Roof insulation: insulating a pitched roof from inside or outside, or the attic floor, and who does it

Warm air rises, and in a home with an uninsulated roof a large part of the heat escapes outside via the attic. Roof insulation is therefore one of the measures that make the most difference to comfort and the energy bill. But there is not just one way to insulate a roof. This article is about the pitched roof of an existing home: you will read which three routes there are, how to prevent damp problems, which materials are available, what the terms Rc and Rd mean, how to find out whether your roof is already insulated, what applies to asbestos, subsidy, VAT and the permit, and who does the work: an insulation company, a carpenter or a roofer. If you have a flat roof, read Replacing a flat roof: bitumen, EPDM or PVC; according to Milieu Centraal (the Dutch independent information centre on energy and the environment), a flat roof is preferably insulated on the outside and interior insulation is not recommended there because of the risk of damp and mould.

Is your roof already insulated?

Start by looking at what is already there. The year of construction gives a first indication. According to Milieu Centraal, homes from before 1975 were usually not insulated when built, and insulation that occupants added later is often thin. Homes from the period 1975 to 1992 usually have a modest layer of a few centimetres, and homes from 1992 onwards a reasonable to good layer that can often still be improved.

You only get certainty by looking. In the attic you can often already see whether there is insulation against the roof boarding, or whether you are looking directly at timber parts or board material. If the roof is already finished, look behind a knee wall or hatch, or have a tradesperson look under a roof tile. Also pay attention to the condition of the roof: damp patches, daylight between the tiles or a weathered underlay must be resolved first. Insulating against a leaking roof only hides the problem.

If there is already a thin layer, you can top it up or replace it. Milieu Centraal considers improving it worthwhile if the existing layer is thinner than about 8 centimetres. There are two ways: add an extra layer, or remove the old layer and replace it with a thicker one. According to Milieu Centraal, which way is possible depends mainly on the space on the inside of the roof. If there is enough space, an extra layer goes in with a climate membrane, because the existing layer is usually already vapour-retarding; if there is little space, the old layer and the finish come off and are replaced by thin boards with a high insulation value. You can read more about the vapour control layer further on.

Three routes for a pitched roof

1. From the inside: against or between the purlins

This is the most commonly used approach for an existing home. The insulation goes on the inside of the roof, without anything having to be done to the tiles. In many Dutch homes the roof boarding rests on purlins, the horizontal beams between the walls; on other roofs sloping trusses or rafters support the roof. There are roughly two methods:

• Insulation boards against the roof boarding. Rigid boards, such as PIR or PUR, are cut to size and fixed between or against the beams. The joints are filled or taped. Thinner boards with a high insulation value limit the loss of space.
• Blankets or soft boards within a battened frame. Glass wool, rock wool or wood fibre is clamped between the beams or between an extra timber battened frame. A vapour control membrane then goes on the inside.

Then comes the finish: plasterboard, gypsum fibreboard, board material or timber cladding, and possibly plastering or painting. According to Milieu Centraal, insulating from the inside is suitable to do yourself or have a tradesperson do. The disadvantage is that you lose some height and space in the attic, and that interior insulation is no solution for a roof with a poor underlay or rotten tile battens.

2. From the outside: if the tiles are coming off anyway

If the tiled roof needs renovating anyway, the insulation can go on the outside on the roof boarding, with new counter battens, tile battens and the tiles on top. According to Milieu Centraal this gives very good insulation and it is always work for a tradesperson. The attic stays the same size and the insulation layer runs continuously. The roof does become higher, so the connection with the gutter and possibly the neighbours has to be redone. How that works, when a renovation is necessary and what then applies is explained in Renovating a tiled roof: replacing roof tiles and insulating.

Milieu Centraal also mentions a variant in which a retrofit insulation company sprays or blows insulation under the tiles, such as PUR foam, glass wool, rock wool or EPS beads. With this method, ask for a clear explanation of how the company deals with the ventilation under the tiles and the vapour permeability of the roof.

