I’m curious if it’s ok to dig next to a trenched footer supporting a slab to add insulation after the concrete is poured? I’m using radiant heat in my slab and the local inspectors are not going to allow ridgid foam insulation under my slab.
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Replies
" I'm using radiant heat in my slab and the local inspectors are not going to allow ridgid foam insulation under my slab."
Why not.
What SPECIFICALLY reason do they give.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
The only reason I was given was a compaction / settling. Basically, in our area (mid Missouri) I don't think they see much insulation under the slab and don't know how to inspect for good installation.
I would use borate treated EPS foam. And it is make in KC, KS. The compnay used to be Contoured Foam, but I think that it changed names.They are a license of R-control foam and you can find them fro their link.You might see if they (or R-control) has any information to educate the BI."The only reason I was given was a compaction "Well foam is not compacted. What is compacted is the gravel base under the foam.I have a copy of the 2000 IRC. This is covered in R506. But realy does not have anything in pro or con about foam under a slab.Later versions might have more details. And if they have adopted an Energy Code check htat also. It might require the insulation.So find out exactly what codes they have adopted.And insulation around the foundation will help, but not near as much as underslab.Do you have the footing/foundation in? And what kind of construction.If no there are a couple of options to look at.Chapt 4 has all kinds of details on Shallow Frost Protected foundations. Bascially it uses foam to keep the footings from freezing and thus you only need then 18" deep. While they probably never heard of that either, the complete details are right in the code.Or if you are using 30-36" footer and stem walls then use IFC for the stem walls..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
i hate to speak in definitives around here, but,
your inspectors are idiots.
most of the people here are just being nice on their behalf by calling this an "unusual" decision on their part.
i typically get around ignorance by having an engineer specify the detail in a drawing and let them try to find a reason to disapprove.
can't imagine ever building a radiant slab structure without insul under and around the slab.
carpenter in transition
As a standin for Rez I award you the How to make friends with the BI award"..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
How to make friends with the BI award
Times 2.
Piz z him off and be right too.
Joe H
You don't have to pizz him off to get an answer. It is all in the approach, and how you ask them to explain the reason behind thier decision. Call it diplomacy or whatever, but most of the time there is a way to not get in a whizzing contest with the guy.
When all else fails, we have an appeals process, but you better have your ducks in a row before you undertake that route.
I'm like Tim on these issues. I'll buck them when I see thier ingnorance is going to cost me either money or quality. If I have to take continueing education classes to retain my licenes, shouldn't they, and that means keeping up with the changes in material and building techniques.
That sounds strange -- insulation is used beneath slabs often enough and is rated for some pretty high compression. Consider walk-in coolers, which can have several inches of rigid beneath the slab.
Do you have to insulate the slab? Are you just trying to insulate the foundation wall? I don't understand why you need to insulate under the slab with heat -- we've designed this condition without insulating beneath the slab.
Hmmmm. How is it efficient to heat the ground under the house when what you actually want to heat is the house? Not insulating makes no sense.
We use Foamular 250 insulation under slabs, heated or not.
I'm just saying that I'm not so sure insulation is absolutely necessary for efficiency. You heat the slab, which warms the air above it, then rises through the space -- what difference does it make if the heat ends up in the earth around the slab? it's still ultimately going to transfer to the air above, right? Hey, maybe not -- it ain't something a lot of people want or do around here (Indianapolis).
Certainly you wouldn't want the edge of a heated slab to transfer through a foundation wall, so i can see insulating between exterior foundation walls and the slab, but that should be done anyway, at least in a cold climate.
More information is probably necessary to determine the best installation anyway -- is this a radiant system alone for heating the entire building? Is it just for a bathroom? I think maybe the manufacturer would be best at determining whether the insulation is worth the cost of installation depending on the circumstances.
Doesn't a "do it all the time" approach ignore the potentially unique nuances of any given situation?"It depends on the situation..."
