Floor insulation and crawl space ground insulation: is it possible, which material and who does it

Walking on a cold floor costs you not only comfort but also heat. If your home has a crawl space under the ground floor, that floor can often be insulated from below, without having to break open the floor indoors. According to Milieu Centraal (the Dutch independent information centre on energy and the environment), floor insulation also means less moisture rising up from the crawl space. There are two methods: insulating the underside of the floor, or insulating the ground of the crawl space. This article explains when each method is suitable, which materials are available and where things can go wrong. Where floor insulation ranks among the other insulation measures is explained in Improving your energy label: which measures really count.

Can your floor be insulated from below?

Three things determine whether insulating from the crawl space is possible: the height, the access and the moisture.

• The height. Anyone fitting insulation under the floor needs to be able to work there. Milieu Centraal's rule of thumb is that the crawl space must be at least 35 centimetres high, measured to the underside of the joists. Insulation companies apply different requirements, so it pays to have several companies take a look. Measure it yourself, at the hatch and in a few other places, from the ground to the underside of a joist or of the concrete floor, because the ground is by no means always level.
• The access. There must be a crawl space hatch through which you, or the tradesperson, can get under the floor, and the hatch must be large enough. If the hatch is under a fixed floor finish or a cupboard, it first has to be made accessible.
• Dry or wet. A crawl space is naturally damp, but there should be no standing water in it. If water regularly stands there, the cause must first be found, such as a leaking pipe, a broken drain or a high water table. For crawl spaces where water sometimes stands, Milieu Centraal mentions materials that can float, such as a special bubble-wrap foil or a layer of polystyrene chips (EPS).

If the crawl space is too low or there isn't one, Milieu Centraal says there are other options, such as filling the crawl space with insulation material or adding insulation on top of the floor.

Floor insulation or ground insulation: the difference

With floor insulation, the insulation material is fitted directly against the underside of the floor, between or under the joists with a timber floor, or against the underside of a concrete floor. The heat then stays in the home, and the crawl space becomes a cold space under an insulated floor. Floor insulation as a rule has the greatest effect, but requires someone to be able to work in the crawl space.

With ground insulation, the material is laid on the ground of the crawl space. This insulates the crawl space itself from the cold, damp soil, so the air under the floor cools down less and becomes less humid. Ground insulation can often be applied from the hatch, which makes it suitable for low crawl spaces. Its effect on the warmth of the floor is as a rule smaller than with insulation directly against the floor, because the ventilated air between the ground and the floor is still cool.

Sometimes the two are combined. In that case, watch out for the subsidy: according to RVO (the Netherlands Enterprise Agency), if ground and floor insulation are fitted directly above each other in the same crawl space, you only get a subsidy for one of the two.

For insulating the underside of the floor, Milieu Centraal recommends an insulation value (Rd) of at least 3.5, and preferably between 3.5 and 5; with underfloor heating, Milieu Centraal recommends an Rd of at least 5. How thick the material needs to be for that varies per material; ask the company to state the Rd value on the quote.

Materials for the underside of the floor

For insulation against the underside of the floor, Milieu Centraal mentions among others thermal insulation cushions, boards of EPS (polystyrene), cork, wood or flax, foam and glass or stone wool. The main types in practice:

• Insulation boards. Boards fixed between or against the joists or against the concrete floor. They are easy to inspect and remove again, but need to be cut and fitted accurately. Gaps between the boards and open spots around pipes reduce the effect. Milieu Centraal stresses that the material must fit closely, without gaps and without skipping any sections.
• Sprayed foam. PUR or PIR foam that is sprayed against the underside of the floor and cures there. It fits closely to an irregular surface and around pipes. The drawbacks are that it bonds firmly to the floor, making later repairs to joists or pipes more difficult, and that according to Milieu Centraal sprayed material is harder to recycle; Milieu Centraal calls PUR foam a less good choice. For an ISDE subsidy, according to RVO, sprayed PIR or PUR must be made with an HFC-free blowing agent.
• Thermal cushions and foil. Thermal cushions are pads made of several layers of foil with air chambers, which are stapled to the joists and taped together. They are light and quick to fit. With reflective foils, don't go only by the promised insulation value: ask which Rd value the product achieves in the build-up in which it is used, and whether it is on RVO's reporting code list (meldcodelijst).

