The short answer: you insulate a pitched roof from the inside by wedging insulation tightly between the rafters (the sloping roof timbers), adding a second layer underneath the rafters where needed to deal with cold bridges, and sealing the whole thing airtight on the inside with a vapour control layer. That membrane always sits on the warm side, so the room side. Then you finish with plasterboard or OSB. The biggest pitfall isn’t the insulation itself, it’s the vapour control layer and the ventilation of your roof. Get that wrong and you’ll get condensation and rot in the structure.
In this article I’ll walk you through the lot: when working from the inside makes sense, which material to pick, how thick it needs to be, how to fit that membrane faultlessly, and what it costs. So that after this piece you can get on with it yourself, or make a deliberate choice to bring in a professional.
From the inside or from the outside: which one?
There are two places you can insulate a pitched roof.
From the outside means the tiles and the roof deck come off, insulation goes on top of the rafters (over-rafter boards, for instance), and everything goes back on. Your loft stays fully usable and you keep the beams on show. But it’s a big job, expensive, and genuinely one for a roofer. It mainly pays off if you’re replacing your roof covering anyway.
From the inside you do without a single tile leaving the roof. You work from the loft, between and underneath the rafters. This is the cheapest route and the only one a handy DIYer realistically takes on themselves.
The honest downsides of working from the inside:
- You lose space. A build-up of 14 to 18 cm plus the finish comes inwards. In a cramped loft that adds up.
- Cold bridges at the rafters. Timber insulates worse than your insulation material. With insulation only between the rafters, those beams stay relatively cold. A second layer underneath the rafters sorts a good part of that.
- Damp risk. You’re building insulation against a cold roof. Without a proper vapour control layer and ventilation, warm, moist indoor air travels into the structure and settles there as condensation. This is the number one reason roofs insulated from the inside sometimes go wrong.
In short: go from the inside if your roof is still sound, you want to save money and do the work yourself, and you can spare the space. Is your roof open anyway, or is the covering due for replacement? Then insulating from the outside is worth considering.
Would you rather insulate the floor of an unused loft, or your ceiling? Those are different jobs with their own rules. This article is purely about the sloping roof itself, from the inside.
Check your roof build-up first (it decides everything)
Before you buy any material, you need to know how your roof is put together. Get up into the loft and look between the rafters at the roof deck.
Is there a sarking layer or a breathable membrane? On newer roofs there’s a water-resisting but vapour-permeable membrane or board underneath the tiles. Water vapour can get out through it, rain can’t get in. In that case you’re allowed to take the insulation right up against that layer.
Do you only see the underside of the tiles, or solid timber or bitumen boarding? Then your roof isn’t vapour-permeable. Moisture can’t pass through it to the outside. Now you need a ventilated air gap between the insulation and the deck, so moisture can be ventilated away (more on that further down).
Unsure about the build-up, or do you see damp patches, surface condensation or rot? Sort that out before you close anything up. Insulating over a problem only makes it worse, and hides it.
Which R-value and which thickness?
You express insulating performance as the R-value: the higher, the better it insulates. Dutch consumer organisation Milieu Centraal advises an R-value of at least 3.8 m²K/W for insulating a pitched roof from the inside. For the Dutch ISDE grant, a minimum R-value of 3.5 m²K/W applies to the insulation material. (Source: Milieu Centraal, RVO.)
