Short answer: start with cheap draught-proofing, then tackle the roof, then the walls or cavity, then the floor or crawl space, and leave the glazing for last. That order follows one simple logic: biggest heat loss first. Most heat escapes through the roof, because warm air rises. After that come the walls, and only then the floor and windows.
But there is nuance to it. It is not only about where you leak the most. It is also about return on investment, cost, payback time and the logical building order. And there is one thing you must never forget: insulating without ventilating properly gives you damp and mould.
In this article I take you through the full order, with target values, realistic costs and the mistakes I see people make again and again. So that by the end you know exactly where to start and why.
Where does most heat escape in a house?
Before you pick an order, you need to know where the heat is leaking away. In an average, moderately insulated older home, the split looks roughly like this:
| Building element | Share of heat loss (indicative) |
|---|---|
| Roof | 25 – 30% |
| Walls / façade | 20 – 25% |
| Ventilation, gaps and cracks | 15 – 25% |
| Floor / crawl space | 10 – 15% |
| Windows / glazing | 10 – 15% |
Note: these are ranges, not laws. The split depends on the year the house was built, the type of property and what has already been done.
A detached house loses relatively a lot through the walls. A mid-terrace house shares walls with the neighbours and loses more through the roof and floor instead. A flat has little wall loss, but sometimes a cold floor above an unheated space.
Do you want to know for certain? Get an insulation survey or a heat loss calculation done. Then you will see in black and white which element is costing you the most.
Is “heat rises, so do the roof first” actually right?
Partly. Warm air does rise, and a poorly insulated roof is often the main culprit. That is why the roof sits at the top of almost everyone’s list.
But it is not an iron law. In a house with a decent roof and a freezing cold crawl space, floor insulation may well deliver more. And the biggest quick win often is not in a building element at all, but in the gaps.
So the right order comes down to four things at once:
- Biggest heat loss first (the most saving).
- Best return and payback (cheap wins first).
- Logical building order (seal the shell first, then the heating system).
- What you can do yourself versus what needs a professional.
The recommended order, step by step
Here is the order I recommend, with the reasoning per step, what you can take on yourself and the ballpark figures.
Step 0: Sealing gaps and cracks (the almost-free win)
Before you start on the big insulation jobs, seal the gaps first. Draught strips on doors and windows, sealing tape, a letterbox brush, sealant along skirting boards and pipe penetrations.
This is dirt cheap, you do it yourself in an afternoon, and it easily tackles 15 to 25% of your heat loss through ventilation and gaps. Payback is often less than a year.
Important: only seal unwanted gaps, not your ventilation grilles. You need those (more on that below).
Step 1: Roof insulation
Biggest source of loss, so usually step one. You insulate the pitched roof, from the inside or from the outside, or the loft floor if your loft is unheated and staying that way. That last option is the cheapest.
- DIY? Insulating the loft floor and tackling an open roof structure from the inside is often within reach for a handy DIYer. The roof from the outside, when the covering is being replaced, is a job for a roofer.
- Target value: at least Rc 3.5 m²K/W, which works out at roughly a U-value of 0.28 W/m²K. If the roof is open anyway, go straight for Rc 6.0. That insulation is going to sit there for the next few decades, so do it properly in one go.
- Cost: roughly €20 to €60 per m², depending on the method.
- Payback: around 4 to 8 years, before any grants.
Step 2: Wall and cavity insulation
After the roof, the second big source of loss. How you approach it depends on your wall:
- Cavity wall (two leaves with an air gap between them): have the cavity filled. Relatively cheap, quickly done, and you feel the comfort straight away. This is work for a specialist firm, not for the DIYer.
-
Solid wall (no cavity): then you insulate from the inside (internal wall lining) or from the outside (external wall insulation). Considerably more expensive and far more disruptive.
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Target value: at least Rc 3.5 m²K/W for retrofit insulation inside or out. Lower values apply to cavity fill, simply because the cavity is only so wide.
- Cost: cavity wall roughly €15 to €35 per m². External wall insulation runs up to €80 to €200 per m².
- Payback: cavity wall insulation is one of the fastest-paying measures there is, often within a few years. External wall insulation takes longer.
Warning on cavity walls: always have the cavity inspected first for damp, penetrating moisture and debris. Insulating a wet or dirty cavity causes damp problems. This is where people make the most expensive mistake.
Step 3: Floor and crawl space insulation
Warm feet, less draught from below. Especially worthwhile above a cold crawl space or an unheated cellar.
- DIY? If the crawl space is accessible and tall enough, fixing boards or insulation blankets to the underside of the floor is something you can do yourself. Insulating the crawl space base, with loose fill or PIR, is usually blown in by a firm.
- Target value: aim for Rc 3.5 m²K/W or higher.
- Cost: roughly €15 to €40 per m², depending on the method.
- Payback: around 5 to 10 years. The comfort gain, no more cold feet, counts for a lot here.
Step 4: Glazing (double or triple)
Glazing is at the back of the queue on purpose. Not because it does not help, but because the saving per euro is usually lower than for the roof and walls, and the costs are high.
- Going from single glazing to modern low-emissivity double glazing is a huge step up and almost always sensible.
- Triple glazing mainly pays off if you are replacing the frames anyway and heading towards a heat pump. Watch out: triple glazing is heavier and often needs stronger or new frames.
