FHB Logo Facebook LinkedIn Email Pinterest Twitter X Instagram Tiktok YouTube Plus Icon Close Icon Navigation Search Icon Navigation Search Icon Arrow Down Icon Video Guide Icon Article Guide Icon Modal Close Icon Guide Search Icon Skip to content
Subscribe
Log In
  • How-To
  • Design
  • Tools & Materials
  • Restoration
  • Videos
  • Blogs
  • Forum
  • Magazine
  • Members
  • FHB House
  • Podcast
Log In

Discussion Forum

Discussion Forum

Insulation under a garage slab.

migraine | Posted in Energy, Heating & Insulation on December 13, 2003 01:09am

I guess this is where this question belongs.

The question came up today whether or not I would like to put an inslation under the garage floor slab.  Most of the 1950 sqft garage is actually going to be a woodworking shop and I am hoping to use it regularly.  Some of it will be used as a game room with a pool table in it. Only about 400sq’ will actually be for cars(or maybe no cars?)  Anyway, I am In the PacNW and the soil is sand.  Does the foam panel really work well as a thermal break when laid under the concrete.  And, are there a problems of deterioration of the panels to cause the slab to crack/sink? Plus, does the foam panels tend to lose their energy efficincy after a few years.    Anybody know any thing about this??

I have even thought about a rfh heat system, but at this time, the piggy bank has only pennies left in it.  So, someday, I’ll be using a space heater

Thank you in advance, Brian

 

Reply
  • X
  • facebook
  • linkedin
  • pinterest
  • email
  • add to favorites Log in or Sign up to save your favorite articles

Replies

  1. MojoMan | Dec 13, 2003 01:33am | #1

    I'll be looking forward to answers on this one as I may be doing a slab job soon, but I have a thought. Perhaps you could put pex tubing in the floor now and hook it up to a hot water heat system when the budget allows.

    Al Mollitor, Sharon MA

  2. bkhy | Dec 13, 2003 02:20am | #2

    Infloor heat is great but I don't have it, however I do have detached garage with wood shop & pool table--highly recommended



    Edited 12/12/2003 6:22:04 PM ET by BKHY

  3. User avater
    BillHartmann | Dec 13, 2003 02:53am | #3

    When I build my house I put in installation under the slab. At that time the recommendation was a vertical piece and only the first 2 ft horizontal. And we only used 3/4.

    My first floor slab has the forced warm air ducts under the slab. While this is not "heated" by anymeans in the middle of a cold winter I can stand on it with sock feet and vinyl floor.

    My basement slab also has it under it. The only heat that I have in their is an electric blower that is used only when I am in there. That floor is cold, but I am sure that it would be colder without the insulation.

    And no problems with cracks are anything.

    I would do it again.

  4. xMikeSmith | Dec 13, 2003 04:16am | #4

    we've put a lot of insulation board under our slabs over the years.. we are constantly researching cost effectiveness... and longevity.., ease of installation..

     ancillary effects on other costs..

     a lot of cement finishers balk at pouring over foam board.. they claim it retards the set..

    anyways : if i was pouring your garage slab...

    i'd use 2" EPS , much cheaper than polyiso... and scandinavian testing shows that eps  has great longevity, and almost zero water absorption..

     i'd put 6 mil poly on top ... and cast about 1" of sand  ..

    i'd tell my cement finisher i want a plasticizer, because i want him pouring a  low water content mix... the plasticizer will give him the workability of a wetter mix  without the water..

     the insulation will make all the difference in the world in  terms of both energy saved and comfort attained..

     the pex for future RFH would be a nice added touch

    Mike Smith   Rhode Island : Design / Build / Repair / Restore

    1. moltenmetal | Dec 13, 2003 07:35am | #5

      Mike:  is this method only good for garage slabs which have a perimeter footing without foam insulation underneath?  Or can you do a "slab on grade" entirely on top of foam insulation?

    2. caseyr | Dec 13, 2003 08:16am | #6

      Mike -

      How do you handle the carpenter ant problem with EPS?  We have only a 2ft footing depth here in Oregon and I am guessing that in the carpenter ant convention center that is my lot, they will want to play games in any EPS - they will even tunnel through fiberglass batts that are left lying on the ground. 

