Key Takeaways
- Painting over mould does not kill it — the source moisture must be fixed first.
- Tile adhesive is not universal; using the wrong type can cause tiles to fail prematurely.
- Skipping primer rarely saves time or money in the long run.
- Caulk is not a structural gap filler — it has specific and limited use cases.
- More insulation is not always better; proper installation matters as much as thickness.
Why DIY Myths Are So Persistent
Home improvement advice travels fast — through neighbourhood chats, social media tutorials, and well-meaning family members. The trouble is that a tip repeated often enough starts to feel like fact, even when it contradicts how materials actually behave. Acting on a flawed assumption can turn a simple weekend fix into a costly repair job.
This article unpacks some of the most common DIY misconceptions so you can approach your next project with accurate expectations. For a broader look at how myths form in adjacent areas, our drywall repair guide and guide to recurring damp patches offer grounded, practical detail.
Myth
Painting over mould kills it and solves the problem for good.
Fact
Paint — even mould-resistant paint — does not kill mould. It only covers it temporarily.
Mould is a living organism that grows when moisture is present. Sealing it under a coat of paint traps it momentarily but does nothing to stop the biological process. Without eliminating the moisture source — a leak, condensation, or inadequate ventilation — the colony continues to grow behind and eventually through the new paint layer.
The correct approach is to identify and fix the moisture source first, treat the affected surface with an appropriate mould-remediation product according to its label instructions, allow the area to dry fully, and then repaint. For recurring damp problems that resist surface treatment, see our guide on recurring damp patches.
Myth
Any tile adhesive will work for any tile installation.
Fact
Adhesive type must match the tile material, substrate, and whether the area is wet or subject to temperature changes.
Using a standard wall adhesive on a large-format floor tile, or a basic mastic in a shower enclosure, are both common causes of premature tile failure. Different adhesive formulations are engineered for different load-bearing requirements, flexibility, and water resistance.
Always check the adhesive manufacturer's data sheet against your specific tile weight, size, and installation environment. In wet areas such as showers or around bath surrounds, a polymer-modified or epoxy-based adhesive is typically required. When in doubt, consult a tile supplier — choosing correctly at the start is far cheaper than re-laying a floor.
Myth
Skipping primer saves time and the extra coat of paint compensates.
Fact
An extra topcoat does not replicate what primer does — and omitting it often means more coats are needed, not fewer.
Primer serves several functions that paint cannot: it seals porous surfaces so topcoat is absorbed evenly, improves adhesion on slick or previously painted surfaces, and blocks stains from bleeding through. Without it, paint can streak, peel, or look patchy — outcomes that require additional coats and often a full redo.
On new drywall especially, skipping primer leads to a phenomenon called 'flashing' where some areas absorb more paint than others, leaving an uneven sheen even after multiple coats. Our article on patchy paint results covers this and other preparation mistakes in detail.
Myth
Caulk can fill any gap around the house — it's a universal fixer.
Fact
Caulk is a flexible sealant designed for narrow expansion and contraction joints, not a structural gap filler or mortar substitute.
Caulk is appropriate for sealing joints between two surfaces that move slightly relative to each other — around window and door frames, at the junction of a bath and tiles, or between baseboard and floor. Filling large voids with caulk leads to cracking and shrinkage as the product cures and cannot support its own volume without backing.
For gaps larger than roughly 6mm (about a quarter inch), a backer rod should be installed first to give the caulk a foundation and prevent it sinking into the void. Structural gaps — such as cracks in masonry or wide separations in drywall — require proper repair materials specific to the substrate. See our drywall repair walkthrough for the correct patching approach.
Myth
Adding more insulation is always better — pile it in and you'll save more energy.
Fact
Beyond a point, additional insulation yields diminishing returns, and poor installation defeats even high R-value products.
R-value measures thermal resistance, and there are recommended levels for each climate zone and building location. Beyond those levels, the energy savings per dollar of added insulation decrease substantially. More critically, insulation that is compressed, misaligned, or installed without adequate air sealing performs well below its rated R-value regardless of thickness.
Air leakage — not poor insulation alone — is responsible for a significant share of heat loss in most homes. Sealing air gaps around penetrations, attic hatches, and rim joists before adding insulation is often more effective than simply layering more material. Our homeowner's introduction to insulation explains R-values, types, and realistic DIY scope.
Mistakes That Compound Over Time
Several of the myths below share a common thread: they offer a shortcut that appears to work initially but creates a larger problem months or years later. Mould painted over will resurface. Tiles laid with the wrong adhesive will loosen. Insulation installed without attention to air sealing will underperform regardless of its R-value.
When DIY Becomes a Safety or Code Issue
Some tasks — including electrical work, gas line repairs, load-bearing structural changes, and certain plumbing connections — are regulated in most U.S. jurisdictions and typically require permits and licensed tradespeople. Undertaking these as unguided DIY projects can create safety hazards, void homeowner's insurance, and cause complications when selling your home. Always verify local permit requirements before starting work that goes beyond cosmetic changes.
Understanding why a correct method works helps you evaluate future advice on its merits rather than on how authoritative it sounds. If you are also reviewing your renovation budget, the principles for renovating on a budget article explains how to build in contingency for exactly these kinds of hidden failures.
~35%
Of home heat loss attributed to air leakage
The U.S. Department of Energy estimates that air leakage accounts for a significant portion of heating and cooling energy loss in typical American homes.
1 in 5
DIY projects that require professional correction
Industry surveys suggest a notable share of DIY home repairs require a licensed professional to fix errors, often at greater cost than the original job.
For paint-specific problems, see why freshly painted walls still look patchy — preparation errors explain most of those outcomes.