How many centimetres that works out to depends on the material and its lambda value (the thermal conductivity, lower is better). To reach R 3.8 you need roughly the following, according to Milieu Centraal:
| Material | Lambda (indicative) | Thickness for R 3.8 | Fire behaviour | Workability |
|---|---|---|---|---|
| Glass wool / rock wool (mineral wool) | 0.032 – 0.040 | ± 14 cm | Non-combustible (rock wool best) | Easy, wedges nicely between rafters |
| EPS boards (expanded polystyrene) | ± 0.035 | ± 14 cm | Combustible | Light, cuts well |
| PIR / PUR boards | 0.022 – 0.028 | ± 9 cm | Combustible, chars | Thin build-up, more cutting to fit |
| Phenolic foam (resol) | ± 0.021 | ± 8 cm | Combustible | Thinnest build-up, pricier |
| Wood fibre / hemp (biobased) | 0.038 – 0.045 | ± 15 – 17 cm | Combustible, but good against summer heat | Heavier, pleasant to work with, strong summer performance |
Thicknesses for R 3.8 based on Milieu Centraal; lambda values vary by product, so always check the data sheet for your specific insulation.
A few practical trade-offs:
- Mineral wool is the classic choice for between the rafters: cheap, springy, wedges itself in place. Rock wool scores best on fire and sound.
- PIR/PUR and phenolic hit the same value with far less thickness. Ideal when space is precious. The downside: rigid boards you have to cut exactly to size, and every gap is a cold bridge.
- Biobased (wood fibre, hemp) performs noticeably well against summer heat in a loft and is more vapour-open, but you need more thickness and more budget. In the Netherlands the ISDE grant also pays a bonus on top for it (see the grant section).
Tight on space? Then combine mineral wool between the rafters with a thin PIR board underneath, for example. That gets you a high value without building your loft in completely.
The vapour control layer: the bit people forget
If you take one thing away from this article, make it this. When insulating from the inside, the vapour control layer matters more than the insulation itself.
Why do you need that membrane? Warm indoor air is full of water vapour (cooking, showering, plants, breathing). Warm air wants to travel to the cold side of your roof. On the way it cools down, and at the cold point that vapour settles out as condensation. That happens right in the middle of your insulation and against your timber rafters. The result: wet insulation that no longer insulates, mould, and rot in the long run.
A vapour control layer on the inside stops that moist air before it can get into the structure.
The three iron rules:
- Always on the warm side. So on the room side, right behind your plasterboard, over the insulation. Never on the cold (roof) side. The wrong way round means you’re trapping the moisture instead.
- Seal it airtight. A vapour control layer with holes and gaps is as good as useless. Overlap the sheets by at least 10 cm and tape every seam with proper vapour control tape. Tape or seal tightly around roof windows, wall junctions, beams and service penetrations too.
- Puncture it as little as possible. Every screw or nail through the membrane is a small leak. Plan your finish so the membrane stays as intact as you can manage.
Mind the difference between a vapour control layer and a vapour barrier. A true vapour barrier is (near enough) vapour-tight; a vapour control layer lets a small amount of vapour through and is the safest choice for most homes. Not sure about a particular roof build-up? Then go for an intelligent membrane with variable vapour resistance, which adapts to the season.
Exception: if you have the roof sprayed with PUR foam, that material is already vapour-tight in itself and the separate membrane isn’t needed. That one really is specialist work though.
Ventilating the roof: let it breathe
On a roof that is not vapour-permeable (solid boarding, or just tiles on battens), any moisture that does get into the structure has to be able to go somewhere. That’s why you keep a ventilated air gap clear between the insulation and the roof deck.
- Keep at least about 2 to 4 cm clear between the top of the insulation and the deck.
- That gap has to be able to draw air in at the bottom (at the eaves) and let it out at the top (at the ridge). That way dry outside air moves through it continuously.
- So do not stuff that gap full. The classic mistake is pushing the insulation all the way up against the tiles “for that little bit extra”. That blocks the ventilation and creates a moisture trap.
Do you have a breathable sarking membrane after all? Then the insulation can go straight against it and you don’t need this gap, because the moisture can escape through the vapour-permeable layer.
Not certain which type you’ve got? Err on the safe side and keep a ventilated gap.
Step by step: insulating a pitched roof from the inside
Here’s the approach, one step at a time. Set aside a few weekends and work neatly; with this kind of job the quality is all in the details.
