Central Oklahoma Workshop; Is Rigid foam insulation needed under the slab?

[ron_marsh]

Hello and Howdy! I'm planning on building a 40x40x10, 3:12 pitch workshop/garage, steel building, in Central Oklahoma. Any feedback on whether, at this latitude and climate, insulating under the slab with rigid foam insulation will provide a worthy return-on-investment? Plan to condition 1000 sf of the 1600 sf space. Thanks!

 

[Randy in Maine]

Yes. 2" please.

 

[homeinthegarage]

...with underfloor insulation, it'll be warmer in the colder months...but also warmer in the summer months.

Unlikely to pay back unless you plan on being there a long time...

 

[DEnd]

If you are planning on conditioning the space 24/7 yes. If you are only planning on conditioning the space for a few hours a week, probably not.

 

[Gerald O]

Most building codes in the US now require under-slab insulation to very specific R-values for any conditioned space with a slab floor-- regardless of how frequently you plan to use it. The slab edges also need to be insulated. If you are planning on 'legally' conditioning it, then check your local building codes.

 

[My Old Tools]

Not down here in the south. Our frost line is 0. The ground doesn't freeze and an ice storm might last 3 days. The next week you have to mow. I haven't seen any slabs insulated here, even on half million dollar homes.

 

[ron_marsh]

Thanks to All of you for your feedback.

The 40x40x10 space will be split into 3 zones: one dedicated to garaging 2 cars; one which will be used on a part time basis; and the third which will be used [ probably lived in ] pretty much full time.

I would like to achieve a tight envelope to optimize energy efficiency in the heating and cooling of the area (the garage will not be conditioned to the extent of the workspaces.)
Oklahoma's climate does not compare with that of the Northern states, but we do get chilly in the Winter and hot in the Summer. Average highs and lows are as follows: Jan, 50/29; Feb, 55/33; March, 63/41; April 72/50; ... July, 94/72. Plus, the summer humidity is definitely a factor.

I can see that there is a spread of opinion (man am I good or what!) and that I will need to determine what is appropriate for my particular requirements, but any and all considerations and feedback are appreciated.

 

[Falcon67]

Not down here in the south. Our frost line is 0. The ground doesn't freeze and an ice storm might last 3 days. The next week you have to mow. I haven't seen any slabs insulated here, even on half million dollar homes.

What he said.

And I know where Quinlan is too

 

[Gerald O]

Did a little checking for yall.
Statewide, Texas is still on the 2009 IECC code, which doesn't require under-slab insulation in zones 1-3. Some parts of northern Texas are in zone 4 and R-10 insulation is required.
https://energycode.pnl.gov/EnergyCodeReqs/?state=Texas
However some cities in Texas have just adopted the 2012 codes, which are tougher:
https://www.energycodes.gov/adoption/states/texas

Oklahoma is further behind on the code cycle. They're using 2006 IECC:
http://www.energycodes.gov/adoption/states
Also most of Oklahoma is zone 3, so underslab insulation is not mandatory for most of the state.
https://energycode.pnl.gov/EnergyCodeReqs/?state=Oklahoma

 

[ron_marsh]

Gerald_O and Falcon67, MyOldTools, Randy-in-Maine, HomeinTheGarage, Dend, and everyone else. Thanks, truly! I'm thinking that, as Ross @ MyOldTools has stated, the frost line is not a factor, so as he had suggested earlier, instead of insulating under the slab (will definitely place thermal barrier netween slab and foundation) I think that instead I'll put that money to good use in the building's shell. Great web site, great forum, and great folks. Thanks for your time and experience.

Falcon67, my first car was a used '65 Rambler. Loved that hunk of junk. Bought it for $65 and my Dad and I worked together to get it running. Not pretty, but man could I go " Fast as a Rambler Goes"... (Kenny Chesney song). Hammer it down, Falcon.

 

[theoldwizard1]

Thanks to All of you for your feedback.

The 40x40x10 space will be split into 3 zones: one dedicated to garaging 2 cars; one which will be used on a part time basis; and the third which will be used [ probably lived in ] pretty much full time.

I would like to achieve a tight envelope to optimize energy efficiency in the heating and cooling of the area (the garage will not be conditioned to the extent of the workspaces.)
Oklahoma's climate does not compare with that of the Northern states, but we do get chilly in the Winter and hot in the Summer. Average highs and lows are as follows: Jan, 50/29; Feb, 55/33; March, 63/41; April 72/50; ... July, 94/72. Plus, the summer humidity is definitely a factor.

I can see that there is a spread of opinion (man am I good or what!) and that I will need to determine what is appropriate for my particular requirements, but any and all considerations and feedback are appreciated.

Common sense.

At a minimum, you want a vapor barrier under all of your slab to reduce humidity. Ground, not directly exposed to the sun, typically has a temperature of about 55F below the frost line. Any space that is "conditioned" should get at least 2" of rigid foam insulation.