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Extra slab insulation "worth it" for sporatically heated space?

strawmyers's picture

I'm building a 26 wide x 36 deep attached garage onto my house this year.  The front part will be for parking the two cars.  It won't be heated all of the time; but I am going to use the back part for a little "shop space" as projects arise and plan to have a propane heater for on demand heat.  It will be 2x6 walls with R-19 and ~14" of cellulose in the ceiling, garage doors will be insulated (R-12, I believe).  My original intent was just the "standard" for this area of 2" (R-10) rigid foam under the slab with 2" around the perimeter for a thermal break.  Had considered 3" (R-15) under the slab so it would be closer to the R-value of the rest of the structure; but that size would have to be special-ordered and wound end up being an additional $600 over the 2".  For this type of application, do you think R15 vs R 10 would make a "felt" difference that merits the upfront investment?

 

[edit] By the way, the garage will all be one space with a 14' ceiling, the "shop space" won't be a smaller space isolated from the parking space because I'll need to keep my tool boxes, etc along the walls beside the vehicles when not in use.   

No way is it worth another (post #207581, reply #1 of 28)

No way is it worth another $600 for the tiny improvement you'd get. At least, as far as payback is concerned, and any difference in comfort would be virtually undetectable.

The R-value of the slab is not really comparable to that of the walls because their respective heat transfer rates are so different.

R-10 meets or exceeds the current requirements of the Energy Conservation Code. BTW, in the coldest climate zones, the Energy Code requires either perimeter vertical insulation to a depth of 48", or perimeter horizontal insulation 48" wide, but not both. 

Insulating under the entire slab would be an impractical expense, if that's what you mean.

I was talking about (post #207581, reply #3 of 28)

I was talking about insulating under the entire slab; but that wasnt' the additional $600.  The extra was the cost of 3" at the perimeter and under the entire slab versus 2" for the same layout.  I'm surprised to read that insulating under the entire slab isn't recommended... seems like a lot of surface area to not have insulated.  At the perimeter, I was just going to do 2" or 3" thick 25psi EPS in strips on it's side vertically down about 12" on the inside of the foundation wall.  This is in central Indiana... do I need to be placing it down the entire foundation wall? (footer will be ~36' deep). 

My initial plan was:

1. Have the footer and wall poured,

2. Then put down a 6mil vapor barrier over the area excavated for the slab and up the part of the wall the slab will be poured up to.

3. Put the strips of 2" thick 12" wide foam sideways along the wall with the lower edge even with the top of the excavated ground and the upper edge even with what the top of the slab will be, bevelled at a 45 at the top to provide a thermal break while still allowing the concrete to be poured nearly touching the wall.

4.   Add 3-4" of pea gravel on top of the vapor barrier.

5. Add a layer of 2" foam over the pea gravel covering the entire area of the slab and butting up to the perimeter foam.  This will hit the perimeter foam 6" down to allow for a 6" slab.

6. Pour wire-reinforced 6" slab.

Input on the above plan?  Would you still not recommend insulation under the slab if the actual slab was heated (radiant)?

The reason full under-slab (post #207581, reply #10 of 28)

The reason full under-slab insulation is not really needed is because dirt is a good insulator if it's thick enough. Perimeter insulation, either vertical or horizontal, provides a shield to the exposed portions of the dirt to allow it to act as a barrier to heat transfer.

Nearly all of Indiana is in Climate Zone 5, which means that if you use vertical perimeter insulation only, you need to extend it down at least 2 feet below grade.

Admittedly, more insulation and full under-slab insulation will provide additional energy savings, but nowhere near enough to pay you back in your lifetime, even if you heated the place continuously.

For the intended use of your shop (intermittent), I can't see the value of installing radiant slab heat, luxuriously comfortable though it certainly is.

I don't want to tell you what to do.............. (post #207581, reply #2 of 28)

but will be happy to suggest blowing your money on a radiant slab.  Seems every time I have shop work-it's 10 degrees outside.  That means pulling out the propane heater and putting up with the fumes and the still cold concrete.  About tomorrow, the room is warm enough to be comfortable.

If I'd have done it right-like we did in the house, I'd have a happy wife with a warm car and a shop that was comfortable b/4 I needed it.

Something along the lines of glycol so no worry of power outage and freezes..............perhaps run off a dom. water heater and a pump.

I'm sure the costs wouldn't be warranted..............

 

However, some constant heat in there because you are insulating it better than most houses, might be a thought.  It's attached, so heat loss out one side won't be a worry.

