I got called to a house in Lucan last year. The homeowner had done everything “right.”
Filled the attic with insulation. Maxed out the Energy Saving Trust grants. Even added extra layers beyond the minimum.
And now he had mould everywhere, rotten timbers, and a £15,000 problem.
The insulation was working exactly as it should—trapping heat below the ceiling. But nobody told him about the second half of the equation: what happens to the moisture that used to escape through that heat.
This is one of the most common and expensive mistakes I see. And it’s not the only one.
Mistake #1: Insulating Without Ventilating
This is the big one. The one that causes structural damage.
The Physics
Your house generates moisture. Cooking, showering, breathing, drying clothes—a typical family produces 10-15 litres of water vapour per day.
In an uninsulated house, that moisture rises through the ceiling, meets the cold roof space, and escapes through natural ventilation. Not ideal (you lose heat), but the moisture exits.
Now add insulation.
The ceiling is warm. The roof space above the insulation is cold. When warm, moist air from the house meets that cold zone, it condenses.
Water droplets form on the underside of your roof felt. On the rafters. On anything cold.
What I See
- Rafters dripping with condensation
- Black mould on timber surfaces
- Felt saturated and sagging
- Insulation soaked (useless and heavy)
- Rot starting in structural timbers
This happens within 2-5 years of badly planned insulation projects.
The Solution
Ventilation must match insulation.
Your roof space needs airflow:
- Soffit vents at the eaves (air in)
- Ridge vent or tile vents at the top (air out)
- Cross-flow allowing moisture to escape
The warm, moist air rises, hits the cold zone, condenses—and then is carried away by ventilation before it can cause damage.
Before you insulate, ensure adequate ventilation is in place. This isn’t optional; it’s physics.
Mistake #2: Blocking the Eaves
I see this constantly with DIY insulation jobs.
What Happens
Keen homeowner buys rolls of insulation. Unrolls them across the attic floor. Keeps going right to the edges.
Now the insulation is blocking the gap between the wall and the roof at the eaves. No air can get into the roof space through the soffits.
Ventilation? Gone. Condensation? Incoming.
The Fix
Leave a gap at the eaves. At minimum, 50mm of clear space.
Better: install proprietary eaves ventilation trays. They create a channel over the wall plate, allowing air in while still letting you insulate over the top.
When I inspect attics, the first thing I check is whether the eaves are blocked. It’s the most common ventilation failure.
Mistake #3: Compressing Insulation
Insulation works by trapping air. Those tiny pockets of air are what resists heat transfer.
When you compress insulation—squashing it into a smaller space—you squeeze out the air pockets. The R-value (heat resistance) drops dramatically.
Common Compression Scenarios
- Stuffing excess into a joist bay that’s too shallow
- Stacking things on top of insulation
- Walking routes across the attic crushing the material
- Cramming insulation around pipes and obstacles
The Result
You’ve got the same amount of material, but it’s performing at maybe 60% of its rated value. You’ve paid for something you’re not getting.
The Fix
- Use insulation appropriate to the depth available
- Create boarded walkways for access (supported above, not compressing, the insulation)
- Cut carefully around obstacles rather than stuffing material around them
- Check manufacturer guidance on depth versus performance
Mistake #4: Leaving Gaps and Cold Bridges
Heat finds the easy route. If your insulation has gaps, that’s where your heat goes.
Where Gaps Happen
- Around hatches (the most common thermal bridge in British attics)
- At pipes and service penetrations
- Where insulation rolls don’t quite meet
- At joist bays that were missed or hard to access
- Around water tanks and pipes
Thermal Imaging Reality
I’ve seen thermal surveys where the insulated sections work beautifully, but a single uninsulated hatch is responsible for 20% of the heat loss.
The Fix
- Insulate the attic hatch itself (available pre-made, or DIY with rigid board)
- Use draught strips around hatch edges
- Ensure rolls overlap or butt tightly
- Fill odd-shaped gaps with loose-fill or cut pieces
- Don’t skip awkward corners because they’re awkward
Mistake #5: Wrong Insulation for the Application
Not all insulation is the same. Using the wrong type for your situation causes problems.
Roof Slopes vs. Attic Floor
- Attic floor: Loft rolls or loose-fill work fine. Doesn’t need to resist compression.
- Roof slopes (room in roof): Needs rigid boards or friction-fit batts that hold position between rafters. Rolls sag and create gaps.
Damp Areas
If you have any moisture issues in the attic, mineral wool insulation will absorb that moisture and become useless. It may never properly dry.
For problematic areas, closed-cell insulation (like PIR boards) doesn’t absorb water.
Pipes and Tanks
In an insulated attic, pipes and water tanks are now in a colder space (heat isn’t escaping up from below anymore).
Pipes can freeze that never froze before. Tanks can freeze that never froze before.
