Building Biology Tips for a Healthy Home
by Sam Davies
by Sam Davies
 | This blog post is in response to an Instagram post we recently saw, where an entire house had suffered incredible black mould damage under hybrid plank flooring. We couldn't help but feel devastated for them, and wanted to share some info so that you can help avoid the same outcome. Moisture is the ENEMY of a building, and should be treated as such. If you’re planning to install hybrid plank flooring* in your home — whether over a concrete slab or a timber-framed floor — it’s essential to get the details right. While hybrid planks are known for their durability and water resistance, improper installation can trap moisture underneath, creating the perfect conditions for hidden mould. |
At Queensland Designer Homes, we’re not just about great home design — we care about creating healthy spaces for you and your family. That’s why we recommend approaching flooring with a building biology mindset, focusing on moisture control, ventilation, and non-toxic materials. Here’s what we recommend to make sure your beautiful new flooring doesn’t become a hidden health hazard!
*This article centers on hybrid plank flooring, as it is the most popular type of flooring. However, the principles discussed herein almost always apply to any other type of flooring also, such as vinyl planks, timber flooring, tiles and more.
How to Avoid Mould Under Hybrid Plank Flooring
For Concrete Slabs:
✅ 1. Start with a Dry Slab
Concrete can take months to fully cure and dry. Moisture that remains in the slab after installation can rise and condense beneath your flooring — the perfect conditions for mould.
✅ 2. Use a Proper Moisture Barrier
Before laying any flooring, it’s essential to install a vapour barrier. This acts like a shield between your slab and your flooring. (This is the black plastic you see before a slab gets poured.)
✅ 3. Choose the Right Underlay
Many hybrid flooring systems require an underlay — make sure you choose one that offers moisture resistance as well as sound insulation.
✅ 4. Avoid Problem Slabs
If your slab has a history of moisture problems — like efflorescence, musty smells, or visible damp patches — address these first.
✅ 5. Leave Proper Expansion Gaps
Hybrid planks expand and contract with changes in temperature and humidity. If installed too tightly, they can trap moisture underneath.
✅ 6. Maintain Indoor Humidity
Your concrete slab isn’t the only source of moisture — your indoor air can also contribute to mould if it’s too humid.
✅ 7. Be Careful with Wet Cleaning
Once your flooring is installed, avoid excessive water when cleaning.
✅ 8. Choose Low-Toxicity Flooring
From a building biology perspective, your floor shouldn’t just be dry — it should also be free from harmful chemicals.
*This article centers on hybrid plank flooring, as it is the most popular type of flooring. However, the principles discussed herein almost always apply to any other type of flooring also, such as vinyl planks, timber flooring, tiles and more.
How to Avoid Mould Under Hybrid Plank Flooring
For Concrete Slabs:
✅ 1. Start with a Dry Slab
Concrete can take months to fully cure and dry. Moisture that remains in the slab after installation can rise and condense beneath your flooring — the perfect conditions for mould.
- Use a moisture meter or humidity probe to test the slab before installation.
- Aim for a moisture content of under 75% relative humidity (RH), or less than 5.5% moisture content, depending on your flooring manufacturer’s recommendations.
✅ 2. Use a Proper Moisture Barrier
Before laying any flooring, it’s essential to install a vapour barrier. This acts like a shield between your slab and your flooring. (This is the black plastic you see before a slab gets poured.)
- Use a 200-micron (μm) plastic sheet, overlapped and taped with moisture-proof tape.
- Alternatively, apply a liquid moisture membrane that bonds directly to the concrete.
- Don’t skip this step — even if your slab "seems dry."
✅ 3. Choose the Right Underlay
Many hybrid flooring systems require an underlay — make sure you choose one that offers moisture resistance as well as sound insulation.
- Look for vapour retardant underlays designed specifically for hybrid flooring over concrete.
- Check that it is compatible with your flooring product and doesn’t void warranties.
✅ 4. Avoid Problem Slabs
If your slab has a history of moisture problems — like efflorescence, musty smells, or visible damp patches — address these first.
- You may need to improve external drainage, add sub-slab ventilation, or apply concrete sealing.
- Installing flooring over a known issue is a recipe for expensive remediation later.
✅ 5. Leave Proper Expansion Gaps
Hybrid planks expand and contract with changes in temperature and humidity. If installed too tightly, they can trap moisture underneath.
- Leave appropriate expansion gaps around all edges, as per manufacturer instructions.
- Don’t seal skirting boards or trims so tightly that no airflow can reach the floor below.
