What To Insulate?!

[conmoto]

I'm having a 24x24, slab on grade, built this summer.

7 courses of block needed for the back as it is partially built into a slope in the back yard. I live in MN and plan to add a 5000 watt electric heater only to the the edge off those really cold days (I'd be happy with 35° in there) and to spot heat on the few days I want to work in the garage.

My questions:

Do I add rigid insulation along the outside of the block/foundation to cut down on condensation for those hot/humid summer days? Will rigid insulation on the outside keep warmth in or is batt insulation under drywall good enough? I plan on finishing the inside someday (drywall).

Do I insulate under the slab? It'll cost $1400 to do so.

I'm struggling the idea that slab insulation will keep the floor from getting damp (condensation...cool ground/humid air), but will the slab insulation also prevent warmer ground temperatures from rising into the garage? Assuming the ground actually helps raise garage temps in winter. I just don't know.

FYI - I chose to go with electric heat only because running gas to the garage will be costly and the handful of days I spend out there in the winter don't justify in slab radiant.

Thoughts?

 

[Radix2]

If you are going to finish it, insulate everything. In MN,the ground is not going to heat the inside, in fact, a big heat loss is to the frozen ground since it is direct conduction.

24x24 is 576 /(4x8)=18... 18 sheets of foam is $1400?

insulation and a vapor barrier will help with inside humidity, but if you open the door to warm moist air when the slab is cold, it will get damp.

 

[conmoto]

Yeah, I wish I knew why it costs $1400. Labor? Is this something i should question the builder on?

Thanks for your input on the slab insulation.

 

[DougWil]

You should go over to GreenBuildingAdvisor and do a bit of research.
http://www.greenbuildingadvisor.com/

Basically the biggest bang for your buck is ceiling and wall insulation, properly installed (a lot is poorly installed with reduced effectiveness), all air infiltration gaps/holes foamed off, and a vapor retarder like 2 coats of latex paint over drywall.

Way down the list is footing and slab insulation.

 

[GYPSY400]

Insulate the slab for sure! My garage stays warm with the heat off for multiple days.. ( it will go down to about 45-50F after 2-3 days) and it never has dampness on the floor, summer or winter. Once warm, the cement will hold heat like a big heat sink.. if it's not insulated it will be a giant ice cube.
The rigid on the outside will help, but if your not heating it constantly I don't think it's necessary ( mine is only Batt R20)

Any consideration of a different heat source like propane? If you can keep it above freezing it will be a lot easier to warm up when you need to.

Sent from my SM-G903W using Tapatalk

 

[conmoto]

Any consideration of a different heat source like propane? If you can keep it above freezing it will be a lot easier to warm up when you need to.

I thought about propane, but what I've read about moisture and carbon monoxide as a byproduct have steered me away. I know electric heat is more expensive, but it's dry and easy. I don't imagine it'll be used too often. Maybe I'm dreaming, but if it's between 20-30° outside, I'm guessing the insulated garage will maintain around 30-40° without any help at all. It's those handful of subzero° days that I intend to let it run as needed.

I'm just old enough to really dislike getting into a rock hard icebox with **** to scrape off the windshield. Plus, I've always felt guilty for starting a vehicle when it's -15°.

 

[slowthump]

I live in northern Minnesota and I would strongly recommend insulating the slab. You can't go back and insulate it later. My garage has a heated and unheated side. I insulated under the slab in the heated part and not the other side. Every spring and early summer the floor in the unheated/uninsulated part is wet and slippery because the the warm, damp, humid air is condensing on the cold slab. Hot air rises and heat radiates. In the winter the floor will **** a ton of heat and transfer it too the soil.

 

[GYPSY400]

I thought about propane, but what I've read about moisture and carbon monoxide as a byproduct have steered me away. I know electric heat is more expensive, but it's dry and easy. I don't imagine it'll be used too often. Maybe I'm dreaming, but if it's between 20-30° outside, I'm guessing the insulated garage will maintain around 30-40° without any help at all. It's those handful of subzero° days that I intend to let it run as needed.

I'm just old enough to really dislike getting into a rock hard icebox with **** to scrape off the windshield. Plus, I've always felt guilty for starting a vehicle when it's -15°.

http://www.reznorhvac.com/en/na/products/products-unit-heaters-udap

I have a unit similar to this . Moisture and carbon monoxide are non issues as the burner is vented to outside. These units can be speced with either natural gas or propane regulators
..I bet you could run comfortable all year on a 100lb tank

Sent from my SM-G903W using Tapatalk

 

[myredracer]

I insulated under our slab with 3" closed cell rigid foam that I got from a building material salvage place. Price was really good and only had a few pieces with chunks missing. Huge improvement over the last garage/shop of 1800 sq. ft. I built which I didn't put insulation under it. One good thing about rigid foam is you can put the poly sheeting under it and not have to worry about it getting punctured during the prep and pour work. I put rebar in our slab and wet-doweled it all around. Have had a skid steer loader on it and no cracking. I made the slab 5" thick and used zip-strips for crack control.

