Standard render adds almost no insulation on its own — it is millimetres of cement-based material, not an insulant. What it does do is keep driving rain out of the wall, and a dry wall loses noticeably less heat than a saturated one. If the goal is genuinely warmer rooms and lower bills, the system that delivers it is external wall insulation with a render finish, not render alone.
- A few millimetres of render has negligible thermal resistance. Anyone selling it as insulation is overselling.
- The indirect gain is real though: wet masonry conducts heat far better than dry masonry.
- External wall insulation is the system that actually changes the U-value, and it finishes in render.
- Solid walls are where the opportunity is — they lose considerably more heat than filled cavities.
The direct answer: render is not insulation
A silicone thin-coat system is perhaps 8 to 10mm thick. Monocouche might be 15 to 20mm. These are cement-based materials with thermal conductivity in the same broad territory as the masonry behind them. The thermal resistance they add is small enough to be within the rounding error of any energy calculation.
So if someone quotes you for rendering and tells you it will insulate the house, treat the rest of the conversation with care. It will not, and the claim is a reliable indicator of a sales approach rather than a technical one.
That is the direct answer. The indirect answer is more interesting, and it is where the honest benefit sits.
Why a dry wall is a warmer wall
Water conducts heat far better than air. Masonry that is saturated with rain loses heat substantially faster than the same masonry when dry, because the water filling its pores provides a conductive path straight through the wall.
There is a second effect on top of that. As moisture evaporates from the outer face of a wet wall it takes latent heat with it, cooling the wall surface in the same way sweat cools skin. A wall that is repeatedly soaked and then dries is losing heat through both mechanisms.
On an exposed, solid-walled property that takes driving rain on one elevation, this is not a marginal effect. Keeping that wall dry through the winter genuinely reduces heat loss through it — and that is exactly what a sound render in good condition does.
So render does help with energy use, but through moisture management rather than insulation. The distinction matters because it tells you where the limits are: on a sheltered elevation, or a wall that was already dry, you will notice very little.
Where the real gains are
| Measure | What it does thermally | Realistic effect |
|---|---|---|
| Standard render on a dry wall | Negligible added resistance | Little to none |
| Standard render on a wall that was getting soaked | Stops the wall saturating | Noticeable on exposed elevations |
| Cavity wall insulation | Fills the cavity in a cavity wall | Substantial, where the cavity is suitable |
| External wall insulation with render | Adds a continuous insulating layer outside the wall | The largest change available to a solid wall |
| Internal wall insulation | Insulating layer inside the room | Effective, but loses floor area and risks interstitial damp |
| Loft insulation | The cheapest heat saving in most houses | Large relative to cost; do this first |
The sensible order is almost always loft first, then draughts, then walls. Wall insulation is the most expensive measure per unit of heat saved, which is why it is worth doing properly when you do it.
External wall insulation: the system that actually works
External wall insulation, or EWI, is insulation board mechanically fixed and bonded to the outside of the wall and then rendered over with a reinforced basecoat and a topcoat. You end up with a rendered finish, so it looks like a rendering job, but it is an insulation job that happens to finish in render.
On a solid-walled house this is the measure that changes the numbers. A solid brick wall has roughly three times the heat loss of a modern insulated wall, and EWI addresses that directly by wrapping the building in a continuous layer.
It has a second benefit that gets less attention: because the insulation is on the outside, the masonry stays within the heated envelope. The wall itself becomes warm, which means it stores heat, there are far fewer cold bridges, and the surfaces inside are less prone to condensation and the mould that follows.
What EWI involves that plain render does not
- Thickness. Typically 100mm or more of insulation plus the render. Window and door reveals, sills, the roofline, rainwater goods, meter boxes and the boiler flue all have to be reworked to suit.
- Eaves and verge detailing. If the roof overhang is shallow, it may need extending so the new wall face is still protected.
- Breathability decisions. On a solid wall the insulation should be vapour-open — mineral wool rather than the commoner expanded polystyrene — so the wall can still dry outwards.
- Detailing at the base. The insulation must stop above the damp-proof course with a proper starter track and drip detail, or you have built a bridge around the DPC.
