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Architecture/Isolating Concrete floor from Exposed foundation blocks and outside winter air


Western Blocks Exposed
Western Blocks Exposed  
Southern Blocks Exposed
Southern Blocks Expose  
QUESTION: Dear Richard,

I have a one and a half year old 30 foot by 30 foot 2x6 stick built garage on 8 inch blocks with 4 inch cap blocks on the top course. The garage was built on sloped ground sloping east to west. There is a dirt floor with a couple inches of gravel right now, but I plan to have 4 inches of concrete poured.

The western block foundation has 18 inches of exposed block on the northwest corner to 3 feet of exposed block on the southwest corner. (Please see attached photo) The concrete slab to be poured will be 4 inches thick level with and on the same plane as the 4 inch cap blocks.

The western and southern walls had their cores poured with solid concrete with rebar to strengthen the wall as the mortar joints were cracking. Thus, the exposed part of the block wall has no air space in its cores but is solid concrete.

Presently there is one-inch thick Blueboard foam board (R5) on the upper 14 inches of the block inside the garage on the western and southern walls.

I plan to heat the garage in the future to no more than 45 or 50 degrees during winter when storing my car and mowers, but I will heat the garage up to maybe 60 degrees when doing occasional repair work. I most likely will use a "pellet stove" or ceiling mounted propane heater. I'm open to suggestions about heat choices for this building. I don’t want to put a lot of money into heating the building.

The 2x6 walls will be insulated with 6 inches of fiberglass and the ceiling with a value of about R20 - 30.

All the exposed block on the western wall concerns me as I live with freezing cold south central Pennsylvania winters where the coldest winter temps can reach into the teens and wind-chill can drive temps below 0 degrees F.

I'm concerned that the cold will penetrate the exposed block on the western wall, make the dirt under the slab cold, and thus make the concrete floor cold. This would suck warm air out of the garage as the concrete floor would cool the air inside the garage. Since I have arthritis in my feet and knees, keeping the cold exterior temperatures away from the slab is on my mind.

My initial plans include putting 1 inch of the Blueboard between the 4 inch cap blocks and floor to isolate the concrete slab floor from the outside block walls.

How much advantage would there be in placing 1 inch of the rigid foam Blueboard under the part of or the entire concrete floor?

What do you recommend to make a thermal insulation break between the concrete floor and the concrete slabs under the two 10 foot wide overhead garage doors?

Thank you for your thoughts.




Foundation Insulation
Foundation Insulation  

No amount of insulation is going to compensate for the air infiltration around the garage door.  But I can tell you that the prescriptive solution for limiting heat loss through the edge of your concrete slab is to install 2" thick rigid insulation, 24" horizontal and 24" vertical around the perimeter.  This would be installed behind your block foundation and underneath the 4" slab.  Obviously, it would help to install rigid insulation underneath the entire slab.  But the cost-benefit is marginal.

If you would like to play with the variables, I would recommend that you download (for free) either the COMcheck or REScheck software from the Department of Energy.  After entering the square feet of North wall, West wall, insulation values for the roof, walls, floor slab, etcetera... then the software will let you know how increasing the insulation for any one of those three elements has the most impact.

I hope that helps.

Richard Burton, AIA
Registered Architect (but not licensed in Pennsylvania)
ICC Certified Plan Reviewer
NFPA Certified Fire Plan Examiner

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QUESTION: Hi Richard,

Thank you for your excellent advice!

With regard to the concrete floor, I did originally consider having part of the floor be "wood" in the garage. The plan was to lay pressure treated 4x4s on 7 mil think plastic over the fine gravel floor. I'd put the 4x4's on 12 inch centers. I would screw into the top of the 4x4's 4x8 sheets of three quarter thick Advantech plywood which the company claims is pretty water proof. Finally I'd paint the surface of the AdvanTech.

I planned on putting this wood floor only in the area of the garage a car would not be parked. In my 30x30 garage, the south wall will be where I build a long work bench. The cars enter from the north wall. I planned to have the wood floor come out 8 to 10 feet from the southern wall towards were the cars pull into the garage.

Standing on wood or sitting on a wooden floor to work on a tractor on a cold winter day would be a LOT warmer than sitting on a concrete floor is my rationale and reasoning for the wood floor.

What are your thoughts on this idea?

Thanks for your guidance!




The entire idea of 3/4" thick plywood over 4 x 4 sleepers @ 12" on center over 7 mil plastic over pea gravel, is a good one.  Many of our early 1900's buildings have wood flooring over basements that were designed to drive vehicles on.  The joist spacing is the same but the wood flooring is 1-1/2" thick.  Just something to think about in case there was an occasion when you might want to drive over a wood floor.  I certainly would not place a car jack or depend on a concentrated point load unless I painted thin lines at 12" on center to remind me the location of the 4 x 4 sleepers.

After having worked for a few months in a beef packing plant, I would agree that anything is better than standing all day on a cold concrete floor.  Smart workers would recycle several layers of cardboard to create a 4' x 4' pad to stand on.

My only comment is that your wood floor is going to be more expensive that your concrete floor but it will be well worth it if you are going to spend a lot of time standing or sitting on it.  I would guess $40 per square foot for the wood floor compared to $6 per square foot for the concrete floor.

I hope that helps.