3. Insulating the attic floor

If you do not use the attic as living space, you do not need to insulate the entire roof. You can also insulate the attic floor, so the heat stays downstairs and the attic becomes a cold space. According to Milieu Centraal this is suitable if the attic is unheated, is used only for storage and can be closed off with a hatch or door. There are walkable insulation boards, or you lay insulation between the floor joists with planks or floor boards on top. Here too a vapour control layer goes on the warm side, i.e. under the insulation. Insulate the hatch as well and seal the gaps around it.

If you might want to use the attic as a bedroom or workspace later, rather insulate the roof. And if the attic is open to the floor below, without a hatch or door, then according to Milieu Centraal attic floor insulation makes no sense.

Damp: why the vapour control layer and ventilation are so important

The air in the house contains water vapour from breathing, cooking and showering. That vapour wants to escape outside through the structure. If warm, moist air ends up behind the insulation against the cold roof boarding, it can condense there. The timber then gets wet, and in the long run mould and wood rot can develop, without you seeing it from the inside.

The principle is therefore: a vapour control layer on the warm side, the inside, and a vapour-permeable but water-resistant layer on the cold side, under the tiles. Any moisture that does get into the structure can then escape outside. If the timber is enclosed between two vapour-retarding layers, moisture has difficulty getting out and the timber can become too damp and start to rot. What that means in practice:

• Know what is under the tiles. If there is no membrane and the roof consists only of tiles, tile battens and timber roof boarding, that roof is vapour-permeable and interior insulation requires a vapour control membrane on the inside. If there is already a vapour-retarding or vapour-tight membrane under the tiles, Milieu Centraal recommends a climate membrane, which adapts to the humidity, so the timber does not end up between two impermeable layers.
• Rigid foam boards are, according to Milieu Centraal, already sufficiently vapour-retarding in themselves; do seal the joints properly. Above a bathroom an extra vapour control membrane is advisable.
• Work airtight. A gap or a sloppy connection at the wall, the ridge, a roof window or a pipe penetration lets moist air into the structure at precisely one spot. Overlap the membrane joints and seal them with tape intended for the purpose.
• No cavity between insulation layers. Anyone adding an extra layer sometimes leaves an air gap between the old and new insulation. Milieu Centraal advises against this, because it does not remove moisture adequately.
• Ventilation under the tiles. The space under the tiles must remain ventilated, so that moisture escaping outwards through the water-resistant layer can be carried away.

And then the ventilation indoors. An insulated roof makes the home more airtight, and the moisture you produce has to go somewhere. Milieu Centraal stresses that ventilation must continue day and night, also after insulating. So check whether vents are open and whether your mechanical ventilation is working properly; more about that in Mechanical ventilation and heat recovery: maintenance, replacement and who does it. If you see condensation or mould after insulating, Recognising damp and mould in the home helps you find the cause.

Which material do you choose?

According to Milieu Centraal, the differences in environmental impact between insulation materials are small. In practice you mainly choose on the basis of the space you have, how easy it is to work with and fire safety.

• Glass wool and rock wool. Available as blankets and boards and easy to clamp between beams. Mineral wool is non-combustible. It does need more thickness than rigid foam boards. When working with it, wear gloves, long clothing, safety glasses and a dust mask.
• PIR and PUR boards. Rigid foam boards that achieve the same insulation value with less thickness, which saves space in a cramped attic. They are plastics that can burn; Milieu Centraal points out that plastic insulation boards must be covered with fire-resistant materials, such as plasterboard. So never leave foam boards exposed without a finish.
• Wood fibre and other bio-based materials. Wood fibre, flax, cellulose or recycled textiles are plant-based alternatives. According to Milieu Centraal you need slightly more thickness of these than of glass wool. Fire behaviour differs per product and treatment; ask about the fire class and the correct finish.
• Reflective foils. Milieu Centraal also mentions heat-reflecting plastic foil with an aluminium coating; according to Milieu Centraal, about 14 centimetres of it is needed for an Rd value of 3.8, the same as with glass wool. Such a foil can also be vapour-retarding; ask how it fits into the build-up with the vapour control layer.

Whatever the material: ask about the fire class of the product and the finish, and preferably have the insulation finished with board material that forms a fire-resistant layer on the inside. It can get hot around a chimney or flue; have the tradesperson check what distance from combustible insulation material is required there and what the manufacturer of the flue system prescribes.