- Planning. EWI changes the external appearance and the building line. It is often permitted development on a house, but not on flats, and not in conservation areas or where an Article 4 direction applies.
- PAS 2035 applies where the work is funded through a government scheme, bringing a required assessment and design process.
Is it worth it financially?
That depends almost entirely on what you are starting from. The honest framing is this: EWI on a solid-walled, exposed, poorly insulated house with high heating use is a genuinely worthwhile measure. EWI on a 1990s cavity-walled house with filled cavities and a well-insulated loft is spending a lot to save a little.
The payback arithmetic on wall insulation alone is usually long — longer than people expect and longer than it is often presented. What changes the picture is that most people doing EWI are also getting a new exterior finish they wanted anyway, and in that case the comparison is not EWI against nothing, but EWI against the cost of re-rendering. On that basis the margin is a lot more attractive.
The non-financial benefits are also real and tend to be what people actually notice: rooms that warm up faster, fewer cold spots, less condensation on external walls, and a quieter house.
What it costs
These are the rendering and insulation figures published across this site:
| Job | Typical Coventry guide price | Notes |
|---|---|---|
| Rendering | £40–£70 per m² | System & access dependent |
| Three-bed semi | £2,500–£5,000 | Full elevation render |
| Detached house | £9,000–£12,000 | Larger area & scaffolding |
| External wall insulation | £90–£140 per m² | Insulated render system |
The gap between the rendering rate and the insulated render rate is the cost of the insulation and the extra detailing it brings. If you are re-rendering anyway, that gap is the real decision rather than the full EWI figure.
What to do first if the aim is a warmer house
- Loft insulation to current depth. It is the cheapest measure by a distance and it is often found to be thin, compressed or missing in patches.
- Draughtproofing. Unglamorous, cheap, and frequently worth more than people expect in an older house.
- Cavity wall insulation, if you have a cavity and it is suitable and unfilled.
- Deal with any damp. A wet wall will undo a good deal of whatever you spend on insulation.
- Then external wall insulation, particularly if the walls are solid and the exterior needs attention anyway.
We will tell you whether the walls are solid or cavity and whether insulated render is proportionate — including when it is not.
Frequently asked questions
Does rendering a house make it warmer?
Only indirectly, and only where the wall was getting wet. The render itself adds almost no thermal resistance, but keeping driving rain out of the masonry stops the wall saturating, and a dry wall loses considerably less heat than a wet one. On an exposed solid-wall elevation that is a real improvement; on a sheltered wall that was already dry you will notice little.
How much insulation does render add?
Essentially none worth counting. A thin-coat silicone system is around 8 to 10mm of cement-based material with thermal conductivity similar to the masonry behind it. If a quote presents standard render as an insulation measure, that is a sales claim rather than a technical one.
Is external wall insulation better than cavity wall insulation?
They apply to different walls. If you have an unfilled cavity in good condition, filling it is far cheaper and should come first. External wall insulation is the measure for solid walls, which have no cavity to fill and lose substantially more heat. A solid-wall property is where EWI earns its cost.
Will EWI cause damp problems?
It can if specified badly, and it usually improves things when specified well. The risks are using a vapour-closed insulation on a solid wall that needs to dry outwards, and bridging the damp-proof course at the base. Done correctly — vapour-open insulation on a solid wall, proper starter track above the DPC — it generally reduces condensation because the inside face of the wall is warmer.
Will rendering improve my EPC rating?
Standard render, barely. External wall insulation, yes and often substantially, because EPC assessments weight wall construction heavily and solid uninsulated walls score poorly. If an improved rating is part of your reason for doing the work, it is the insulation that delivers it, so make sure that is what you are buying.
Do I need planning permission for external wall insulation?
For most houses it is permitted development, but it changes the external appearance and the building line, so the exceptions matter: flats and maisonettes are excluded, and listed buildings, conservation areas and Article 4 areas all need checking first. Where work is funded through a government scheme, PAS 2035 assessment and design requirements also apply.
Warmer walls, or just better-looking ones?
We will tell you which of the two your house actually needs, and whether insulated render is proportionate to what you would gain. Free survey, fixed written quote.