I'm in western Washington where the climate is mild and we are prohibited from pouring slabs without insulation if the space will be heated. It makes zero sense to my to leave it out. Someone smarter than I am could calculate how much energy you will be wasting by not insulating. I doubt it is trivial. Martin Holliday (he frequents the JLC forums) could comment on this, I may ask him.
"Martin Holliday "And here, those not as frequent.But he has a current question on the forum..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
David,
You're right. Unless you live somewhere where the average ground temperature is 72 degrees F, a heated slab will lose heat to the soil continuously all winter long. Here in northern Vermont, the soil stays a relatively constant 40 to 45 degrees when you dig down about 4 or 5 feet. Why would anyone want an uninsulated slab in a heating climate? You just bleed heat continuously all winter long. Let's face it, the planet Earth is a big chunk of rock and soil. Even though it's warm at the center, that heat is a long way off from your slab. Up here on the surface, that dirt and rock is pretty cold. All that cold rock and soil will happily pull heat from your slab all winter long, and trust me -- you're not going to bring it up to room temperature during the first day of heating season.
I have radiant heat (hydronic) in the floor. It's a DIY project for myself. In the past I've been able to insulate under the slab (it's my third house) without any inspection problems. I guess it's a new rule this time. They made me pull all the insulation out before we could pour concrete. I didn't insulate trenched footers ahead of time and now want to go back later and add foam board. I just am a bit concerned that I've disturbed the soil around them and a don't want unexpected settling.
It sounds like it might all be in place.In that case the you can dig down to the base of the footing. And from there out and down at a 45* angle for common soils..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Thank you very much. It sounds like a good plan.
Unless you can guantee a clear 6" or so between the top of the foam and any wood you should use the borate treated foam in this area..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Welcome back to BT.
What code is used in you area?
Many will tell you it is not good to question an inspector. I disagree. You have the right as a HO or builder to know why and for what reason he is not allowing it. He needs to give you, chapter and verse, the scource of his dissaproval. After all you are paying not only his inspection fee, but the taxes that help support the code inforcement department that employs him.
He might not have ever seen an insulated slab, but if it is not specifically prohibited by you state or local code, he has to do some research, not just issue some edict that says you can't use it. Look into you appeal process before you pour your slab.
OTOH edge insulation is a good thing. A high percentage of the heat loss occurs at the slab edge and within the first four feet under the slab. Having both the edge and the bottom of the slab insulated is your best combination, if possible.
IRC 2006
Easily dig down to the bottom of the footing and then at an angle if you need to go deeper. Use pink/blue/grey board. Flash the top up and uner the siding.
I agree ... strange he didn't want you to insulate under the slab ... I did it on my house. I think for radiant floors in the northwest they HAD to do it by the energy code (I may be corrected on that ... it's been a while).
Heat travels evenly in all directions. A mass that is at temperature will not 'stratify'. Heat loss down in a slab will be greater than heat loss up ... assuming ground temp cooler ... but as it reaches equilibrium, it will slow ... but the loss is still to the ground ... that is, the ground will have a tendency to 'suck heat' from the slab ... it is a very large heat sink. You will likely enjoy the heated slab, though ... just tweak the thermostat down a bit.
I hope you already haven't poured your slab... The inspector is going to cost you thousands and thousands of dollars in higher energy bills over the life of the house if you do what he says. I'm an HVAC engineer and have never spec'd a radiant job without slab insulation (2" minimum). I don't have the calculations in front of me, but with insulation, you can expect to see a 10% loss through the insulation into the ground. Without insulation, the equations get very convoluted based on where you live, the water table, and soils data. I'd bet 20-30% energy losses minimum. I really hope you don't live in an area where it rains a lot or has a high water table. I've heard of horror stories of people who live above ponds and didn't insulate. The water percolates through the soil, around and under the slab, down the hill, and into the pond. End result, 50degree cold house, 70degree warm pond. If I were you and the inspector wasn't cooperating, I'd call your Pex manufacturer and rigid insuation manufacturer. your Pex manufacturer will give you written data that it is absolutely needed and your insulation manufacturer will give you information stating that their insulation is made to go under slabs.
I really wouldn't let this one slide if I were you.
Ryan