Insulating a slab

[Jon E]

Been thinking about building a garage, here in Vermont frost depth is between 4-5 feet depending on the winter, so I was going to put in full frost walls and a slab inside the walls.

Now my budget went to crap and I still want to do this but I can only afford to put in an "Alaskan Slab" - basically a thickened at the edges, slab that floats. I need to find a way to insulate this enough that I can run radiant tubing through it from my OWB. The guy that owns land next door put in a barn and a fairly thin, normal-looking slab and put just some thin insulation down underneath it and radiant tubing. I cannot see how he won't lose a pile of heat through the edges of the slab.

Anybody done this effectively?

 

[HEAVY FUEL]

Do some research on Frost protected shallow footings. Haven't done it myself but it's getting more popular.

 

[VINIFIREWOOD]

The standard here in MN is 2 inch high density foam underneath. some guys dig in full sheets around the outside standing on edge but unnecessary, hard to believe but frost does not travel sidways.

 

[Khtsr44]

Slab insulation

It is a monoslab.
Been around for 40 years.
Poly under and 2" of foam and you will not believe how much it will NOT take to heat it.
Use rebar and not wire.
You are lucky to be in the warm part of the world!

 

[Streblerm]

The standard here in MN is 2 inch high density foam underneath. some guys dig in full sheets around the outside standing on edge but unnecessary, hard to believe but frost does not travel sidways.

This is an interesting comment, care to elaborate? I'm not saying it isn't true, just that it is counter to what I have read. Heat will move to any colder area, so insulating the edge seems like a good idea. Isolating the slab should equal better heat retention. Around here it is code to insulate basement walls.

My research led me to insulate around my slab, down to the footings and then 4' in from the edges under the slab. The center of the slab has no insulation. I was trying to create a "heat sink". If I run my heat a couple times a week, my slab stays around 50 degrees.

I have radiant tube heat at the ceiling, not in the floor. I did read that if you have radiant floor heat, you want to insulate under the entire slab to reduce the response time for temperature rise, and not to loose heat to the ground.

 

[loadthestove]

When I built my house in 01 I poured my footers below the frost line(24 inches) then poured my slad on top of that.I then bricked from the footers up.My brick mason left a small amount of airspace between the brick and the actual slab.This gives me a dead air space below the frost line.I have ductwork directly in my concrete slad some my floors stay very warm.
This works well for me in southeastern ky,but compared to a lot of places we bascially just have a cold spell for winter.rarely do we ever see below zero tempatures,and if we do its is only for a few (Very few) days.

 

[logbutcher]

It is a monoslab.
Been around for 40 years.
Poly under and 2" of foam and you will not believe how much it will NOT take to heat it.
Use rebar and not wire.
You are lucky to be in the warm part of the world!

Pretty much it. It's called "sandwich" slab here: compact the base first, poly for moisture, 2" or more foam sheets under AND over the slab, then edge foam for security.

P.S. Where did the " frost doesn't run horizontally" come from ? Never heard of this. Then again..........

 

[GRAYSTONE]

Kawartha Lakes Slab Floor

I just finished building a house on a floating 40 x 40 slab with radiant heat.

Entire Floor is done with rebar on 12" centers. 5 Perimeter bars were added to hold everything in place. Slab was 14" thick at the side and we placed 6" of durafoam insulation under the slab. 2" Under the edge and stepped in to act as a footing. Edges of slab were covered with 2" of durafoam and durafoam was extended 3 feet out from the slab all around the perimeter. Keeps frost from penetrating under slab. Radiant floor heating tubes were installed on 12" centers attached to the rebar. Tubes were set 2 1/2" below the finished surface.

I also recently did a slab garage floor with 8" thick slab and 2" of foam under the slab and around the perimeter as described above. Rebar was used on the perimeter and mesh on the remainder of the floor.
Insulation on the perimeter is more important than insulation under the floor.

