Radiant slab insulation

John Abbott

I had a customer ask if 4" of blue board foam underslab insulation would save energy more than the 2" I normally install.The application is slab on grade.I am in NH and there is no ledge or groundwater present.I have read somewhere that after a certain R value in any application there is a very diminished return on investment the slab is 40 X 73 and cost is not as much of a consideration as energy efficency.Your thoughts would be appreciated.

John

Unknown

Insulation R-Value

John,

The answer to your question is, of course, it depends.

Start off with, what type of heat are you using. Radiant? Baseboard? Forced air?

When it cones to the insulation let's look at a ceiling. For instance, in my area (philadelphia) we have a 14F outside design temp and a ground temp of 55F. I find that when upgrading from R30 to R38 ceiling insulation the cost of the insulation nearly doubles but the heatloss reduction to the envelope is less then 1%.

For a slab, when it comes to radiant heat, another contributing factor will be your floor covering. Rules of thumb say that minimum you should have 5 times the floor covering for slab insulation. Meaning with a heavy carpet and thick pad your r-value can be (or exceed) R2.5, so you would want R10 (or 2") insulation so your heat from the slab is directed upward. Of course, if this is a forced air heated house, the R2.5 heavy carpet and pad would be added to your slab insulation and AID in keeping yor heat in the house.

When looking at a slab, it is most important to consider edge and perimeter insulation. 90% of your slab on grade loss is held here. Your edge insulation NEEDS to be below frost line. In my area the minimum edge insulation is 2 feet.

If you do an actual design you may find that going from 2" to 4" insulation will be a marginal benefit to your customer. Do a heatload calculation or have someone run the numbers for you using your local environmental condistions. In my area there is no significant change to heat savings to go the extra 2".

Best regards,

Steve
aka wheels

John Abbott

I neglected........

to say it was a radiant slab. Colored concrete stamped with no floor covering.

Thanks,
John

Unknown

heat load

John,

The following is as per Manual-J 7 calculation software:

Lets say that your building load using 1" was 56,000btuH, then it would be 46,900btuH with 2" and 42,311 with 4". These based on my earth temp here in Philadelphia.

Here is what changed, Load per square foot was 12.9btuH. Downward back loss per square foot show 6.3BtuH @ 1", 3.1BtuH @ 2" and 1.6BtuH @ 4".

Does it help, sure. Is it worth it, thats your call :-)

Steve

John Abbott

Thank you.......

for the info.I had a gut feeling that this would be the case but I don't have the technical ability to quantify it as you have.I now will be able to advise my customer properly thanks to your help.

John

John Abbott

Thank you.......

for the info.I had a gut feeling that this would be the case but I don't have the technical ability to quantify it as you have.I now will be able to advise my customer properly thanks to your help.

John

Unknown

no problem

John,

No problem, glad to help.

This is and always will be a question customers ask, like do I need ANY insulation. Kind of a fun question.

Now if only I could solve world peace and get the answer to the universe (and 42 may be the answer)

Steve