You are one ambitious s.o.b.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


My ambition comes from (post #207581, reply #4 of 28)

My ambition comes from spending way too many nights in my parents' unheated, uninsulated pole barn in the middle of winter working on some unexpected vehicle problem, etc and being just miserable the entire time.  The goal when I get this thing built is to move all of my tools over to my place and have a tolerable working space regardless of the weather.  There will actually be a garage door on the side near the back of the garage to allow pulling a vehicle in/out of the back without having to move the daily drivers.

I had considered the in-slab radiant heating using a residential water heater; but had a few concerns:

1. If the garage is only being heated during "project time", seems like the slab wouldn't get heated up fast enough as compared to a large tube propane heater placed on the ceiling directely above the "shop" part of the barn.

2. If slab is heated all of the time, scares me to think what my added electric costs would be to run the dedicated residential water heater during the winter months.

3. I want to put in a lift eventually; but not sure on the exact location and if it will be a 2 or 4 post lift.  Worried about hitting a Pex line down the road when drilling for the anchor bolts for the lift.  How deep into the 6" slab would the Pex be placed?

4. I'm only running a 100 amp subpanel into the garage from the house.  Electric water heater + pump running, lights on, then the big 220V air compressor kicks on while I'm running the plasma cutter... am I going to overtax the subpanel? 

5. Would an electric residential water heater be able to heat the water fast enough to supply a nearly 1000 ft^2 slab?  I installed the geothermal to get away from ridiculous propane bills... don't really want to get back on the bottle with a 60-100 gallon propane water heater.

Here is what I did for my (post #207581, reply #5 of 28)

Here is what I did for my shop that has radiant heat.

  1. Rented trencher and put 2” Styrofoam vertically in trench.  3 feet below grade and 1 foot above.
  2. Built a 2 x 4 frame around the perimeter. 
  3. Put 8” section of 2” Styrofoam  6” inches in and supported by a metal stud on risers. Used plastic spacer ties to keep the foam separated.
  4. Put down gravel, vapor barrier and installed 2” foam
  5. Laid down 6” by 6” wire mesh on risers
  6. Attached Pex pipe in proper configuration a foot apart with 3 zones
  7. Poured the whole thing at once as a floating slab as it is detached.
  8. Used water heater as a heat source. Used glycol in case heater quit. Since yours is attached to the garage you may not need to do this or use a weaker glycol.
  9. There are many details which I am omitting but you get the idea. 

Even if you don’t think you want to do the radiant heat now, Pex is cheap and I would throw it in.  Good luck

Sorry, I think I'm following (post #207581, reply #6 of 28)

Sorry, I think I'm following what you're saying; but I'm not confident of it.  Here's what I'm getting out of the above:

You poured the "walls" (8" deep x 6" wide perimeter) and slab at the same time.  2" foam is on the outside of the poured wall from 1' above grade to 3' below grade.  2" foam is on the inside of the "wall"; but only extends 8" below the top of the finished concrete and does not extend above the finished concrete.  Is it just below the top of the finished concrete?  2" foam is under the entire slab, on top of the vapor barrier, which is on top of the gravel.  Does all of that sound correct?

Some questions: How thick is the slab and how deep is the Pex in the slab?

                             How many square feet is the slab?

                             How long is each run of Pex?  (I'm assuming 1/2" Pex)

                             How big is the water heater?  NG, LPG, or electric?

                             What pump do you use to circulate the fluid and how do you have it "zoned"?  T-stat probe in the floor of

                              each zone or on the wall?

                             What did you do to "finish" the outside above-grade foam from an appearance standpoint? 

Here's my 2 cents worth. (post #207581, reply #7 of 28)

A cold slab is not going to warm quickly from unheated.  My unscientific thinking is adding more insulation under the slab may make it colder.  This is because the ground temperature can be warmer than the ambient air temperature.  Adding a lot of insulation under the slab is useless, because heat rises, and the ground temp is generally closer to room temp than outside air in winter.  Even if the slab was heated, R10 or less would suffice for insulation. 

I'm with Calvin on the heated slab idea, since that seems to be what you want.  Beware that it takes a long time to warm a slab, so sporadic heating may be a problem.  I would keep some heat going all the time if you use the shop a fair bit.  I haven't heard many complaints about heated slabs at all.

Sorry, I think I'm following (post #207581, reply #8 of 28)

Sorry, I think I'm following what you're saying; but I'm not confident of it.  Here's what I'm getting out of the above:

You poured the "walls" (8" deep x 6" wide perimeter) and slab at the same time.  2" foam is on the outside of the poured wall from 1' above grade to 3' below grade.  2" foam is on the inside of the "wall"; but only extends 8" below the top of the finished concrete and does not extend above the finished concrete.  Is it just below the top of the finished concrete?  2" foam is under the entire slab, on top of the vapor barrier, which is on top of the gravel.  Does all of that sound correct?

Kind of. See attached drawing

Some questions: How thick is the slab and how deep is the Pex in the slab?