      1. User avater
        Mongo | Dec 13, 2003 04:31pm | #9

        Casey,

        Ask around about PerformGuard EPS, it's a 2# RFBI treated with borates.

      2. xMikeSmith | Dec 13, 2003 10:04pm | #11

        casey...one of the main reasons i dropped polyiso is because of vermin..had some real scares in the past

        in the  chit-chat here  i learned about Performguard EPS....  now all we use in anything connected  to the foundation is PerformGuard ... they treat the EPS with borates in the manufacture..

        in the south ... a lot of the jurisdictions require PerformGurard or the equivelent  for  ICF's

        Mike Smith   Rhode Island : Design / Build / Repair / Restore

        1. andybuildz | Dec 14, 2003 04:11am | #12

          I'll be pouring the 30x30 garage/shop slab in the spring and considered getting some prices to put plex tubing in the concrete but it occorded to me today .....what if the gargage gets so cold when the heat isn't on....will the tubing freeze and crack?My life is my practice!

          http://CLIFFORDRENOVATIONS.COM

          1. xMikeSmith | Dec 14, 2003 06:09am | #13

            dats why you use pex  ( it won't split... and has a memory )... and probably anti-freeze in the boiler Mike Smith   Rhode Island : Design / Build / Repair / Restore

          2. DennisS | Dec 14, 2003 09:27am | #14

            Andy -

            One of the heating guys that worked on my place tried an experiment - he filled a short piece of PEX with water, put fittings on both ends to seal it up and left it in his freezer for a week. The tubing expanded with the water/ice as it froze but did not rupture. Took it out of the freezer, let it thaw out and it went back to it's original shape.

            Now - the thing to worry about with freezing tubing in the slab isn't the tubing rupturing but the expansion caused but the freezing water to spall the concrete! I suspect some type of anti-freeze would be in order, something like the potable RV stuff they use for winterizing campers. But - in a well designed system your boiler is hooked to your domestic water system - with an anti-siphon valve to be sure. This is to provide for make-up water periodically. Thus the anti-freeze over time may become diluted and not be as effective as need be.

            I should think it would depend on your locality. If it gets cold enough to freeze water in a concrete slab inside a garage where you live, I don't want to know how cold it must be outside! (grin)

            ...........

            Dennis in Bellevue WA

            [email protected]

          3. FrozenMark | Dec 15, 2003 10:01pm | #15

            Living here in the interior of Alaska I haven't heard of anyone having trouble with the Pex swelling and causing problems with the concrete floor slab.

            Several replies from other people mentioned the use of antifreeze(glycol) in the heating system. That is how most of the plumbers here in interior Alaska (Fairbanks) set up garage slab tubing. However, they don't use straight glycol in the heating system. The plumbers use a mix of glycol and water to get the best of both worlds for heat transfer and freeze protection. Water is bettter than glycol for transfering heat, but straight glycol has a freezing point that's not much lower than water. Just like your car, you use a mix of glycol and water. Talk to a plumbing or mechanical contractor to figure out the specific mix needed for your area.

            One final comment on the glycol. Be sure and use the non-toxic glycol in your system. This is especially important if you use your boiler to make your domestic hot water. A rupture in the boiler could allow glycol into your domestic water supply.

            Good luck with the slab! It is a balmy -5F in Fairbanks today.

          4. DavidThomas | Dec 15, 2003 11:02pm | #17

            "But - in a well designed system your boiler is hooked to your domestic water system - with an anti-siphon valve to be sure. This is to provide for make-up water periodically."

            Yes, but in a well installed system, you don't need make-up water because there aren't any leaks.     ;-)       That's my perspective, but I realize that my design and practice in that regard is in the minority.  At least my gycol is never diluted without someone knowing it.

            FrozenMark:  I agree about the water/gycol mix.   More water for heat capacity and lower cost.  But more gycol if you need a lower freezing point.   Look at the label for the perentages.

