If there’s one part of a self build or renovation that’s easy to underestimate, it’s insulation. Most of us know it’s essential for keeping the house warm and reducing energy bills, but the reality is that insulation isn’t just about stuffing some material into walls or roofs and calling it a day.
Done wrongly, it can lead to damp, drafts, mould, and a home that’s far less comfortable than it should be.
Over the years, I’ve seen plenty of errors – the kind that quietly undo all the hard work and expense of insulating in the first place. If you’re planning an upgrade or new build, here are the most common insulation mistakes and how to avoid them.
1. Not checking if your home is suitable
Before any type of insulation is installed, its vital that it’s checked for suitability. Too often, self builders rush in and start insulating without checking if the application (cavity walls, timber frames, solid masonry walls etc) can be insulated. For example, using cavity wall insulation on a timber frame building, or spraying foam between the rafters of a roof space.
Without proper ventilation, moisture can get trapped causing rot, and insulation that gets wet or suffers thermal bypass will perform far below expectations, wasting money rather than saving it.
The solution is to always start with a professional advice. This might be a survey or an energy assessment from a qualified surveyor or designer who’ll assess the building, spot potential problems (like damp or poor ventilation), and recommend the best materials and insulation methods.
2. Choosing the wrong type of insulation
Insulation isn’t a one-size-fits-all product. Different parts of a house, such as the loft, walls, floors, or roof, will require different approaches and usually a different insulation product.
For example, fibreglass is great for dry lofts but not suitable for damp spaces. Rigid insulation boards are really effective under floors and in walls, but they’re less suitable if they need to fit awkward shapes or uneven surfaces. An obvious alternative is spray foam insulation and whilst this might look attractive, if its installed incorrectly, it can cause major problems
Most self builders will look for a product that’s very thermally efficient, but it’s also important to consider moisture resistance, fire safety, and breathability as well. Again it might be best to consult a professional who’ll understand the product and will be able to compare it with other alternatives.
Alternatively, take time to do your own research, look at the best ways to insulate a home, and read the manufacturers literature and technical information such as installation instructions and declarations of performance (DOP’s).
3. Poor installation — gaps and voids
Even the best insulation product won’t perform properly if its not installed properly. Gaps, overlaps, or over compression of quilts will affect an insulation material's actual performance. Neatly insulated lofts will be compromised by something as simple as tiny gaps, resulting in cold spots and heat loss, higher bills and potentially condensation and mould.
It’s important to pay attention to detail. Insulation should fit snugly, without being squashed or stretched. Cavities should be consistent and well-ventilated and rigid insulation materials must be cut precisely around timbers and joists.
Achieving a good job requires care, so measure twice, cut once. To help get things right, invest in innovative products such as compressible gap fillers including Compriband or Gapotape, available from Insulation Online, which will fill gaps and prevent air movement and thermal bypass.
Compressive band of Charcoal Grey Combriband, 20mm wide, 4.3m long that expands from 8mm to 28mm
4. Neglecting airtightness
Insulation works by slowing the transfer of heat from a warm to a cold space. But insulation on its own doesn’t stop air movement. If there are gaps around windows, floorboards, or service penetrations, heat will still escape and cold air will find its way in.
As well as installing the correct insulation product, it’s essential that airtightness is also dealt with. This means installing airtight membranes and vapours control layers along with airtight tapes and sealants to close gaps. A simple smoke test with an air pressure test will check that an installation meets the appropriate level of airtightness, and will help identify where leaks are happening so they can be sealed properly.
Try these products for insulating around windows and doors
Window and door seal strip, self adhesive in 6m length
Suitable for sash windows and doors, a 3m length of self adhesive insulation
Quick and easy to install, this strip fits to the bottom of doors to keep out the cold
5. Blocking ventilation paths
A common misconception is that sealing up every possible gap will make a home more energy efficient. Whilst there is some truth to this, some airflow is essential for adequate ventilation – particularly in lofts and cavities. Blocking eaves vents or air bricks prevents moisture escaping and can quickly turn a dry, healthy space into a damp, mouldy mess.
The solution is simply to maintain ventilation paths when installing insulation. With loft ventilation, cross flow vent trays fitted between the rafters at the eaves will keep air moving. For cavity wall insulation, make sure any air bricks or weep holes are not obstructed when the insulation is pumped in. A good installer will check these things both before and after an installation.
6. Ignoring existing moisture problems
Insulation and moisture typically don’t mix. If there’s an underlying damp problem, insulating over it is like painting over rust – it hides the issue but doesn’t fix it. Moisture trapped behind insulation can be even worse, causing damage to the insulation, structural members and encouraging different types of mould to grow. Once again moisture will also cause the insulation to lose its effectiveness.
The key here is to always carry out a survey and address any damp issues first. Fix leaks, improve drainage, and maintain ventilation. Once the area is dry and sound, the insulation can be installed with confidence. In moisture-prone areas like basements or ground floors, opt for specialist closed-cell insulation products designed for wet conditions. These materials won’t absorb water and will not deteriorate over time.
7. Over-insulating without proper planning
Adding layer upon layer of insulation might sound like a good idea, but can easily backfire. Beyond a certain point, extra insulation provides very little additional benefit, meaning it's important to understand how much is too much insulation. Get it wrong and it can actually create problems by restricting ventilation paths, trapping heat and in some circumstances, becoming a fire risk.
The best approach is to seek expert advice and discuss how a building element should be insulated before making alterations. A good professional will know the best option and when old, tired insulation should be removed. They’ll recommended appropriate products with thermal resistances to such the location and climatic conditions. The objective should be to achieve the right balance of insulation, ventilation, and airtightness so it properly works together. If the building becomes overly airtight, consider mechanical ventilation with heat recovery (MVHR) to maintain healthy air circulation and retain heat.
8. DIY disasters and uncertified installers
Installing insulation might look straightforward, but there’s actually much more to it than meets the eye. DIY efforts can easily go wrong – from leaving gaps and overly compressing quilts, to using the wrong type of material.
Hiring someone who isn’t properly certified or competent can also lead to problems such as wrongly installing spray foam insulation causing structural damage and resulting in insurance and saleability issues. Poor workmanship wastes money, reduces performance, and in some cases, requires costly remediation. In extreme cases properties can even become unmortgageable.
Always seek professional advice and hire qualified, experienced contractors. Look for certifications from reputable bodies such as the National Insulation Association (NIA) or TrustMark. Ask for references, check reviews, and don’t be afraid to ask questions about the appropriateness of materials, ventilation strategies, and guarantees. Professional contractors will appreciate the conversation and will be just as interested as you are in safely making improvements to your home.
For more advice on fitting insulation in the right way, take a look at our guide to installing internal wall insulation in a renovation and make sure you've allocated sufficient funds in your budget to your loft insulation costs.