- Measure up and settle on your build-up. Measure the centre-to-centre spacing between the rafters and their depth. Choose your material and thickness based on your target R-value (at least 3.8) and the space you have.
- Fix damp and defects first. Check for leaks, rot and mould. Repair those before anything else. Never insulate over a wet or rotten roof.
- Sort the air gap (if needed). No breathable sarking membrane? Then make sure a clear, ventilated gap of 2 to 4 cm stays behind your insulation. Use spacer battens or counter battens if you need to.
- Cut the insulation to size. Cut mineral wool about 1 cm wider than the space between the rafters, so it wedges in tightly without folding. Boards, on the other hand, you cut to an exact fit and seal the joints. Too tight leaves cold bridges, too much stuffing squeezes the insulating value out of it.
- Wedge the insulation between the rafters. Fill the full depth neatly, with no voids. Work from the top down and make sure the runs butt up properly against each other.
- Add a second layer underneath the rafters (recommended). Screw battens or metal profiles across underneath the rafters and fit an extra layer of insulation into them. This covers the cold timber rafters, lifts your total insulating value considerably, and gives you space to run cables and pipes in front of the vapour control layer.
- Fit the vapour control layer airtight. Stretch the membrane tightly over the whole lot on the warm side. Overlap the sheets by at least 10 cm and tape every seam and junction. This is the most important step, so take your time over it.
- Finish with plasterboard or OSB. Screw the boards to the battens, ideally without puncturing the membrane unnecessarily. Seal the edges and junctions at walls, floor and roof windows airtight.
- Check every junction. Go round the roof windows, the ridge, the eaves and the gable walls once more. That’s exactly where the gaps appear that let warm air out and moisture in.
Common mistakes (and how to avoid them)
- Leaving out the vapour control layer, or fitting it on the wrong side. The most expensive mistake there is. Without a membrane, or with it on the cold side, you get condensation in your roof. Always on the warm (inside) face.
- Not sealing seams and gaps airtight. A vapour control layer that isn’t taped up doesn’t work. Gaps at walls and roof windows are leaks too.
- Blocking the air gap. Pushing insulation up against non-breathable boarding blocks the ventilation and brings damp. Keep the gap clear.
- Insulation cut too tight or too loose. Too tight leaves voids (cold bridges); pressing it in too hard ruins the insulating value. Cut mineral wool slightly oversize so it wedges, and boards to an exact fit.
- Ignoring the cold bridges at the rafters. Insulating only between the rafters leaves the timber beams cold. A second layer underneath fixes that.
- Insulating without ventilating the house. The tighter your roof, the less your home ventilates on its own. Keep trickle vents open, otherwise you move the damp problem into your living space.
What does it cost and what does it save?
The figures below come from Milieu Centraal, for an average end-of-terrace house where an uninsulated pitched roof is insulated from the inside. Treat them as ballpark numbers; your roof, your area and your choice of material decide the real cost.
| Item | Indication (end-of-terrace, Milieu Centraal) |
|---|---|
| Cost through a contractor | ± €6,500 |
| ISDE grant (via a contractor) | ± €900 |
| Net cost through a contractor | ± €5,600 |
| Doing it yourself (materials only) | ± €1,700 |
| Saving per year | ± 340 m³ of gas, ± €480 |
Run the numbers on the DIY scenario: around €1,700 in materials against roughly €480 saved a year gives a payback time of broadly 3 to 4 years. That’s quick, and it’s the big trump card of insulating your roof yourself. Have a contractor do it and, after the grant, you’re looking at more like 10 to 12 years, but you do get a guarantee and the grant money.
Per square metre of material you’re often around €10 to €25 per m² for mineral wool, and higher for PIR/PUR boards, depending on thickness and brand. Milieu Centraal compares the return on roof insulation to about 5 percent interest on a savings account, and that’s before you count the comfort (a cooler loft in summer, a warmer one in winter).