 How many square feet is the slab?    24 x 40 

 How long is each run of Pex?  (I'm assuming 1/2" Pex)  Each run is about 280 ft.  Don’t want to run more than 300 ft.

How big is the water heater?  NG, LPG, or electric?  40 gallon electric.  I built the shop behind one of my rental houses.  Only have electricity to it.  You may want to explore the mini-boilers.  When I built it, that was the cheapest option.  By going to a min-boiler you cut down on glycol costs.

What pump do you use to circulate the fluid and how do you have it "zoned"?  Just have one pump hooked to all 3 zones.  Pump is hooked to thermastat. Any radiant heat pump will work,

T-stat probe in the floor each zone or on the wall?  No probes

What did you do to "finish" the outside above-grade foam from an appearance standpoint.   You can do stucco or I just used aluminum flashing.   This is not part of my house so I didn’t get real fancy.

 I can send you some drawings of the layouts if you want when I have more time. This details the slab, radiant heat, wiring etc.

 

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Shop_Floor.pdf95.46 KB

I do think I'll install the (post #207581, reply #11 of 28)

I do think I'll install the Pex lines before the concrete pour.  Even if I don't utilize it right away; for no more added cost than it will be, it will be nice to have it as an option down the road.

Do you know how much heat is required? (post #207581, reply #19 of 28)

A concrete slab 24 x 40 feet at 4 inches thick weighs in at 17 tons.

It takes 0.2 Btu to raise one lb of concrete 1 degree F.

If the days temperature is 40F and you raise the slabs temperature to 70F that takes 229,718 Btu.

But, you only have 2 inches of insulation underneath and none above or round the edges - not a good idea.

The heat loss will be very high - it cannot make sense to do this?

Another option, for your (post #207581, reply #9 of 28)

Another option, for your workspace, is to lay OSB or some such over the floor, to provide a bit of a thermal break from the concrete.  Of course, you wouldn't want to do this in the vehicle area.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

I say forget heating it with (post #207581, reply #12 of 28)

I say forget heating it with a water based system, install and electric radient system near the top and tile over it.  Keep the heating wires as close to the top as possible.  Set the witing up on a simple timer, so you hit the switch and come back in an hour and the floor will be warm.  That combines with an air heater will make your shop very pleasent to work in ON DEMAND.

Cheap to install and run too.

YAY!  I love WYSISYG editing!  And Spellcheck!

____________________________________________________

UFH warm garage? (post #207581, reply #13 of 28)

As written elsewhere UFH concrete slab takes forever to warm and to cool down.

To run UFH economically you need at least four inches of closed cell insulation below and arround. Concrete has the capacity to absorb a tremendous amount of heat before it starts to emit.

A better way to heat your work shop is Radiant Quartz short wave heating.

The joy of quartz heating is its like the Sun! Switch it on and the heat is there instantly, switch it off and its cold.

It only heats the space you are working in, move away and the area is cold.

Its cheap to run.

Perry (post #207581, reply #16 of 28)

I'm asking again for you to explain your expertise related to Hydronic heating.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


I'll have to admit that, for (post #207581, reply #17 of 28)

I'll have to admit that, for a shop with intermittent use, electric radiant heat from above is a very attractive option.  It's essentially instant, and it keeps you warm enought to work comfortably.  The downside (besides operating cost) is that the heat is uneven -- one side of you is warm and the other cold.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Frost Protected Shallow Footing? (post #207581, reply #14 of 28)

I'm on the fence about the extra insualtion being useful for occasional heat. However, I wonder if you've considered doing a frost protected shallow footing? I did one with my garage (Check out the article in JLC, I think December, 2012). Essentially, extending the foam under the footers and out a few feet from the building saves the concrete and excavation of a frost footing. The IRC describes this for heated buildings, but you have to get the ASCE document that part of the IRC is based on for the details to use with an unheated building.

Andy Engel

Never try to teach a pig to sing. It wastes your time and annoys the pig.

I don't see how that could (post #207581, reply #15 of 28)

I don't see how that could work very well for an unheated building.  At least not in places subject to serious frost.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

The idea is that controlling (post #207581, reply #20 of 28)

The idea is that controlling groundwater and trapping the geothermal heat from below prevents the ground below the footings from frost heaving. In my case, I had to insulate the ground 4 feet out from the building. Piece of cake. The building has been through three winters and there are no apparent issues. And as my original post mentioned, it's an engineered design put out by the American Society of Civil Engineers. The insulation details depend on the climate zone.

Andy Engel

Never try to teach a pig to sing. It wastes your time and annoys the pig.