Rc and Rd: what do these values mean?

The insulation value is expressed as thermal resistance, in m²·K/W. The higher the number, the better the insulation. There are two terms that are used interchangeably:

• the Rd value is the thermal resistance of the insulation material itself, at a given thickness. It is stated on the packaging or in the product information;
• the Rc value is the thermal resistance of the entire construction: roof boarding, insulation, any air layers and the finish together. The former Bouwbesluit (Building Decree), now the Bbl, calculates with the Rc value.

Milieu Centraal recommends an Rd value of 3.8 or higher for a roof, preferably with a KOMO certificate (a Dutch product quality certificate). That is a bit more than the ISDE requires, but anyone who insulates properly now will not need to do it again later. According to Milieu Centraal, you achieve an Rd of 3.8 with about 14 centimetres of glass wool or rock wool, about 15 centimetres of plant-based material or about 9 centimetres of PIR or PUR board. Have the company state the thickness and Rd value of the material in the quote, and if possible the Rc value of the whole.

What does the Bbl require?

For insulation during alterations, the Besluit bouwwerken leefomgeving (Bbl, the Dutch Building Decree) applies. The starting point for alterations is the rechtens verkregen niveau (legally acquired level): the quality after the alteration may not be lower than before. For insulation this has been tightened. When renewing or replacing insulation layers, a roof must have a thermal resistance (Rc) of at least 2.1 m²·K/W, or the legally acquired level if that is higher. In addition, a general minimum of Rc 1.4 m²·K/W applies to alterations, or the legally acquired level if that is higher (article 5.20, first paragraph, Bbl). With the ISDE requirement and Milieu Centraal's advice, in practice you are well above these values.

The higher new-build value does not apply to ordinary roof insulation. It does apply to a major renovation: if more than 25 per cent of the total building envelope (roof, façades and floor together) is renewed integrally, including all layers. The new-build value then applies to the renewed parts, which for the roof of a home is Rc 6.3 m²·K/W. The new-build values also apply to a new or completely renewed dormer (article 5.20, third paragraph, Bbl); see Installing a dormer: permit-free or permit required? If you combine roof insulation with a dormer or attic conversion, then according to Milieu Centraal that is a good moment to insulate properly straight away.

Asbestos and protected animals

Asbestos. In homes from before 1994 there may be asbestos in the roof, for example in roof boarding or board material under the tiles, in ceiling or wall panels in the attic, or in fire-resistant boards and sealing cord around a chimney. Drilling, sawing or screwing into such material can release fibres. If in doubt, first have a survey carried out by a certified company before the insulation is fixed. If asbestos has to be removed, this must be reported in advance via the Omgevingsloket (the government's online permit portal) (article 7.10 Bbl). How that works is explained in Recognising asbestos when renovating.

Bats and birds. Protected animals may live under the tiles and behind the roof boarding. Milieu Centraal advises checking whether there are joints or openings of 7 millimetres or more in the roof boarding or at the connection to the façade, and calling in an ecologist if there are signs such as droppings. If you come across a bat or bird during the work, stop and contact an ecologist. Bats and the nests of birds such as the house sparrow and the common swift are protected (articles 11.37 and 11.46 Besluit activiteiten leefomgeving, the Dutch Environmental Activities Decree). If the work affects protected animals or their nests or roosts, an omgevingsvergunning (environmental permit) for a flora and fauna activity may be required, which you apply for via the Omgevingsloket. In a number of provinces and municipalities a scheme for nature-friendly insulation applies under which the work must be reported in advance. Ask the insulation company or the omgevingsdienst (regional environmental service) which approach applies in your municipality.