            And to add to what you said about toxicity:  Ethylene Gycol (Prestone, etc) is the highly poisonous stuff.  Oxalate crystals forming in the kidney cause renal failure.  Propylene Gycol  (Sierra is one brand) is so non-toxic it is in a variety of food at the supermarket.  I've only ever used propylene gycol in a residential setting - solar DHW or hydronic heat.  20 years ago, you had get it in drums from speciality suppliers.  Now Wal-Mart has it in the automotive section.

            Edit: it was, "in the kidney cause liver failure"  Jeez!  Well, "liver" looks almost completely like "renal" if you turn it upside down.  In simpler langauge: hard chunks form in the kidney (which extracts urine).  Chunks are bad.

            David Thomas   Overlooking Cook Inlet in Kenai, Alaska

            Edited 12/15/2003 4:50:44 PM ET by David Thomas

  5. DennisS | Dec 13, 2003 08:36am | #7

    Migraine -

    Do the insulation thing! You won't be sorry. It won't help a great deal to keep the floor "warm" but will retard heat loss into the ground, of course.

    With respect to the radiant heating - install the PEX tubing now while you can. You don't have to install the heating system 'til later when the piggy bank grows up a little. I did that for my new shop. It's not too terribly expensive - as I recall the tubing alone ran about $300 for a 24 x 48 slab. I installed it myself. Just use plastic ties to tie it to the rebar or mesh so it doesn't float up out of the concrete - it's full of air at the outset ya know! (grin)

    I used 2" pink EPS under this slab which was in turn over 6" of pea gravel. No 'undesturbed' soil, either. Rough grade under the insulation was done with the pea gravel. The finisher just specified 1% calcium (an accelerator) in the mud to compensate for the lack of water absorption into the ground due to the rigid insulation.

    But by all means, try to get the tubing in if you can possibly afford it. I have a price from my heating guy of about $1800 for a small on-demand type water heater he says will do the job once I get the money to have it installed. That's for a propane heater - I'll still have to purchase a small-ish tank separate from the house tank but ... sure will be nice once I can get some heat in that shop!

    ...........

    Dennis in Bellevue WA

    [email protected]

  6. FrozenMark | Dec 13, 2003 09:43am | #8

    I live up here in the frigid interior of Alaska. Radiant concrete slabs are very popular in new construction. The standard approach I have seen is grade the base of your excavation, lay down 6 mil poly then 2" of Extruded PolyStyrene (pink or blue board) and then securing the pex tubing on top of the foam with the handy little staples the manufacturer makes.

    I've seen people use the tubes to support the slab reinforcement, but the reinforcement should really be centered in the slab to be most effective for stopping the concrete slab from cracking excessively.

    All the plumbers I've talked to pressurize the pex tubing before placing the concrete. They claim that the tubing could collapse or get bent if the tubing is not pressurized during the placement of the concrete.

    Adding the radiant tubing in the floor slab now makes the most sense. Don't forget you'll also want to have some kind of unit/cabinet heater for quick make up of lost warm air when you open the garage doors in the winter. The concrete slab takes awhile to adjust to the loss of the heated air. But I may be thinking in the extreme sense. Our high for almost a week was -20F in the first part of December. The low for that time period was -42F.

    Good luck with your concrete slab!

    1. migraine | Dec 13, 2003 05:00pm | #10

      Does anybody know what the life expentancy of the "pink or blue foam" is??  Is it 15,20,30,40,or 50 years?  I don't want to be jack hammering a slab out now or in a few years when I'm older.  I hate using ones of those things now and I'll hate it even worse later.  Plus, by then, my boys will be up and gone and I won't have anyone to help me schlep the waste away.

    2. DavidThomas | Dec 15, 2003 10:50pm | #16

      Another Alaskan response:  I mostly agree with FrozenMark, but I haven't missed not having aa additional heat source to quickly respond to opening the doors.  The entire floor is at 72F and weighs 50 pounds per square foot.  That much concrete has 11 BTUs per F.  The 10-foot column of air above it only weighs 12 ounces.  Needs only 0.17 BTU per F.  Heat capacity-wise, the concrete is where it's at, 65 to 1.