- Target value (U-value, lower is better): modern double glazing around U ≤ 1.2 W/m²K, triple glazing around U ≤ 0.7 W/m²K.
- Payback: longer than the other measures, and mainly driven by comfort and the end of that cold downdraught by the window.
Priority table: at a glance
| Order | Measure | Rc/U target | Cost (indicative) | DIY? |
|---|---|---|---|---|
| 0 | Seal gaps and cracks | n/a | Very low | Yes |
| 1 | Roof / loft floor | Rc ≥ 3.5 (ideally 6.0) | €20 – €60/m² | Partly |
| 2 | Walls / cavity | Rc ≥ 3.5 (cavity lower) | €15 – €200/m² | No (cavity/external) |
| 3 | Floor / crawl space | Rc ≥ 3.5 | €15 – €40/m² | Partly |
| 4 | Glazing (double / triple) | U ≤ 1.2 / U ≤ 0.7 | High | No |
Rule of thumb: cheap, big wins first. Gaps and cavity walls come top on return, glazing comes bottom.
Insulating and ventilating: do not treat these separately
This is the most important warning in the whole article. The better you insulate and seal, the less your house ventilates by itself.
A draughty house used to blow itself dry. A well-insulated house does not. Moist air from cooking, showering and breathing stays inside, settles on cold spots and gives you mould.
So insulating and ventilating go hand in hand.
- Keep ventilation grilles open, in winter too.
- Consider mechanical ventilation or a heat recovery system if you are insulating heavily.
- Always ventilate during and after cooking and showering.
Insulating without ventilating is not a saving. It is a mould problem in the making.
Insulating before a heat pump: shell first
Thinking about a heat pump or low-temperature heating? Then the order is clear: shell first, system second.
A heat pump runs at lower flow temperatures than a gas boiler. That only stays comfortable if your home loses very little heat. Insulate properly first, and the heat pump can run at low temperature, which lifts both efficiency and comfort.
Do it the other way round and you buy an expensive system that cannot get a leaky house warm. A waste of your money.
Watch the heat emitters too: at low temperature you need more emitting surface, so bigger radiators, convectors or underfloor heating. Test that before you rip the old boiler out.
Common mistakes
- Filling a cavity without a damp check. Insulating a wet or dirty cavity pushes moisture through to the inside. Always have it inspected first.
- Insulating without sorting out ventilation. The classic: house sealed up tight, and three months later there is mould in the corners and on the frames.
- Ignoring thermal bridges. Wherever insulation is missing, at a concrete edge, at the floor-to-wall junction, at a window frame, you get condensation. Insulate as continuously as you can.
- Wrong order with a heat pump. Buying the system first and insulating afterwards. Flip it around.
- Looking only at the saving. Comfort counts too: no cold feet, no cold downdraught at the window, an evenly warm house. That is half the win.
- Doing half a job on the roof. If the roof is open anyway, go straight to Rc 6.0. It costs next to nothing extra and you benefit for decades.
Grants: keep it short and check the current rules yourself
There are grant schemes for insulation in most countries, and the details vary a lot by where you live. The common threads look like this:
- Support is aimed at measures on the building shell: roof, walls, floor and windows.
- The insulation has to meet minimum performance values, usually set as a minimum R-value or a maximum U-value per element.
- The work often has to be done by a certified installer, and in many schemes you have to apply before the work starts.
- Some schemes only pay out if you combine several measures, or if an energy assessment has been carried out first.
The amounts and conditions change regularly. I deliberately do not quote a fixed figure here, because it will be different again next year.
Always check the current amounts and conditions with your national scheme and look at whether your local council offers its own grant or a low-interest energy loan on top. Count any grant as a discount on your payback time, not as a reason to change your order.
Frequently asked questions
What order should I insulate my house in?
Start with draught-proofing, then the roof, then the walls or cavity, then the floor or crawl space, and the glazing last. That order follows the biggest heat loss and the best return per euro.
Do I really have to start with the roof?
Usually yes, because most heat disappears through the roof. But if you already have a good roof and a freezing cold crawl space, floor insulation may deliver more. An insulation survey settles it.
What is the cheapest insulation with the fastest payback?
Sealing gaps (almost free, paid back in under a year) and cavity wall insulation (often within a few years) are the quickest wins. Glazing takes the longest to pay for itself.
Why do I need to ventilate when I have just sealed everything up?
Because an insulated house can no longer shed moisture on its own. Without ventilation you get condensation and mould. Keep grilles open and ventilate actively, especially after cooking and showering.
What Rc value should I aim for?
For existing homes, Rc 3.5 m²K/W is a good minimum. If your roof is open anyway, go straight to Rc 6.0. For glazing, look at the U-value: modern double glazing around U 1.2 and triple around U 0.7.
Insulate first or get a heat pump first?
Insulate first. A heat pump runs at low temperature and needs a well-insulated home to work comfortably and cheaply. Shell first, system second.
Want to know exactly where the heat is escaping from your house, and what order to tackle it in without paying through the nose? In our Energie Verpulveraar we show you step by step how to insulate smartly, ventilate properly and save tens of thousands on energy. Take a look here: https://energie-verpulveraar.fredsbouwtekeningen.nl/