- Order matters: roof, glazing, walls, floor, then ventilation, then installations.
- Every insulation measure changes how moisture moves through the construction.
- Once the house is airtight, ventilation stops being optional.
- Cavity fill is excellent in the right building and a liability in an exposed, driven-rain location.
- Internal insulation of a solid wall needs careful detailing or it moves the condensation point inwards.
A pre-war Dutch house loses heat through every surface, and improving that is transformative — warmer, quieter, cheaper to run. It is also where we are most often called in to fix somebody else's work, and the cause is nearly always the same: the insulation was installed correctly and the moisture was not considered at all.
Where the heat actually goes
For a typical uninsulated Dutch terrace, roughly in order of magnitude:
- The roof. Heat rises and the roof is a large surface, often with nothing in it at all.
- Windows. Single glazing performs very poorly, and frames leak air as well as heat.
- Walls. A large area, though loss per square metre is lower than through glass.
- The ground floor. Often a timber floor over a ventilated crawl space, which is effectively outside.
- Air leakage. Around frames, at the eaves, through letterboxes, chimneys and service penetrations. Frequently a larger factor than people expect.
Note that party walls barely feature: your neighbours are heated too. That is why a mid-terrace house performs better than an end-terrace of identical construction, and why the end-terrace deserves attention to the exposed flank.
The order that works
| Step | Measure | Why here |
|---|---|---|
| 1 | Roof insulation | Largest single loss; often accessible from inside |
| 2 | Glazing and frames | Comfort transformation; addresses air leakage at the same time |
| 3 | Wall insulation | Large area; method depends on construction |
| 4 | Floor insulation | Straightforward where the crawl space is accessible |
| 5 | Airtightness detailing | Cheap, and it makes the previous steps perform |
| 6 | Ventilation | Now essential, not optional |
| 7 | Heating installation | Sized for the improved envelope, not the old one |
Doing step 7 first is the most common and most expensive sequencing mistake. A heat pump in an uninsulated house disappoints, and then the technology gets the blame.
Cavity walls: when filling is right and when it is not
Most Dutch housing from roughly the 1930s onwards has a cavity wall. Filling that cavity is quick, relatively inexpensive and often excellent value. It is also the measure most likely to cause problems in the wrong building.
It works well when:
- The cavity is clean, of consistent width and free of mortar droppings.
- The outer leaf is sound: good pointing, no cracks, functioning gutters.
- The building is not severely exposed to driving rain.
It causes problems when:
- The outer leaf lets water through. Fill provides a path for that moisture to cross the cavity to the inner leaf, and you get damp patches internally that were not there before.
- The building is on an exposed coastal or open elevation with heavy wind-driven rain.
- The cavity is obstructed, giving patchy fill and cold spots that then attract condensation.
Inspect the cavity before filling it — usually with an endoscope through a drilled hole — and repair the outer leaf first. Filling a cavity behind defective pointing converts a maintenance problem into a damp problem.
Solid walls: internal or external
Pre-war housing often has solid masonry with no cavity. Then there are two routes, and both need care.
Internal insulation is usually the only option in a terrace or a protected townscape. It works, but it moves the dew point: the masonry behind the insulation becomes colder, and any moist internal air reaching it will condense there. That makes the vapour control layer and its detailing essential rather than optional — continuous, taped, and carefully handled at floors, ceilings, window reveals and party walls. It also costs internal space and creates cold bridges at every junction.
External insulation is technically superior: a continuous layer with no cold bridges and the masonry kept warm. It is rarely possible on a street facade in the Netherlands, for planning and streetscape reasons, but it can be viable on a rear elevation or a flank wall.
Either way, solid-wall insulation is design work, not just installation. Done without thought it is the most reliable way to create damp in a house that did not have it.
Roof and floor
The roof can be insulated from inside, between and under the rafters, or from outside when the covering is being replaced. From inside is far more common and works well, provided two things happen: a second layer beneath the rafters to break the timber as a cold bridge, and a properly taped vapour control layer on the warm side with a service void in front of it so that every socket does not puncture it.
In older roofs without a breathable membrane, a ventilated air gap must remain between the insulation and the boarding. Packing insulation tight against the boarding in such a roof is the classic cause of timber decay, invisible for years.
The floor is often the easiest win. Where there is an accessible crawl space, insulation can be fixed to the underside of the floor with relatively little disruption. Check the crawl space first: standing water or very high humidity is a separate problem, and insulating over it simply hides it.
Ventilation: the step that gets skipped
A draughty house ventilates itself. That wastes heat, but it also removes the moisture produced by cooking, washing, drying and breathing. Insulate and seal that house and the losses stop — and so does the moisture removal.
This is why condensation and mould so often appear after an insulation upgrade, and why homeowners conclude that insulation caused damp. It did not; the missing ventilation did.
What is needed depends on the house, but the principle is constant: controlled supply and controlled extraction, sized for the dwelling. Mechanical extraction from kitchen and bathroom, discharging outside rather than into a roof void or a defunct duct, is the minimum. Never block existing ventilation openings because they cause a draught; find the cause of the draught instead.
The mistakes we are called in to fix
- Cavity filled behind a defective outer leaf, producing internal damp patches within a year or two.
- Insulation packed tight against roof boarding in a roof with no breathable membrane, leading to timber decay.
- Vapour control layer omitted, punctured or untaped, so moist air enters the construction and condenses.
- No second insulation layer under the rafters, leaving visible cold-bridge lines on the ceiling.
- Ventilation ignored, producing mould in a house that never had it.
- Internal wall insulation without detailing at floors, reveals and party walls, creating cold spots exactly at the junctions.
- A heat pump installed first, into an envelope that cannot support low flow temperatures.
Summary
Insulating an older Dutch house is the highest-value work available, and the order matters: roof, glazing, walls, floor, airtightness, ventilation, then installations. Every measure changes how moisture moves through the construction, which is why the vapour control layer, the ventilated gap in an older roof and the detailing at junctions are not extras. Inspect a cavity and repair the outer leaf before filling it. And treat ventilation as part of the insulation project rather than as something to look at later, because a sealed house without it produces exactly the damp problems that insulation gets blamed for.
Next: energy labels or our window frames and insulation service.