You need to:
- Insulate pipes thoroughly (lagging)
- Insulate around (not under) water tanks
- Consider electric trace heating for vulnerable pipes
I’ve attended burst pipe emergencies that were directly caused by insulation upgrades that forgot about the plumbing.
Mistake #6: Ignoring the Existing Condition
Before adding insulation, assess what’s already there.
What I Find
- Old insulation saturated with water (add more and you’re trapping the moisture)
- Rodent contamination (nesting, droppings—health hazard before you start)
- Asbestos-containing insulation in pre-1980s properties (requires professional removal)
- Electrical wiring in poor condition (buried under insulation = fire risk)
- Existing timber damage (insulation won’t fix rot, but will hide it)
The Right Approach
A proper insulation upgrade includes:
- Assessment of existing conditions
- Remediation of any problems found
- Ventilation check and upgrade if needed
- Then, and only then, insulation installation
Jumping straight to “add more insulation” without this assessment causes expensive problems down the line.
Mistake #7: DIY in Room-in-Roof Situations
Attic floor insulation is relatively DIY-friendly. Unroll stuff between joists. Done.
Room-in-roof situations—where the attic space is (or will be) living space—are completely different.
The Complexity
You need:
- Insulation between rafters AND/OR above/below
- Vapour barrier on the warm side
- Ventilation gap between insulation and roof covering
- Airtightness at all junctions
Get this wrong and you’ve got condensation forming inside your roof structure—invisible, damaging, expensive.
My Advice
If you’re insulating a habitable attic or converted loft:
- Get professional design advice at minimum
- Seriously consider professional installation
- Building regulations require specific U-values—DIY often fails to meet them
The grant doesn’t pay for doing it twice.
Mistake #8: Chasing Maximum Depth Without Considering Returns
More insulation is better, right?
Up to a point.
Diminishing Returns
- 100mm insulation: Good basic improvement
- 200mm insulation: Excellent performance
- 300mm insulation: Current regulation standard, very good
- 400mm insulation: Diminishing returns begin
- 500mm+ insulation: Minimal additional benefit for significant extra cost
The first 200mm does most of the work. Doubling that again gives a small percentage improvement.
Where the Money Goes Better
Instead of going from 300mm to 400mm in the attic, you’d get better results from:
- Insulating the attic hatch properly
- Sealing air leaks around services
- Improving wall or floor insulation
I’ve seen people obsess about maxing out attic insulation while ignoring a draughty hatch that’s undoing half the benefit.
Mistake #9: Forgetting Building Regulations
If you’re doing significant work to your home—and an insulation upgrade can qualify—Building Regulations may apply.
What’s Required
- Minimum U-values for thermal elements
- Moisture and ventilation requirements
- Compliance with Part L (Conservation of Fuel and Energy)
When It Matters
For most attic floor insulation, regulations are advisory rather than mandatory (unless you’re doing it as part of a larger project).
But for room-in-roof insulation, extensions, or major renovations, compliance is required—and inspected.
DIY installations that don’t meet regulations can:
- Cause problems when selling (compliance certs missing)
- Void insurance (non-compliant works)
- Actually perform poorly despite the expense
What Good Looks Like
A proper insulation upgrade includes:
- Assessment: Condition check, moisture check, ventilation check
- Remediation: Fix any problems first
- Ventilation: Ensure adequate before insulating
- Materials: Right type for the application
- Installation: No gaps, no compression, proper attention to details
- Ancillaries: Hatch insulation, pipe lagging, tank insulation
- Documentation: For grants, compliance, and future reference
This costs more than “unroll some stuff and call it done.” But it actually works, and doesn’t create problems that cost more to fix than you saved.
The Energy Saving Trust Grant Reality
Energy Saving Trust grants for attic insulation are generous. And the registered contractors doing the work are supposed to follow standards.
But I still see problems:
- Ventilation not checked (not in the contractor’s scope, apparently)
- Existing moisture issues not addressed
- Grant conditions met, but actual performance compromised
If you’re getting grant-funded insulation:
- Ask about ventilation specifically
- Ask about existing condition assessment
- Ask what they do about eaves, hatches, pipes
A good contractor will cover these. A box-ticking contractor might not.
Final Thought
Insulation is brilliant. It reduces heat loss, cuts energy bills, makes your home more comfortable.
But it’s not as simple as “more is better.”
The mistakes I’ve described—blocking ventilation, ignoring moisture, compressing material, forgetting the details—these turn a good investment into an expensive problem.
Done properly, insulation repays itself many times over.
Done badly, it creates rot, mould, and regret.
Know the difference before you start.
Want to insulate properly?
We can assess your attic, check ventilation, identify issues, and ensure the insulation actually works.
Or call: +44 89 981 9675
Seamus O’Brien has been fixing insulation disasters for years. He’d rather help you avoid them.
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