I will post some pictures when I get a chance. I have lots of pictures if anyone wants to PM me I will email you out a bunch.

 

[barkeatr]

Im in the design trades here in northern NY. I have used the shallow frost protected slab for years. Simply overhang your wood two by six wall two inches and then turn up the ridgid insulation to meet the PT sill. Cover the exposed insulation with flashing or vinyl product they make for that purpose,or you can stucco it.

it may be good to also extend a wing of insulation out from your buiilding to keep frost from getting under the wall and lifting. two inches thick two feet out works but there are formal guidlines available.

I had a fellow architect question the structural strength of overhanging the wall two inches..and I explained you still have the same "meat" on the wall as you do with a two by four wall...he didnt get it and has since moved south.

Code requires that horizontal insulation wing to be 18 inches down and that is a good idea otherwise grass wont grow well if the insulatin is just a few inches below.

good luck.

 

[barkeatr]

sideways

frost wont travel sideways thats right but it will freeze the ground under the wall where the heat loss from the building is begins to run out..and that ends up being a 45 degree angle of frost under a building if there is no insulative wing and there is water there. Many uninsulated slabs work but water has to be removed from the equiation. YOu need to get rid of water or freezing to be frost free, most of us try to do both for redundancy. Low budgets may dictate less reduncancy...

the edge of the slab is the most important because its subject to the coldest extremes..but in northern vermont...i would insulated under the whole slab for sure...

barkeater

 

[Marine5068]

Concrete Floor Slab Building Code

What GRAYSTONE posted is great information.
We both live in Ontario and get some EXTREME Winter weather here.
One Winter day it's cold and dry and next it's snowing sideways and visibility is near 0.(By cold I mean down to minus 35 degrees Celsius)
Frost can be in the ground from October to May sometimes in Ontario.
You should always check your local Building Code for proper foundation depths and insulation techniques when doing any construction.
The actual technical term for a thickened edge, reinforced slab-on-grade is called a perimeter frost wall.(sometimes also called a grade-beam)
More insulation is always better. Don't cheap-out here. After all, you can't re-do it and it's the foundation of the whole structure so it's VERY important to do it right, the first time.
Do some Internet research on cold climate heated concrete floor systems (that's a mouthful too). I'm sure there are company websites for this type of construction too and may give you good info. or contact info. to call someone and talk about heated floor systems.
Or....Just Ask MIKE HOLMES.
~Stan (Madoc, Ontario)
-Architectural & Mechanical Technologist
-Certified Welder / Fitter

 

[davec]

These are what you are looking for:


http://www.toolbase.org/PDF/DesignGuides/revisedFPSFguide.pdf


http://www.buildingfoundation.umn.edu/MHFAfrostFoundation.htm

Yes, there is still a lot of science out there on building. It just isn't as well-distributed as would be best.

-Dave

 

[chart321]

Would love to see some photos

Would love to see some photos of a shallow frost wall with radiant heating.

I just finished building a house on a floating 40 x 40 slab with radiant heat.

Entire Floor is done with rebar on 12" centers. 5 Perimeter bars were added to hold everything in place. Slab was 14" thick at the side and we placed 6" of durafoam insulation under the slab. 2" Under the edge and stepped in to act as a footing. Edges of slab were covered with 2" of durafoam and durafoam was extended 3 feet out from the slab all around the perimeter. Keeps frost from Would love to see some photos of a shallow frost wall with radiant heating.



penetrating under slab. Radiant floor heating tubes were installed on 12" centers attached to the rebar. Tubes were set 2 1/2" below the finished surface.

I also recently did a slab garage floor with 8" thick slab and 2" of foam under the slab and around the perimeter as described above. Rebar was used on the perimeter and mesh on the remainder of the floor.
Insulation on the perimeter is more important than insulation under the floor.

I will post some pictures when I get a chance. I have lots of pictures if anyone wants to PM me I will email you out a bunch.