edges. So if the heat loss is too bad, it might just be possible to insulate just there and get most of the benefits of insulation.



Remember, this is just my impression, combined with a bit of wishful thinking. I wish someone had built two nearly identical houses next door and insulated the slab of one and not the other. Then a comparison could be made.



The lower the temperature of the water in the floor, it seems to me, the lower the hat loss would actually be in any case. Another justification for modulating boilers with outdoor reset.

hot_rod

It's possible

some of the radiant homes around here had that asphalt coated Cellotex under and around them. Depends on who they were listening to back then.



The heatloss down depends a lot on what the subgrade material is. Gravel or stone fill does not conduct like solid granite. Sand is a poor conductor of heat.



But the larger loss may be the edge insulation or lack of. Sometimes this can be added without too much trouble.



So the insulation efficiency, heat source efficiency, and the distribution material efficiency all need to be looked at.



hr

TonyS

Siggy wrote a good article

on this subject a while ago in PM magazine. He gave the formula and the math. Using 2 inches of poly board instead of 1 had a payback in 3 years. The loss was quite substantial. I have to buy that disk of his articles as they are then searchable.

Gordy

Not Likley

 If you look back to the Era. Fuel was cheap, and conservation un heard of.



My ranch was built in 52, and the basement is radiant like JD's. They did how ever not go all the way to the outside walls with the tubing lay out. They stayed back about 30" which makes sense any way since furniture, laundry equipment is in that perimeter anyway. My ceiling radiant only had an initial as built 5" of vermiculite in the attic can you say back loss.



 Does insulation make a difference? Certainly in response time, and driving heat to where it should go. Will a non insulated slab perform? Certainly it will just be less responsive, and retain its heat for shorter time periods depending on soil temps. Of course this is not to say one should not insulate its just to say that the times did not have xps insulation, the knowledge we do today, and the cost of fuel. 



 I also have a house on a slab on the property which is a combination of radiant, and baseboard. With perimeter side walks it doubles as a snowmelt system.





Gordy

Jean-David Beyer

Fuel oil prices...

When I bought my house in 1976, #2 fuel oil delivered to my house was $0.40 per gallon.



I found a table of inflation-adjusted gasoline prices.

In 1950, a gallon cost 1.91

In 1960, a gallon cost 1.79

In 1970, a gallon cost 1.59

In 1975, a gallon cost 1.80

In 1980, a gallon cost 2.59

In 1985, a gallon cost 1.90

In 1990, a gallon cost 1.51

In 1995, a gallon cost 1.28

In 2002, a gallon cost 1.31

In 2007, a gallon cost 3.26



It is my impression that #2 oil used to cost less than gasoline by about the amount of the gasoline tax, but recently it has cost even more than the price of gasoline at the pump. So really, fuel oil may have only doubled in the last 50 years or so, using inflation adjusted dollars.

Gordy

NG costs

 Does that table give NG costs JD?  Original fuel was fuel oil converted to NG on original boiler time slot unknown.



Gordy

Jean-David Beyer

No, sorry.

I was looking for a table for fuel oil prices, but none went back to the 1950s.

D107

inflation calc vs consumer price index

the site in the link below--check bottom of page on the left--has comments that dispute this methodology of relative price.



http://mjperry.blogspot.com/2009/05/real-price-of-gas-1919-to-2009-gas-is.html

Gordy

2008 dollars

 You have to be careful when making these type of comparisions. After all the 2008 dollar has lost its value considerably compared to the say 50's dollar. So with that being said. I dont ever remember anyone complaining about the cost of various petroleum products until the gas shortage in the early 70's till present.



The consumer price index would be a better way for comparisons. Some times I think these charts are oriented twords the fuel companies to justify thier pricing makes it seem not so bad to pay 2.50 a gallon.



Gordy

bob_46

Loss down

We used to figure 4btu/ft.sq. with no insulation. That was what B&G recommended . None of the jobs I installed in the 60's had insulation.

CMadatMe

Heat Loss for Non Insulated Slabs

I've ran into this question numerous times over the years. What I have found is that a non-insulated slab, meaning no insulation under or perimter has a heat loss of just about twice the heat loss of the same slab that is insulated. Basically, if you needed 10,000 btu's for a insulated slab would need 20,000 btu's for an uninsulated slab. The extra 10K go to mother earth or bascially up the chimmney. It would be safe to say that depending on how the radiant is controlled you could possibly burn upto twice as much fuel.

Jean-David Beyer

I wonder why that is.

"What I have found is that a non-insulated slab, meaning no insulation

under or perimter has a heat loss of just about twice the heat loss of

the same slab that is insulated. Basically, if you needed 10,000 btu's

for a insulated slab would need 20,000 btu's for an uninsulated slab.

The extra 10K go to mother earth or bascially up the chimmney."



It seemed to me that provided there was not a lot of water in the earth under the slab, that the slab would heat that earth up to about the same temperature as the slab itself, and the heat loss would be minimal except at the edges. Now if the water table came up close to the slab, this would not be true.



So the loss seems to me to be at the edges near the interface between the earth and the air immediately above it. You can pretty much see that from the diagram and text I cited earlier in this thread.



That does not make you wrong. For you to be right, the losses around the edges, the perimeter near the surface, would have to account for the enormous heat loss you report. That could be true; it just surprises me.