Insulation under a heated slab

billyc

I am getting ready to install a heated slab. I am getting mixed information from two different contractors. One says I don't need insulation under the heated slab because of my base material being good gravel. The second contractor says I need at least 2" of insulation under the heated slab. Both contractors agree on placement of insulation on the slab perimeter and 18" extending vertically with a slight downward slope about 18" below the top surface of the slab. I live in Alaska south of Anchorage and we get winter temperatures well below 0 F. We have a minimum of 5 months of winter average temperature during the coldest temperatures is around 18 F. Please advise as I am confused and need advise.

Gordy

Insulate

   I would insulate. The insulation under the slab will help prevent downward losses into the ground, and decrease the response time of the slab. You only get one shot before you pour the floor to install it. What is gained for the cost of the insulation I think makes it worth doing. That is less energy consumption by directing the radiant energy into the structure. The decreased response time is a side benefit of this because the energy is concentrated into the slab rather than the ground.





Gordy

meplumber

Check the manufacturer's suggested installation.

http://www.uponorpro.com/en/Technical-Support/Manuals.aspx



This link is to a list of installation manuals for the brand of tubing that I prefer.  Most all of the manufacturers that I am aware of require/recommend insulation under the entire heated portion of the slab. 



I have heard this argued many times before.  Usually by budget conscious builders.  Put the insulation in.  I am in Maine and we always use at least 2" foam under the slab.  -10 deg design temp for us here.



Good Luck.

EricAune

Run the numbers...

I have and it consistently shows a increase of 10 or more percent in loss if the under-slab insulation is omitted. That's a 10%+ load increase for the life of the system.  Unless your heat source is actually free, requires no amount of work to input and endless, the numbers do not lie that savings are to be had immediately. 

k_2

re

As the guys said on GBA. you need insulation. Pretty much more=better, up to a point.





wouldn't think it'd be so hard to convince a guy in Alaska of the value of insulation......

Mark Eatherton

If operating under the IBC, it's not an option...

The IECC requires insulation below the slab and slab edge insulation.



These guys (installers) are not responsible for your fuel bill. Do the right thing while the gettings good.



I'd avoid the one who claims it is not necessary. If he's willing to cut corners here, where else is he cutting corners...



ME

TWar

code

Depending on your local building codes insulation may be optional.  In Alberta we are required to have R-5 minimum with temperatures regularly reaching 0 degrees F.  I agree with other comments as the payback is immediate.  Another thing to think about is boiler life.  The lower the temp and less cycling the longer the life of the system.  If the one contractor only says its needed on the perimeter ask him for an explanation.  In heating like anythhing you pay for what you get.  If one contractor wants to cut corners on insulation where else is he willing to do it.  Generally the contractor who does his homework and comes to you with the best possible system and a relevant explanation for the extra costs is trying to give you the best deal.

Brad White

Insulation Bathtub

I would insulate, R10 at the perimeter and R10 under the slab. A big insulation bathtub. The ground under your slab will be cooler than your slab so there is your answer. Yes, it eventually will warm up -at your eternal expense. Insulate and use a vapor barrier.



I forget who said it first here, but insulation is fuel that you pay for once.

meplumber

I've heard that too Brad.

"You insulate once.  You buy fuel every year."

jp_2

round here, 3in foam is code

thats for a foundation in northern Mich.



I tend to think R10 is a bare minimum at the perimeter anymore.



I like this idea for perimeter in the pic below, I know it shows no underslab.

Brad White

Agreed

I should have said, "minimum"!

Thanks, JP.