Should you fully insulate under concrete slabs on or below grade?
Copyright (c) 2010, Robert Bean, R.E.T., P.L.(Eng.) and www.healthyheating.com

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1. Soil characteristics including soil type, moisture content and density below and adjacent to slabs, moisture storage function, porosity, conductivity and specific heat capacity.

2. Water table characteristics - historical data i.e. risk assessment for change.

3. Temperatures above grade and below grade*

4. Temperature desired for space conditions

5. Temperatures in pipe

6. Slab characteristics including thickness, conductivity and elevation in relation to grade.

7. Enclosure performance above and adjacent to the heated slab, including effects from solar gains. The greater the performance above the slab, the more significant the slab heat loss.

8. Tube spacing, slab conductivities (upward and downward)

9. Tube patterns i.e. warmest fluid vs. coolest fluid in relation to edge.

10. Aspect ratio of building that being either the width to length ratio or area to perimeter ratio i.e. rectangular buildings vs. square buildings.

*Mean ground temperatures typically run between 2 and 6 K above mean annual air temperatures.

Figure 2. Simulation of a heated slab below grade with and without insulation. Temperature scale indicated in degrees Celsius (°C). Horizontal line with white markers is a reference plane at approximately 2.4m below the heated slab. Illustration is representative of a specific condition at steady state ergo it does not represent every project and system.

Figure 3. Simulation of a heated slab on grade with and without insulation. Temperature scale indicated in degrees Celsius (°C). Horizontal line with white markers is a reference plane at approximately 2.4m below the heated slab. Illustration is representative of a specific condition at steady state ergo it does not represent every project and system.

What do the building codes say....

First we should note there are many codes. There are codes which are enforced federally or regionally. Often the regional codes are based on federal codes as is the case in Canada but there is freedom to change or modify codes to suit regional conditions that may not be addressed adequately or otherwise in Federal codes. Earthquake requirements or naturally ventilated spaces for mild climate zones are two examples where regional differences may prevail over Federal codes. Secondly, we should note that building codes are not energy codes unless the building code includes provisions to address energy over and above that based on minimum requirements, i.e. the status quo.

What do we mean by this?

In North America there are energy standards which exist on a voluntary basis outside the building code - for example; there is no mandatory requirement to build homes to the Passivhaus or R2000 standards or to construct buildings and systems to or better than ASHRAE’s 189.1(2) or ASHRAE 90.1(2). Buildings constructed to meet or beat these standards would be better than code. They would be called (wait for it), “better than code buildings”, or in some cases - high performance buildings, but this is confusing since even these are undefined.

When you build to code you essentially get a “D” grade – that is, you didn’t fail but you missed the opportunity to earn an A or A+. If you don’t understand the words, ‘missed opportunity” then you have never considered what it takes to increase the insulation under a concrete slab after it has been poured.

Alright let’s get into it - when it comes to under slab insulation don’t expect a North American team effort – ya know the old college, “all for one - one for all” stuff.

Consider this; the March 2010 version of the International Green Construction Code (IGCC), Table 606.1.1 Prescriptive Building Thermal Envelope Requirements calls for climate based perimeter heat loss factors (F-factor) for slab-on-grade floors between 0.730 Btu/hr·ft·°F to 0.373 Btu/hr·ft·°F; or the Washington State Legislature, WAC 51-11-1002, Section 1002: calling for anywhere between 0.42 Btu/hr·ft·°F to 0.69 Btu/hr·ft·°F for various slab depth and wall construction . Residential Requirements of the 2009 International Energy Conservation Code provides in Table 402.1.3 Equivalent U-values ( in units of Btu/hr·ft²·°F) for floors of 0.028 to 0.064 depending on climate zone and in Table 402.1.1 a minimum R-10 (in units of h·ft²·°F/Btu) perimeter insulation but if it’s heated an additional R-5 is required with the height/depth subject to climate zone. For commercial buildings, edge insulation of R 7.5 to R-20 is required for all heated slabs again depending on climate zone. In Canada the current CSA - B214 References CSA B214 – 07, requires no manufactured insulation, allowing for equivalent R-5 based on resistance between underside of slab and top of water table but this is overridden (and rightly so) by at least two regional codes. Furthermore  the proposed National Energy Code for Buildings (NECB) is calling for complete under slab insulation for all heated slabs - of a value dependant on climate zone. But as far as we can tell it doesn't apply to buildings heated with overhead infrared. No interior insulation is required for unheated slabs except in the coldest climate.

If you read this carefully the first thing that should get your attention is the inconsistency in requirements and inconsistency in insulation description…F-factors, U values or R values….maybe it's time we all got on the same page...

So what is our position at www.healthheating.com?

Well first off - to get a “D” – you have to comply with code. In the absence of an enforced energy code (i.e. it’s up to you to chose an “F”, “D” or better) we’re saying use a minimum R-10 type 3 or 4 rigid. We’re also saying in the absence of enforcement and in the presence of ‘value engineering’, anything with a footprint exceeding 5000 ft² should have a robust slab heat loss analysis to see if it makes sense to reduce insulation.

Why 5000 ft²?

Because 2500 ft² is not out of the ordinary and 10,000 ft² is commercial space by our definition. When you hit the 5000ft² and you’re thinking you want to swap the one time capital cost (i.e. insulation dollars) for long term operational energy dollars then you need to do a robust analysis. You may not agree - but this is our website and our opinion and we’re sticking to it unless convinced otherwise.

For commercial buildings again we’re saying follow the codes to get a “D” but to get an “A” or an “A+” do a full system analysis based on the combined features of the architectural systems and mechanical systems. There is a case to be made for reduced insulation values in the interior spaces depending on the project conditions.

What do we mean by this?

Well if you’re building a square 100,000 ft² shopping center on top of a well drained rise formed primarily with dry sandy soil and less than 10% chance of flood waters ever approaching 15 ft. to 20 ft. under the slab there is little performance benefit to insulating under the entire slab interior. However, move that same structure next to a well known flood plain, turn that square into a long narrow rectangular building and you’ll have completely different project to evaluate.

Our final thoughts - you get one chance to insulate under a slab - do a thorough analysis to ensure the best decision is made before the concrete is placed - and remember once everything is installed the only wild card left is the cost of energy. If you have the potential for robust downward heat losses but are protected by insulation and energy costs go up (and they will) then you're laughing...if energy costs go down then you will be paying less for less energy. If you have no insulation and have robust downward losses and energy costs go up then - in very technical engineering talk - you're screwed.