Grants: the ISDE for roof insulation
In the Netherlands you can get a national grant for roof insulation through the ISDE, administered by the RVO. The main conditions at the moment:
- A minimum R-value of 3.5 m²K/W for the insulation material fitted.
- At least 20 m² of roof area insulated.
- The material is on the RVO’s registered product list (the meldcode list).
- You are both the owner and the occupier of the home.
Pay close attention to this point: you’re not allowed to fit it yourself. The ISDE requires the insulation to be carried out entirely by a building or installation company. DIY therefore doesn’t qualify for this grant. (Source: RVO, Milieu Centraal.)
That makes the trade-off black and white, frankly: do it yourself and it’s far cheaper but without a grant, or go through a contractor and get the grant but pay more. On top of that, if you combine two insulation or ventilation measures within a set period, the grant per m² is higher than for a single measure on its own. Choose biobased insulation (wood fibre or hemp, say) and there’s an extra bonus per m² on top.
The exact amounts per m² change regularly. Always check the current conditions and rates with the RVO (rvo.nl) before you apply, and see whether your local council offers an additional grant or an energy saving loan. Live outside the Netherlands? Then your own national scheme applies, so check its current terms before you order materials or book a contractor.
When are you better off calling in a professional?
Doing it yourself is fine for a handy DIYer with a sound, easily accessible loft. Call in a professional in these cases:
- A steep, high or hard-to-reach roof where working safely is difficult.
- Old roof boarding suspected of containing asbestos. Boarding and roof sheets from before 1994 can contain asbestos. Never cut or drill it yourself; have it tested.
- Doubts about the roof build-up, damp or rot. Get visible defects assessed first.
- If you want the ISDE grant. That requires the work to be done by an approved contractor.
- Planning permission or listed status. Special buildings can come with their own rules; check with your local authority.
Frequently asked questions
How do you insulate a pitched roof from the inside?
You wedge insulation tightly between the rafters, ideally add a second layer underneath the rafters to deal with cold bridges, and seal the whole thing airtight on the warm (inside) face with a vapour control layer. Then you finish with plasterboard or OSB. On a roof that isn’t vapour-permeable, make sure there’s a ventilated air gap behind the insulation.
What R-value should roof insulation have?
Milieu Centraal advises at least R 3.8 m²K/W for a pitched roof insulated from the inside. For the ISDE grant the minimum is R 3.5. That works out to roughly 14 cm of mineral wool or 8 to 9 cm of PIR/PUR/phenolic.
Which side does the vapour control layer go on?
Always the warm side, so the inside or room face, over the insulation and behind the finishing board. On the cold (roof) side you trap the moisture instead and you’ll get condensation.
Do I need to keep an air gap behind the insulation?
On a roof without a breathable sarking membrane, yes: keep 2 to 4 cm of ventilated space clear, able to draw air in at the bottom and let it out at the top. If you do have a breathable membrane, the insulation can go straight against it.
What does it cost to insulate a roof from the inside yourself?
For an average end-of-terrace house, Milieu Centraal puts the materials at around €1,700 if you do it yourself. Have a contractor carry it out and it’s about €6,500, on which you can claim roughly €900 through the ISDE.
Do I get a grant if I insulate the roof myself?
No. The ISDE grant for roof insulation only applies if an approved building or installation company does the work. Fitting it yourself doesn’t qualify. DIY is cheaper, but without the grant.
How much will roof insulation save me?
According to Milieu Centraal, insulating a pitched roof in an end-of-terrace house saves around 340 m³ of gas and about €480 a year. Alongside the saving you gain plenty of comfort: less heat in the loft in summer and a warmer space in winter.
Want to tackle not just your roof but your whole house, in the right order, so you save as much as possible without spending money you didn’t need to? Insulation, ventilation and building all hang together, and the biggest gains come from planning it properly from the start. Have a quick look at Fred’s DIY Plans. Explained plainly, do it yourself, exactly the way Fred would go about it.