We were down to around 12-15F this winter (very unusual) for extended periods and it was comfy warm and I worked in a T-shirt the whole time.

 

[adam0872]

Insulate insulate insulate everything!

 

[beakie]

You should go over to GreenBuildingAdvisor and do a bit of research.
http://www.greenbuildingadvisor.com/

Basically the biggest bang for your buck is ceiling and wall insulation, properly installed (a lot is poorly installed with reduced effectiveness), all air infiltration gaps/holes foamed off, and a vapor retarder like 2 coats of latex paint over drywall.

Way down the list is footing and slab insulation.

I didn't check that site, but that can not be right.

you can heat all you like, but if your feet are cold it will **** to work in.


2" around the slab, even go down further than the slab if the option is there.
2" between the block & earth too.

insulating a foundation in an area that get's freezing temps for extended periods of time is never a waste of money.

 

[ZacSpade]

How come your worried about when it's hot so much? It's when it's cold that it's going to cause bigger issues imo.

A slab has large thermal mass and whatever it is outside it will transfer the temp straight in. Think of it like a 24x24 radiator vs your heater.

You can heat the slab up in winter time by having the sun hit it. Whether that is having a portion of your slab outside on the sunny side of your garage or use windows. This will also help with the moisture.

You can put down carpet offcuts where you are working to help with insulation and comfort for your legs/feet after the fact. Working on concrete for long periods of time isn't great for you.

Hope this helps

 

[DougWil]

I didn't check that site, but that can not be right.

you can heat all you like, but if your feet are cold it will **** to work in.


2" around the slab, even go down further than the slab if the option is there.
2" between the block & earth too.

insulating a foundation in an area that get's freezing temps for extended periods of time is never a waste of money.

I said biggest bang for your buck.

Insulating the slab w/o insulating the walls and ceiling is a far lower rate of return on your investment.
Sure if you have the money, insulate everything.

But you know it is a lot cheaper wearing a good pair of insulated boots then trying to heat an entire slab to keep your feet warm.

 

[DougWil]

Maybe I'm dreaming, but if it's between 20-30° outside, I'm guessing the insulated garage will maintain around 30-40° without any help at all.

If it is 20-30 degrees outside, eventually it will be 20-30 inside with or without insulation.
Insulation only slows the heat transfer, doesn't stop it or make heat.

 

[conmoto]

I appreciate all the input. I've decided to insulate the slab. Still undecided on the heat source. Vented propane heater seems nice, but expensive, and a hole in the wall. Leaning towards 5000 watt electric and hoping that it will rarely get used to counter the really cold days.

Now, how to tactfully ask the builder what I can do to minimize cost by doing things myself.

 

[beakie]

I said biggest bang for your buck.

Insulating the slab w/o insulating the walls and ceiling is a far lower rate of return on your investment.
Sure if you have the money, insulate everything.

But you know it is a lot cheaper wearing a good pair of insulated boots then trying to heat an entire slab to keep your feet warm.

having worked in a few un-insulated garages I would argue that point.
the OP said this space will be heated only now and then (take edge off, or if working out there)

if I wanted to heat a building, and keep it heated I would insulate the walls and ceiling to retain all the heat. but just popping out to work for a day, the heater will be on for a while anyway, and the heat won't just dissipate through the walls/ceiling in a matter of minutes.

I have worked in heated but un-insulated garages, the heater runs a bit longer, but it's still warm.
Un-insulated slabs, those **** to work on for longer periods of time.


my shop right now (there when we bought the home),24x36', no slab insulation, 2x4 insulated walls, 8" of blown in in ceiling.
in winter the floor is always cold, even if I keep the shop at 15*C for a week.
in summer the floors are damp/wet much of the time.




just my opinion but I like a warm floor, or at least not a near frozen floor.

 

[matt_i]

My thinking on the floor has changed.

If you run the R = (delta T)/Q(dot sub A) equation for concrete with R=1 and a delta T = 60-50 = 15 degrees, the magnitude of the heat flow value is 15 per unit area. Take the wall at R = 19 but a delta T of 60-20 = 40 degrees, the magnitude of the heat flow value is 2.1 per unit area.

There's a convective value modifier missing for the fact that the heated air rises but you can see for well insulated walls, my conclusion is that the biggest heat flow is going to be thru the floor.

Some more info here
https://en.wikipedia.org/wiki/R-value_(insulation)