Materials for the ground of the crawl space

On the ground of the crawl space, mainly loose materials and blankets are used:

• Shells. A layer of shells blown into the crawl space through the hatch via hoses or pipes. Shells let air through and are often used in damp crawl spaces. A noticeable insulation value requires a substantial layer thickness, which in turn takes up space in a low crawl space.
• EPS beads or chips. Polystyrene beads blown onto the ground. They hardly absorb any water and can float if there is water in the crawl space.
• Insulation blanket or ground membrane. A blanket or membrane rolled out over the ground, sometimes combined with an insulation layer. A membrane on the ground mainly slows down moisture from the soil. Milieu Centraal recommends using a vapour-retarding plastic membrane, preferably made of PE, because PVC can contain harmful substances. In theory an ordinary membrane can float too, but according to Milieu Centraal water often stays lying on top of it in practice.

For ISDE, the material for ground insulation must also achieve an insulation value of at least 3.5 and be on the reporting code list; see the subsidy conditions below.

Ventilation and moisture: keep the crawl space healthy

The most important rule for any form of crawl space insulation: the ventilation openings in the foundation walls must stay open. Without fresh air, Milieu Centraal says radon, moisture, mould and wood rot can develop. After the work, check that all vents are clear and not covered with insulation material, membrane or beads.

After floor insulation the crawl space becomes colder, because less heat leaks down from the home. That is the intention, but it has consequences. The air in the crawl space can become more humid, and water pipes that run through the crawl space can freeze more quickly in severe frost. So have the water pipes in the crawl space insulated, and ask the company to include the crawl space and the hatch in the plan. Milieu Centraal recommends also insulating the crawl space hatch itself and filling the gaps around pipes passing through the floor.

With a timber floor, moisture deserves extra attention. Joists and boards in a damp, poorly ventilated crawl space can start to rot, and in such conditions dry rot (echte huiszwam) can also occur. With an old timber floor, have the joist ends and the underside of the floor checked in advance. Report a springy floor, a musty smell or damp patches to the insulation company beforehand; Milieu Centraal advises reporting floor problems in advance. Rotten parts must first be repaired by a carpenter. How wood rot develops and how it is distinguished from dry rot is explained in Repairing wood rot in window frames and exterior woodwork.

Pipes, the gas pipe and asbestos in the crawl space

Water pipes, the sewer pipe, electricity cables and the gas pipe often run through the crawl space. The insulation company works around them and must prevent pipes from being damaged, put under strain or made inaccessible. The gas pipe needs extra care: it must not be damaged, and joints must remain accessible for inspection. If you smell gas, don't go looking yourself and call the grid operator. If a pipe is outdated or leaking, have it repaired or rerouted before insulating, because afterwards the pipe is harder to reach.

Homes built before 1994 may contain asbestos, including in the crawl space. Think of remnants of asbestos cement sheets left behind during construction, asbestos-containing panels or sheets on or around the crawl space hatch, and old insulation around pipes. You cannot tell with certainty whether something contains asbestos. If in doubt, don't touch it and have it tested before the insulation company starts; a good company will ask about this itself too. How to recognise asbestos and which steps then apply is explained in Recognising asbestos when renovating.

How the work is done

The process differs per method, but roughly looks like this:

1. Inspection. The company measures the height, looks at the access, the moisture, the condition of the floor and the joists, the pipes and the ventilation, and looks out for suspect materials. Milieu Centraal advises inviting at least two companies for an advisory visit at your home.
2. Preparation. The hatch is cleared, rubble and debris are removed from the crawl space if necessary, and if there is water, the cause is tackled first.
3. Installation. With floor insulation, fitters work in the crawl space and fix the boards or cushions, or spray the foam. With ground insulation, the material is usually blown in or rolled out from the hatch. For low crawl spaces, some companies use a remote-controlled robot that sprays insulation against the underside of the floor; in that case ask how the company checks the quality, for example with camera footage.
4. Finishing and inspection. The hatch is insulated, the ventilation openings are checked and the gaps around pipes are sealed. Ask for photos of the work; you will also need them for the subsidy application.

