The 6-Point Quality Checklist for Underfloor Heating Installation (From a Compliance Manager Who's Seen It Go Wrong)

Who This Checklist Is For (And Why It Exists)

If you're installing Karndean LVT over underfloor heating for the first time—or even if you've done it before but want to avoid the kind of callback that costs time and reputation—this checklist is for you.

I'm a quality compliance manager at a flooring distributor. I review every batch of planks before they go out the door, and I've rejected roughly 12% of first deliveries in 2024 due to dimensional instability or moisture content being off. I've also seen the aftermath of installations that went wrong: loose-lay planks curling at the edges, glue-down failures where the adhesive never fully cured, and click-lock systems that buckled because the subfloor wasn't fully acclimated.

This isn't a general flooring installation guide. It's a specific checklist for underfloor heating, drawn from 4 years of auditing claims, vendor specifications, and real-world failures. Six steps. Follow them in order.

Step 1: Confirm the Heating System Is Compatible (Do Not Skip This)

Not all underfloor heating systems work with LVT. Water-based (hydronic) systems are fine. Electric systems with thin heating mats and a smooth surface are fine. But old-school electric cables with pronounced ridges? Or systems where the surface temperature exceeds 27°C (80°F)? Not fine.

The official spec from Karndean's technical data sheet says surface temperature should not exceed 27°C for continuous exposure. I've seen installers ignore this because "it's only 30 degrees" and ended up with planks that developed a permanent wavy texture within 6 months.

The quick check: Run the heating at maximum for 48 hours before starting. Measure the floor surface temperature at multiple points. If it hits over 27°C, stop. You need a system adjustment or a different product.

Step 2: Acclimate the Flooring in the Room (Not in the Garage)

This is the step that 60% of installers I've audited get wrong. They leave the boxes in the garage or the hallway, assuming the temperature is close enough. It's not.

The Karndean spec requires the room to be at a stable temperature (18-27°C) for at least 48 hours before installation, and the planks need to be in that room for the same period. I once rejected a batch of 200 boxes because the installer had stored them at 12°C overnight. By morning, the planks had contracted by 0.3mm—well within spec, but the adhesive wouldn't bond properly if we proceeded cold.

Roughly speaking: bring the boxes into the room. Open one end of each box. Wait 48 hours. Don't skip this.

Step 3: Prepare the Subfloor (The Part Everyone Rushes)

The subfloor must be smooth, dry, and structurally sound. For underfloor heating, there's an extra requirement: the heating system should have been operational for at least 14 days before installation. This allows the screed to fully cure and the moisture content to stabilize.

If I remember correctly, the standard tolerance for subfloor flatness is 3mm over a 2m straightedge. For heated floors, I'd tighten that to 2mm. Any unevenness will telegraph through the LVT and, worse, create air pockets that ruin heat transfer efficiency.

One thing I wish I had tracked: The number of callbacks due to moisture vapor transmission. If you're installing over a concrete slab with underfloor heating, the moisture can literally be pushed upward. Test the moisture content before you start. If it's above 75% RH (relative humidity), use a vapor retarder.

Step 4: Turn Off the Heating 24-48 Hours Before Laying

This feels counterintuitive—you've been running the heating to prepare the floor, and now you shut it off? Yes.

The floor needs to be at a stable room temperature (around 18°C) when you install. If the heating is on, the floor surface will be warmer than the air, and the adhesive (or the loose-lay friction) won't bond correctly. I only learned this after witnessing an installation where the glue failed because the floor was still warm from the previous day's heating cycle. The cost to redo that room? $2,200. The lesson: turn off the heat, let the floor cool, then install.

Step 5: Use the Correct Adhesive (And Apply It Correctly)

Karndean recommends a specific adhesive for underfloor heating applications: usually a pressure-sensitive adhesive like Karndean's own K-100 or a full-binder system. Do not use the same adhesive you'd use for non-heated floors—the thermal cycling expands and contracts the floor, requiring a more flexible bond.

I'm not an adhesive chemist, so I can't speak to the molecular differences. What I can tell you from auditing product returns is that using the wrong adhesive caused 34% of LVT failures over heated floors in our 2023 analysis. Get the spec from the manufacturer. Use exactly that.

Step 6: After Installation, Wait 48 Hours Before Turning the Heat Back On

Most installers know this, but many ignore it because the client is cold or in a hurry. The adhesive needs time to cure at stable temperature. Turning on the heat early will soften the adhesive, cause it to shrink unevenly, or—in the worst case—create a vapour barrier effect that lifts the planks.

When you do turn the heat back on, ramp it up gradually: 5°C per day until you reach the target temperature. Sudden temperature changes cause rapid expansion. That expansion is how you get the dreaded 'ridge' that runs across the middle of the room.

Common Mistakes I See in Quality Audits

Mistake #1: Not measuring the subfloor moisture. I've rejected 8,000 units worth of installation prep because the installer hadn't tested the slab. The moisture reading was 95% RH. The job would have failed within weeks.

Mistake #2: Ignoring the expansion gap. LVT expands with heat. The recommended expansion gap is 6-10mm around the perimeter. For rooms over 10m in any direction, you need an expansion joint in the middle of the room as well. Ignore this, and you'll get buckling.

Mistake #3: Letting the heating run during installation. I already mentioned this, but it's worth repeating. A warm floor feels good for your knees. It's also the fastest way to destroy your adhesive's integrity.

That's the checklist. Six steps. Follow them, and the underfloor heating installation will perform as designed. Skip one, and you're gambling with time and money.