No expert - Hot water under floor heating - a potted history. (post #207581, reply #18 of 28)

I bought my first house in 1972 - gutted it, did a complete renovation, lining the walls and ceilings with inch thick polystyrene panels. Very advanced stuff in those days - Dow had only brought Styrofoam to market a few years earlier, (52  years ago) -  I could see how it would make a difference.

I fitted base board heating round all outside walls, with the heating in each of the main rooms controlled by its own thermostat and motorized valve, with the exception of the conservatory where I fitted UFH a reinforced 4 inch thick concrete slab, with four inch thick Styrofoam under to control the downward radiation and conduction, divided into three zones, controlled by two thermostats and motorized valves,  I had floor to ceiling patio doors all round and a plastic roof. The house worked well, being one of six identical houses and has since been more comfortable and cheaper to run than the others. The only part I got wrong, was the conservatory and its UFH! It worked perfectly during the winter and most of the year but, when the sun came out it turned into an oven, with temperatures often over 50C - you couldn't breath it was so hot, no shade, you just had to have all the doors open. Since then, no more plastic/glass roofs. just solid roofs with 8 inches of polyurethane foam to keep the temperature under control.

I am no expert, merely someone who has been renovating homes for a long time.

Just thought of something...  (post #207581, reply #21 of 28)

Just thought of something...  so I plan to run the foam on the outside of the foundation wall from the footer to 1' above grade, as earlier instructed.  On the inside, I'll be doing a ~12" tall, 2" thick perimeter strip of rigid foam with the top bevelled 45 degrees to allow the concrete to be poured nearly up to the foundation walls. 

Question is, what about at the garage door and service door openings?  I'll have a total of 33 linear feet of door openings.  How should the interior and exterior perimeter foam be applied in these areas?  If just bevelled, would the thinner concrete not break when driven over?  If left 3-4" shy of where the top of the concrete will be for strength, wouldn't that allow the kind of thermal bridging that the insulation is being installed specifically to prevent?  Thank you to everyone for your continued help!

An important point to (post #207581, reply #22 of 28)

An important point to remember is that if you've got a garage door then fancy insulation for the rest of the structure is just about useless.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

They're going to be 2" thick (post #207581, reply #23 of 28)

They're going to be 2" thick R-17.6 garage doors.  I wouldn't think that would negate insulating the rest of the structure.

No, but it does make it silly (post #207581, reply #24 of 28)

No, but it does make it silly to invest a lot in insulating the rest.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Why do you think it silly? (post #207581, reply #25 of 28)

If he maintains the seal at the bottom and the vinyl WS's on the other three sides-where's the silly?

You need a man door so you lose little heat upon entry.

Of course, you lose heat by opening and pulling cars in/out-but with a radiant floor-you don't lose the slab heat in that short of time.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/

 


The seals will never that (post #207581, reply #26 of 28)

The seals will never that good (at least not for long), and the true R value will never approach 17.  And if 1/6th of your surface is R17, it doesn't matter much what the rest is, beyond about R24.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

There will be a 36" exterior (post #207581, reply #27 of 28)

There will be a 36" exterior door roughly in the middle of the east wall.  Front (south) wall will have the two 10' wide doors we'll use for the daily drivers.  West wall will have a 10' door toward the back so I can pull my off-roady truck or the tractor in/out of the "shop" space without moving one of the vehicles.  Thought about putting a man door on that wall, too; but I need the wall space for tools/work bench more than I need another means of egress.  East (back) wall will be connecting wall to the house and the existing 36" exterior door will become the door to the garage.

 

... so...... any input about how to lay out the interior/exterior foam in all of these doorways before the concrete pour? 

I thought about this, but did not do any study for you. (post #207581, reply #28 of 28)

But-

Use the idea of laying foam beneath the slab-out under the apron to the exterior.

Under the door-place a 1/2" vertical pc of foam-you might find a "U" channel that will fit over that foam-set the foam and "U" to the screed ht.  Edge along that metal strip-after the pour-pull up the "U" and fill depression with Urethane caulk to top of concrete.

or

Instead of just a 1/2 inch foam verticle, use 2" and taper it up from 2" wide at an angle, narrower at the top (to a 1/2 inch).  Hold this a half inch below the screed line.   On top-pin a 1/2 by 1/2" pc of wood.  Use an edger on either side to break the bond (like you'd do along a form).  After set-pull the wood out and fill that depression with Urethane caulk.

or

take a bit of time and seach for thermal break in a concrete slab.   Somebody is doing it for sure.  Some may have perfected it.

 

edit:   here's one- http://www.greenbuildingtalk.com/Forums/tabid/53/aff/14/aft/77192/afv/topic/Default.aspx

a good place to look-GreenBuildingAdvisor:  take a close look at number 6 below

http://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/19854/slab-garage-door-how-insulate-hold-back-pex

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


http://www.quittintime.com/