Richard Burton, AIA
Registered Architect (but not licensed in Pennsylvania)
ICC Certified Plan Reviewer
NFPA Certified Fire Plan Examiner

---------- FOLLOW-UP ----------

QUESTION: Hi Richard!

Thank you for your excellent insights on the garage floor helping me think through it all! After considering the costs of wood floor plus limits to physical abuse to the floor such as jacking a vehicle, hammering, etc. I’ve decided to follow the “beef packing plant” plan for concrete floors by laying a three foot wide piece of Adventech plywood in front of the entire length of the workbench. When working on the mower, tractor, or other items on the concrete floor, I will keep a couple of pieces of Adventech plywood handy to sit or lay upon. I may also look for a composite material mat to utilize. This would be much more cost effective for my budget.

In your first reply you suggested putting 2” thick rigid insulation vertically covering the inside of the block wall and horizontally under the 24 inches of the floor from the block wall. Since the fill is already in the garage, my contractor dug down either 14 or 18 inches placing 1 inch thick ridged Blueboard foam insulation on the inside of the block wall. Wish I talked with you first! Since there is only 1 inch on the walls should I only put one inch under the 24 inches of floor closest to the wall?

My block foundation wall uses 8 inch wide blocks with a 6 inch by 4 inches high capblock to top. This capblock leaves two to three inches of the top course of the 8 inch blocks exposed. My plan is to have a two inch wide strip of one inch thick rigid foam Blueboard lay over that exposed two inch part of the top course of the 8 inch blocks and also have the interior side of the 4 inch high block covered by one inch thick Blueboard .

This all means that two or up to three inches of the edge of the floor slab will only be 3 inches thick. How much of a structural weakness concern is only having 3 inches along the block walls?

My contractor will not put foam between the concrete overhead garage door thresholds and the floor slab. He says they always use a “Double Bubble” reflective foil with mylar bubbles sandwiched between two layers of foil as a break between the floor and thresholds. Does foam degrade in this area? Is Double Bubble the best solution for this area or is there something better?

Richard, you’ve been a blessing to me with regard to the planned garage floor insulation! Many many thanks to you!

With much appreciation,



The contractors that employ my architectural services always argue with me that something similar to the "double bubble" with an R-Value of only R4 is sufficient while placed vertically behind the foundation wall.  But the truth is that if I am not stopping by the job site on a regular basis, they do not put any insulation in place behind the foundation.  While they agree that insulation is a "good idea", they consider it to be a nuisance during construction.  Sometimes I get frustrated with building inspectors that do not require my contractors to install a minimal amount of insulation according to my plans.  Why are we paying for code enforcement if there is no enforcement?

The link below is related to a $700,000 law suit where the structural engineer from Texas tried to argue that insulation was not required under the shallow footings because the heat emanating from the building would also heat the ground around the perimeter.  My colorful illustration suggested otherwise with a temperature of only 17 degrees under the slab edge.  The illustration also depicts that the temperature of the concrete slab that is far from the exterior walls will be an average between the room temperature and 54 degrees.  So if you are going to keep your garage heated to 60 degrees, then the center part of your concrete slab is going to be (60 + 54)/2 = 57 degrees.  Placing insulation under the center portion of your concrete slab makes very little difference in this case.  It is the perimeter near the exterior walls that makes the most difference.  And I would recommend doing whatever it takes to install 2 inch thick x 24 inch wide horizontal insulation around the perimeter.  The garage door locations may required digging down a little deeper.

In your case, I would not worry too much about the 3 inch thick slab around the perimeter but know that you can expect hair-line cracks every three feet instead of every four feet.  I would recommend that you place your control joints at 3 feet on center around the perimeter and 4 feet elsewhere.

I think that answers your questions and let me know how things go for you.

Richard Burton, AIA
Registered Architect
ICC Certified Plan Reviewer
NFPA Certified Fire Plan Examiner


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Richard Burton AIA


A combined total of 25 years experience with construction, architecture, and building code enforcement. Ask me about residential and commercial design. Ask me about design aesthetics, structural methods, and building something that will withstand severe weather conditions. Ask me my opinion about YOUR design ideas and I will tell you the merits of good design and challenge your thinking about practical issues. Ask me how to best find and work with your local architect and approach general contractors to get the most value for the least amount of headaches. You can ask me about improving your energy efficiency and things that are both green and practical. But don't ask me about solar panels and wind turbines unless you are currently paying your utility company 35 cents per Kilowatt. Otherwise the math does not justify the initial cost and return on investment. Ask me any building code question and how to get a building permit from the most difficult enforcement agencies.


Here in the midwest, we design and build "green" in ways that make sense. My construction methods prioritize weather resistance, ease of maintenance and durability. While a graduate student in San Diego, I taught drafting and history of architecture. After working ten years for other architecture firms, I have started my firm Arrow Architecture in 2008. More than half of my work involves commercial office buildings. But my portfolio of work also includes custom homes, residential additions, home remodels, and second story additions.

President-Elect of American Institute of Architects - Lincoln, NE

B.S. Architecture - (Interior Design) University of Nebraska Lincoln 1997 Master of Architecture - (Urban Design and Professional Practice) NewSchool of Architecture - San Diego 2004 Graduated Summa Cum Laude ICC Certified Building Plans Examiner

Awards and Honors
AIA Henry Adams Medal and Certificate

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