✅ 6. Maintain Indoor Humidity
Your concrete slab isn’t the only source of moisture — your indoor air can also contribute to mould if it’s too humid.
- Aim to keep indoor relative humidity between 40% and 60%.
- Use exhaust fans, open windows, or dehumidifiers as needed — especially in humid climates or poorly ventilated homes.
✅ 7. Be Careful with Wet Cleaning
Once your flooring is installed, avoid excessive water when cleaning.
- Use a dry mop or a barely damp cloth to clean the surface.
- Never pour water directly onto the floor — even though hybrid flooring is water-resistant on top, it’s not waterproof underneath.
✅ 8. Choose Low-Toxicity Flooring
From a building biology perspective, your floor shouldn’t just be dry — it should also be free from harmful chemicals.
- Choose hybrid planks that are low-VOC, formaldehyde-free, and certified for indoor air quality (look for labels like GreenGuard Gold or Blue Angel).
- If possible, use floating floor systems rather than glue-down products to avoid adhesives and make future removal easier.
Preventing Mould with Raised or Suspended Flooring Systems
For Timber Floors:
Timber-framed floors — especially those with a crawl space or subfloor void — present different challenges. Moisture can rise from the ground below or become trapped within the structure if not well ventilated. Here’s how to reduce the risk of mould beneath your hybrid flooring when installing over a timber base:
✅1. Inspect and Ventilate the Subfloor Space
✅2. Check for Existing Moisture or Dampness
✅3. Install a Moisture Barrier or Breathable Underlay
✅4. Leave Expansion Gaps and Avoid Over-Sealing the Perimeter
✅5. Ensure Indoor Humidity is Controlled
🌿 Bonus Tip: Use Healthy, Low-Toxic Flooring Materials
Whether you're installing over concrete or timber, choose hybrid planks and underlays that are:
For Timber Floors:
Timber-framed floors — especially those with a crawl space or subfloor void — present different challenges. Moisture can rise from the ground below or become trapped within the structure if not well ventilated. Here’s how to reduce the risk of mould beneath your hybrid flooring when installing over a timber base:
✅1. Inspect and Ventilate the Subfloor Space
- Make sure your subfloor has adequate cross-ventilation to allow air to move freely.
- Blocked or missing vents can lead to stagnant, humid air that promotes mould on joists and underside of flooring.
- In some cases, installing mechanical subfloor ventilation may be necessary — especially in humid climates or low-clearance areas.
✅2. Check for Existing Moisture or Dampness
- Look for signs of mould, rot, musty smells, or cupping in existing timber flooring or joists.
- A timber moisture content of under 12% is typically safe for installation.
- If moisture is present, identify and resolve the cause (e.g. plumbing leaks, poor drainage, blocked vents) before proceeding.
✅3. Install a Moisture Barrier or Breathable Underlay
- Over timber, it’s essential to strike a balance between moisture protection and vapour breathability.
- Avoid completely sealing timber in with plastic sheeting (unless specified by manufacturer), as it can trap moisture within the timber.
- Instead, use an underlay designed for timber floors — preferably one that allows some vapour diffusion while still protecting the planks above.
✅4. Leave Expansion Gaps and Avoid Over-Sealing the Perimeter
- Just like with slab installs, allow expansion joints around all walls and fixed objects.
- Avoid silicone-sealing or tightly boxing in skirting boards, which can reduce airflow under the floor and increase mould risk.
✅5. Ensure Indoor Humidity is Controlled
- If your home tends to trap moisture (older homes, low ventilation, etc.), use dehumidifiers and exhaust fans as needed.
- Ideal indoor relative humidity sits between 40–60% year-round.
🌿 Bonus Tip: Use Healthy, Low-Toxic Flooring Materials
Whether you're installing over concrete or timber, choose hybrid planks and underlays that are:
- Low in VOCs (volatile organic compounds)
- Formaldehyde-free
Final Thoughts
Hybrid plank flooring can be a fantastic choice for modern homes — but it’s only as healthy as the conditions beneath it. Whether you’re laying over a concrete slab or timber frame, the key to long-term performance and indoor air quality is moisture management.
By following the advice above, as well as all Building Codes, Australian Standards, the manufacturers specifications and building biology practices, you’ll significantly reduce the risk of hidden mould and ensure your flooring remains beautiful, functional, and safe for years to come.
Need help designing a home that supports your wellbeing?
At Queensland Designer Homes, we blend great design with smart building practices — so your home not only looks good but feels good to live in. Let’s talk about your project.
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