ISDE subsidy and VAT

For roof insulation and attic floor insulation, as an owner-occupier, i.e. an owner who lives in the home themselves, you may qualify for the Investeringssubsidie duurzame energie en energiebesparing (ISDE, the investment subsidy for sustainable energy and energy saving) from RVO (Netherlands Enterprise Agency). The conditions change regularly; check them on rvo.nl before you award the contract. At the time of writing, the conditions include the following:

• it concerns insulating the existing roof, or the floor of an unheated attic or loft; no heating facilities may be present in such an attic; insulation of a new dormer or extension does not qualify;
• the newly applied insulation material has an Rd value of at least 3.5 m²·K/W and is on RVO's meldcodelijst (list of approved products); insulation already in the roof does not count towards the required Rd value;
• the insulated surface area of roof and attic or loft floor together is at least 20 m²;
• a roof and attic floor above one another in the same space only earn a subsidy for one of the two;
• a construction or installation company carries out the work in full; insulating yourself does not qualify;
• the home was built before 1 January 2019, or the omgevingsvergunning was applied for before 1 July 2018;
• you apply for the subsidy after the work has been carried out, within 24 months.

If you carry out a second insulation measure within 24 months, such as cavity wall insulation or floor insulation, or combine insulation with a heat pump, solar water heater or connection to a heat network, then according to RVO the subsidy amount for the insulation doubles. A combination with ventilation alone does not double the amount. Ask the company for an invoice showing the date, the surface area, the Rd value, the type of material and the registration code. How the ISDE relates to other schemes is explained in Sustainable renovation: subsidies and options, and what roof insulation does for your label in Improving your energy label: which measures really count?

As for VAT: the labour costs of applying insulation material to roofs and floors of a home older than two years fall under the reduced rate of 9%; the insulation materials themselves fall under 21%. Work that does not fall under it, such as replacing tiles, demolition work or carpentry for a new roof extension, also remains 21%. The invoice must therefore split this. More about this in 9% or 21% VAT when renovating.

Permit, VvE and rented home

Insulating from the inside does not make the home larger. According to the Informatiepunt Leefomgeving (IPLO, the government information point for the living environment), for an ordinary home you do not need an omgevingsvergunning for this, as long as you do not change anything in the load-bearing structure; the work must, however, comply with the technical rules of the Bbl. For a rijksmonument (national listed building), an internal alteration is only permit-free if the part has no heritage value (article 13.11 Besluit activiteiten leefomgeving); for a municipal listed building or a home in a protected townscape or village view, the rules are set out in the omgevingsplan (local environmental plan). In those cases, consult the municipality beforehand. Insulating from the outside makes the roof thicker and therefore the home larger; according to IPLO, an omgevingsvergunning is often required for this because of the omgevingsplan. Do the permit check in the Omgevingsloket. What applies here is set out in the guide on the tiled roof and in When do you need a permit?

If you live in an apartment under the roof, the roof generally belongs to the communal parts and the vereniging van eigenaren (VvE, owners' association) decides on insulation on the outside; see Renovating a VvE apartment. If you rent, insulating is as a rule a matter for the landlord. If you want to have insulation installed yourself, first ask for written permission.

Who does the work?

• An insulation company insulates the roof from the inside or the attic floor, often including the vapour control layer and finish, and knows the ISDE conditions and registration codes.
• A carpenter is the logical choice if carpentry is involved too: a battened frame on the purlins, knee walls, a new attic hatch, a walkable attic floor or the finish with boards or cladding. Many carpenters insulate the roof from the inside themselves.
• A roofer is needed for insulation from the outside, for repairing the underlay, tile battens and lead flashing, and to fix leaks first.
• A contractor can coordinate the whole job if roof insulation is part of a larger renovation, such as an attic conversion with a dormer.

Milieu Centraal recommends requesting quotes from at least two companies, preferably after a home visit, and asking whether the company is certified or a member of a trade association. Check that the quote states: the chosen route, the surface area, the material with thickness and Rd value, the vapour control layer and how the joints and connections will be sealed, the finish, what happens if asbestos, damp or wood rot is found, who clears and cleans the attic, and an invoice that splits insulation labour and materials for VAT and the ISDE. How to compare quotes is explained in Comparing quotes: how to read a building quote.

In short: if you use the attic, insulate the roof, from the inside or from the outside if the tiles are coming off anyway. If the attic is cold and unused, the attic floor is often the simplest choice. Always pay attention to the vapour control layer on the warm side and to good ventilation. On VerbouwPro you post your job free of charge and carpenters, roofers and insulation companies from your region respond.