under slab insulation

Wayco Wayne_2

preparing a site for a pour. It will be 1/2 pex in slab. I told the contractor to insulate around the perimeter with 1 inch eps blueboard and to put 2 in in under the slab. However... the slab needs a pier/footing halfway down the room to support a bearing wall, since the span is too long for the joists for the room above. I've seen these monolithic footers in garage pours. The contrator just trenches deeper across the bottom of the slab across the room so the bearing wall has a solid foundation to exert pressure on. I just insulate the trench vertically and at the bottom. However the building inspector won't hear of it and wants a signed report from an engineer before he will allow the pour that it is OK to insulate the footer. Am I nuts? This doesn't feel like a problem to me. However I'm not a concrete foundation engineer either. What is the experience from those out there on the Wall? Thanks for any input. WW

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kevin coppinger_4

Is the footing...

in the middle of the basement? if so I don't think that Not insulating around it will make all that much diff. in the heat loss. Just insulate on top of the footer before they pour the slab...I hope thats what you meant..good luck Wayne. kpc

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freon12

pour

why dont they pour the footers 1st and then you insulate,lay mesh, pex, then pour the slab....

Thats the only way I have ever seen it done....maybe I'm just lucky :P

Constantin

I'd do it in two pours...

In my house, we did all the foundation footings in one pour, the walls in the second pour, the slab on the third.

The footings in the middle of the house were actually strip footings to allow the folks placing the new posts for the main beam complete freedom as to where to site them. Concrete is cheap and the long strip footing is actually easy to make - may be faster than making individual footings for the posts.

Anyway, I wouldn't bother insulating below the pole footing. Just do that one first, put in the pole, put a surround around the pole (to isolate it). Then put in your blueboard and vapor barrier.

I suppose that if you dig down deep enough for the footing, you can even lay the insulation on top of the footing concrete and around the pole. Otherwise, just butt up to the footing. The extra heat loss from a small area in the center of a basement space is miniscule.

kevin_5

I've seen it done \"the hard way\"

I tell them with no insulation it is going to cost them a lot of BTU's, but usually the engineer is "trump".
I use slab shield flexible foam, and usually I can at least let it hang down in the sides if not the bottom of those things. Kevin

Wayco Wayne_2

Sounds like

the strip footings that Constantine described. I told the HO that there's not as much loss in the middle of the slab but keep in mind, this is the HO that took all of his walls and drywall down so he could foam the whole house. He is determined to get the least amount of heat loss possible. I will be interested to see what his engineer says on this matter. WW

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Constantin

Here is some clarification,

... just in case I wasn't clear enough. Here I am showing a sectional view of our foundation strip footings with a pole in place. Note how the pole is not attached to the floor in any manner - it only exerts its weight on the footing below.

This way, the floor will be less likely to crack where you don't want it to (it will crack, it's concrete, but the trick is to make it crack microscopically and only in places where you won't see it). Remember to isolate the pole from the slab with a sheave or somesuch, then seal the sheave to the pole.

This setup is simple to achieve, just excavate a little more, and you get all the XPS goodness under the slab you're looking for.

Constantin

Speaking of which...

... think long and hard about cracks. If the customer cares (I did), you can incorporate breaks into the floor at the time of pour that allow the slabs to crack where the customer is least likely to notice.

For example, in our house, we put breaks on center between the poles that hold up the house above. Since the poles disappear into non-load-bearing walls, the cracks running between them disappear beneath those walls was well. The openings between rooms will have thresholds, so the concrete cracks there will be invisible also.

These breaks do not have to penetrate all the way through the slab. Ours were about 4" high with the wire mesh stopped on either side. This creates a weak spot where the concrete is likely to break because it cannot rely on the steel to help it with tension and the 2/3 break acts like a notch to weaken the spot even further. Worked great!

Unknown

in addition to what others are saying, you want to flip your insulation requirements; 2" perimeter and edge, 1" underneath. edge and perimeter is where the highest heat loss is.

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Wayco Wayne_2

Picture is great

but not how this will be done. A monolithic pour means the slab and the footing will be contiguos(sp?). In other words, all the same pour. The HO knew an engineer who laughed when he heard of the inspectors concern. According to his charts the Blue Extruded 2 inch styrofoam had a higher pressure rating than compacted dirt, which is acceptable. The engineer will sign off on it so the inspector doesn't have any responsibilty. He wants the footing to be 8 inches deep and 18 inches wide. I believe the bearing wall being built will be concrete building modules, what we used to call cinder blocks. Alls well in the kingdom. All that;s lost is a couple days and the price of an engineers report. WW

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Constantin

Wow, that's a small footing...

... our three story house has 24" wide by 12" high footings... which are reinforced with rebar. The walls tie into the footings with even more cast-in-place rebar. The only thing that "floats" is the floor.

Given the previous foundation failing after only 130 years, it was high time for 12" of 3500 psi cement, #4-60 rebar on 16" V. & 14" H. centers, extra rebar around re-entrant corners, etc. to ensure that the new foundation lasts a bit longer. :-)

Wayco Wayne_2

You must remember

this is not the main footing around the perimeter. This is a strip across the middle of the room to support the middle of a span. Also here in MD our frost line is only 18 inches at worst, (usually no more than 6 inches) and they require 24 inche footers on the perimeter. WW

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jbplumber

Wayne

Its more of a " Grade Beam" then a footer Transmitting the stress and weight to the outside footer walls. Correct yes? Best Wishes J.Lockard

Constantin

Very good points!

I prefer separate footings and slabs for the simple reason of crack-propagation.

A monolithic pour is much more likely to create cracks when the concrete shrinks as it dries because part of the floor is "pinned" by the footing. Granted, you can minimize the size of the cracks through the generous use of rebar which is inexpensive... Plus, if the space isn't that big, then there won't be any huge cracks anyway.

On the other hand, I have been to homes where the basement floor could only be characterized as a crater-field. Such homes could benefit from the application of muriatic acid, followed by a thickened epoxy to fill in the cracks and make them dissappear a bit. I may yet coat the basement floor in our home with such an epoxy finish (usually found in car garages) to prevent the concrete from being stained by the usual vagaries of life.

As you can probably tell, I have a bit of a cracks-in-concrete fetish. :-)

Wayco Wayne_2

Thanks Jim

I know what it is but I don't know what to call it. :P WW

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