      I do try to close the doors quickly and it does take 5 to 10 minutes for the heat to bounce back.  But that's okay with me.David Thomas   Overlooking Cook Inlet in Kenai, Alaska

  7. DavidThomas | Dec 15, 2003 11:05pm | #18

    Guess I forgot to say in earlier post an obvious but important point:

    RFH in a garage is a wonderful thing.  Gets the snow and ice off the car quickly.  Evaporates the snowmelt or rainwater off the floor.  With many other heating systems, you can get boxes FROZEN to the floor while T-stat is happily at 70F five feet off the floor.

    David Thomas   Overlooking Cook Inlet in Kenai, Alaska
    1. User avater
      johnnyd | Dec 16, 2003 12:05am | #19

      Ditto on the SOP for type of insulation under the slab here in Minnesota. 2" rigid extruded...pink or blue.  If you're really concerned about durability and if you might be having something really heavy in there, the board comes in two densities..150 and 250 I think...get the 250 for really heavy stuff like tow trucks, big tractors, or something.

      Don't forget to insulate the PERIMETER, down as far as you can, and/or horizontally 24 " or so out form the slab.

      Panicked the other night 6 degrees F outside, opened the garage door to give my SAAB a treat, looked at the wall thermometer after closing the door.  38 degrees!

      Thought maybe I had underdesigned and the water heater couldn't keep up, or something else.  This is a new system this fall and uses a slab sensor to keep the slab at 55 - 60 degrees first time it's got really cold.  Shrugged, went down to work on the house awhile...came back up 45 minutes later...air temp 58 degrees....what Dave said:

      That slab is a BIG radiator, a little cold air doesn't even phase it.

Log in or create an account to post a comment.

Sign up Log in

Become a member and get full access to FineHomebuilding.com

Video Shorts

Categories

  • Business
  • Code Questions
  • Construction Techniques
  • Energy, Heating & Insulation
  • General Discussion
  • Help/Work Wanted
  • Photo Gallery
  • Reader Classified
  • Tools for Home Building

Discussion Forum

Recent Posts and Replies

  • |
  • |
  • |
  • |
  • |
  • |
View More Create Post

Up Next

Video Shorts

Featured Story

Mortar for Old Masonry

Old masonry may look tough, but the wrong mortar can destroy it—here's how to choose the right mix for lasting repairs.

Featured Video

Builder’s Advocate: An Interview With Viewrail

Learn more about affordable, modern floating stairs, from design to manufacturing to installation.

Related Stories

  • Fire-Resistant Landscaping and Home Design Details
  • A New Approach to Foundations
  • A Closer Look at Smart Water-Leak Detection Systems
  • Guest Suite With a Garden House

Highlights

Fine Homebuilding All Access
Fine Homebuilding Podcast
Tool Tech
Plus, get an extra 20% off with code GIFT20

"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Fine Homebuilding Magazine

  • Issue 332 - July 2025
    • Custom Built-ins With Job-Site Tools
    • Fight House Fires Through Design
    • Making the Move to Multifamily
  • Issue 331 - June 2025
    • A More Resilient Roof
    • Tool Test: You Need a Drywall Sander
    • Ducted vs. Ductless Heat Pumps
  • Issue 330 - April/May 2025
    • Deck Details for Durability
    • FAQs on HPWHs
    • 10 Tips for a Long-Lasting Paint Job
  • Issue 329 - Feb/Mar 2025
    • Smart Foundation for a Small Addition
    • A Kominka Comes West
    • Making Small Kitchens Work
  • Issue 328 - Dec/Jan 2024
    • How a Pro Replaces Columns
    • Passive House 3.0
    • Tool Test: Compact Line Lasers

Fine Home Building

Newsletter Sign-up

  • Fine Homebuilding

    Home building tips, offers, and expert advice in your inbox.

  • Green Building Advisor

    Building science and energy efficiency advice, plus special offers, in your inbox.

  • Old House Journal

    Repair, renovation, and restoration tips, plus special offers, in your inbox.

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters

Follow

  • Fine Homebuilding

    Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.

    Start Free Trial Now
    • Facebook
    • Instagram
    • X
    • LinkedIn
  • GBA Prime

    Get instant access to the latest developments in green building, research, and reports from the field.

