Before you pour your slabs, you probably stop and wonder how that pre-pour inspection is going to go. But did you ever stop to think whether your buyers will be as satisfied with these slabs as the building inspector?
In this post, we’ll address the break between the existing energy codes and occupant comfort. We’ll also look at a few insulation products that can help you bridge this gap.
Occupant Comfort vs. Energy Efficiency — It’s a Different Standard
Energy codes dictate the insulation requirements in our buildings, but they’re not designed with occupant comfort in mind. Instead, they mandate builders to hit the less stringent requirements needed to conserve energy.
Most uninsulated slabs feel cold beneath our feet because the ground has a temperature that stays consistently below what we consider “pleasant.” That’s why underslab insulation is essential to the comfort of a home’s occupants.
On the other hand, underslab insulation is only ever vital to conserving energy if its purpose is to:
- Protect a heated slab
- Insulate a slab in a colder climate zone, at or above the 4-foot frost line depth — this is where the atmospheric temperature is most likely to affect that of the soil
In all other situations, underslab insulation does little to boost a home’s energy efficiency: warm indoor air expands and pushes heat to escape upwards, not down into the soil.
Residential Underslab Building Code Requirements — Not Enough For Comfort
Since our codes are less concerned with comfort than curbing energy use, their underslab R-value requirements only aim to meet the lower energy efficiency standard. Sadly, this standard does nothing to ensure that concrete slabs are pleasant to live, work and walk on.
Here is what the residential codes mandate for underslab insulation in Canada and the US:
Canada, Unheated Slab:
- Above 4-foot frost line depth — R-11.1 in Climate Zones 4-8
- Below 4-foot frost line depth (applicable to most basements) — no requirements
Canada, Heated Slab:
- Below grade — R-13 to R-16, depending on the climate zone; Ontario — only R-10
- Slab on Grade — R-11 to R-21, depending on the climate zone
US — Heated, Unheated, Slab-on-Grade, Below-Grade:
R-10, 2 feet around the slab’s perimeter
Passive House Requirements — Much More Stringent
Where energy codes fail to consider the well-being of a home’s dwellers, passive house requirements are more robust — and holistic. The standard elevates the design criteria for a building’s envelope with both efficiency and comfort in mind, focusing on eliminating all thermal bridging and ensuring airtightness.
To meet their strict design guidelines, passive homes’ underslab insulation must hit between R-20 and R-40 in most of Canada, as well as US marine zones 4-8.
While most builders gripe about strict, expensive efficiency clauses in residential codes, passive house contractors pursue a different approach.
Let’s hear what one such passive expert has to say.
“For us, R-10 is the bare minimum; we like to go to R-30 or better. Although, as the level of insulation increases, the incremental ROI decreases (you do get diminishing returns). For instance, you get great ROI by investing in R-10 underslab insulation versus zero, but less incremental ROI when opting for R-30 versus R-10, and so on. But the benefit — and the value — is comfort. Bosiak Construction is a passive house builder with roots going back to the R2000 program in the 80s, so occupant well-being is a big factor for us.”
Good, Better, and Best — Underslab Insulation Options for Occupant Comfort
So, what are the best underslab insulation options for meeting code AND ensuring your dwellers’ well-being?
We’ve ranked them from “good” to “best”:
Good — Chrome GPS. These graphite polystyrene (GPS) panels have a thin profile but robust thermal resistance at R-5 per nominal inch. Though they’re unlaminated, these boards are highly resistant to deterioration, making them perfect for the unaccessible underslab location.
Best — Halo Subterra Plus. Just like Subterra, but comes with a super-tough cross-woven laminate that helps it withstand heavy jobsite traffic and other punishing conditions that could otherwise cause damage. Also, it serves as an effective Radon barrier. Subterra Plus is only available in the Upper Midwest US and Western Canada.
Density Considerations For Underslab Insulations — No Matter Which Option You Select
Concrete slabs weigh a lot, support heavy loads, and thus exert a great deal of pressure on the underslab insulation. This is why only the toughest products — specifically those with high compressive strength — are suitable for this application. The minimum acceptable compressive strength for underslab insulation is 16 PSI.
Luckily, all of the products we’ve listed above meet this criterion. Chrome GPS and the Halo Subterra series panels come in compressive strengths of 16, 25, and 30 PSI, and you can request a higher value to be custom-made.
Wrapping It Up
Our residential codes go far in ensuring energy efficiency but leave home comfort out of the equation. So it’s up to you, the builder, to ensure that buyers feel well when they live in the homes you construct.
The insulation options we’ve ranked above go above and beyond the code requirements; you won’t need these products to satisfy your building department. But they’ll surely help you win over your buyers.