    Start Free Trial Now
    • Facebook
    • YouTube
  • Old House Journal

    Learn how to restore, repair, update, and decorate your home.

    Subscribe Now
    • Facebook
    • Instagram
    • X
  • Fine Homebuilding

    Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.

    Start Free Trial Now
    • Facebook
    • Instagram
    • X
    • LinkedIn
  • GBA Prime

    Get instant access to the latest developments in green building, research, and reports from the field.

    Start Free Trial Now
    • Facebook
    • YouTube
  • Old House Journal

    Learn how to restore, repair, update, and decorate your home.

    Subscribe Now
    • Facebook
    • Instagram
    • X

Membership & Magazine

  • Online Archive
  • Start Free Trial
  • Magazine Subscription
  • Magazine Renewal
  • Gift a Subscription
  • Customer Support
  • Privacy Preferences
  • About
  • Contact
  • Advertise
  • Careers
  • Terms of Use
  • Site Map
  • Do not sell or share my information
  • Privacy Policy
  • Accessibility
  • California Privacy Rights

© 2025 Active Interest Media. All rights reserved.

Fine Homebuilding receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.

  • Home Group
  • Antique Trader
  • Arts & Crafts Homes
  • Bank Note Reporter
  • Cabin Life
  • Cuisine at Home
  • Fine Gardening
  • Fine Woodworking
  • Green Building Advisor
  • Garden Gate
  • Horticulture
  • Keep Craft Alive
  • Log Home Living
  • Military Trader/Vehicles
  • Numismatic News
  • Numismaster
  • Old Cars Weekly
  • Old House Journal
  • Period Homes
  • Popular Woodworking
  • Script
  • ShopNotes
  • Sports Collectors Digest
  • Threads
  • Timber Home Living
  • Traditional Building
  • Woodsmith
  • World Coin News
  • Writer's Digest
Active Interest Media logo
X
X
This is a dialog window which overlays the main content of the page. The modal window is a 'site map' of the most critical areas of the site. Pressing the Escape (ESC) button will close the modal and bring you back to where you were on the page.

Main Menu

  • How-To
  • Design
  • Tools & Materials
  • Video
  • Blogs
  • Forum
  • Project Guides
  • Reader Projects
  • Magazine
  • Members
  • FHB House

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Popular Topics

  • Kitchens
  • Business
  • Bedrooms
  • Roofs
  • Architecture and Design
  • Green Building
  • Decks
  • Framing
  • Safety
  • Remodeling
  • Bathrooms
  • Windows
  • Tilework
  • Ceilings
  • HVAC

Magazine

  • Current Issue
  • Past Issues
  • Magazine Index
  • Subscribe
  • Online Archive
  • Author Guidelines

All Access

  • Member Home
  • Start Free Trial
  • Gift Membership

Online Learning

  • Courses
  • Project Guides
  • Reader Projects
  • Podcast

More

  • FHB Ambassadors
  • FHB House
  • Customer Support

Account

  • Log In
  • Join

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Follow

  • X
  • YouTube
  • instagram
  • facebook
  • pinterest
  • Tiktok

Join All Access

Become a member and get instant access to thousands of videos, how-tos, tool reviews, and design features.

Start Your Free Trial

Subscribe

FHB Magazine

Start your subscription today and save up to 70%

Subscribe

Enjoy unlimited access to Fine Homebuilding. Join Now

Already a member? Log in

We hope you’ve enjoyed your free articles. To keep reading, become a member today.

Get complete site access to expert advice, how-to videos, Code Check, and more, plus the print magazine.

Start your FREE trial

Already a member? Log in

Privacy Policy Update

We use cookies, pixels, script and other tracking technologies to analyze and improve our service, to improve and personalize content, and for advertising to you. We also share information about your use of our site with third-party social media, advertising and analytics partners. You can view our Privacy Policy here and our Terms of Use here.

Cookies

Analytics

These cookies help us track site metrics to improve our sites and provide a better user experience.

Advertising/Social Media

These cookies are used to serve advertisements aligned with your interests.

Essential

These cookies are required to provide basic functions like page navigation and access to secure areas of the website.

Delete My Data

Delete all cookies and associated data