ISDE subsidy and VAT

Floor and ground insulation fall under the Investeringssubsidie duurzame energie en energiebesparing (ISDE, the investment subsidy for sustainable energy and energy saving) from RVO. The conditions change regularly; check them on rvo.nl before you commission the work. At the time of writing, the conditions include the following:

• the ground and floor insulation together cover at least 20 square metres
• the material has an insulation value of at least 3.5 and is on RVO's reporting code list
• a construction or installation company carries out the work; insulating it yourself does not qualify
• the scheme is intended for owner-occupiers: you own the home and live in it yourself
• the home was built before 1 January 2019, or the omgevingsvergunning (environmental permit) was applied for before 1 July 2018
• you apply for the subsidy after the work has been carried out, within 24 months
• you only get a subsidy once each for floor insulation and for ground insulation
• owner-occupied homes in the province of Groningen and a few municipalities in Noord-Drenthe are not eligible for the ISDE insulation subsidy; separate schemes apply there

If you combine floor insulation within 24 months with another insulation measure or a heat pump, this can increase the subsidy for the insulation. Ask the company for an invoice showing the date, the surface area, the Rd value, the type of material and the reporting code. How the ISDE fits together with municipal schemes and the Energiebespaarlening (energy-saving loan) is explained in Sustainable renovation: subsidies and options.

As for VAT: the labour costs for fitting insulation material to the floors of a home more than two years old fall under the reduced rate of 9%; the insulation materials themselves fall under 21%. The invoice must therefore split the two. The Belastingdienst (Tax and Customs Administration) does not explicitly say whether the low rate also applies to insulation on the ground of the crawl space; ask the company how it invoices this. More on this in 9% or 21% VAT on renovations.

Who does it, and what to look for in the company?

Floor and ground insulation is usually carried out by a specialist insulation company. Repairs to a timber floor or to joists beforehand are work for a carpenter. If you want to combine insulation with other work, such as a new floor, cavity wall insulation or a renovation of the ground floor, a contractor can coordinate the work and subcontract the insulation to a specialist.

When choosing a company, look for experience with your type of floor and crawl space. There are KOMO certificates (Dutch quality certification) for retrofitting floor insulation, issued among others through SKG-IKOB; ask the company whether it is certified and to which guideline. For spraying PU foam against the underside of the ground floor, there is also a separate process certificate based on BRL 2131 for thermal retrofit insulation with PU spray foam. Also ask about the warranty, references from similar homes and whether the company supplies photos of the work. Many points to watch are the same as for cavity wall insulation.

VvE, rented homes and listed buildings

If you live in a ground-floor flat or an apartment, the floor and the crawl space are often shared parts, and the vereniging van eigenaren (VvE, owners' association) decides on insulation. Exactly what is shared is set out in the splitsingsakte (deed of division). Moreover, the crawl space often continues under several homes, so in practice the work is done for the whole building at once. Put the proposal forward at the meeting. More about decision-making is explained in Renovating in a VvE flat.

If you rent, insulation is as a rule a matter for the landlord. If you want to have the floor insulated yourself, first ask for permission in writing; this is as a rule required for changes that you cannot easily undo at the end of the tenancy (Article 7:215 of the Burgerlijk Wetboek, BW, the Dutch Civil Code). If the landlord has the floor insulated as an improvement, they may propose a reasonable rent increase (Article 7:255 BW); that does not apply to repairing a defect. Make sure you are properly informed about this in advance.

For an ordinary home, floor insulation is usually permit-free. For a rijksmonument (national listed building) that is different, partly because the interior and the historic floor structure may be protected. There, the only permit-free work is necessary regular maintenance aimed at preserving the heritage values, as long as detailing, profiling, design, type of material and colour do not change, and an internal alteration to a part of the monument that has no heritage value (Article 13.11 of the Besluit activiteiten leefomgeving, the Dutch Environmental Activities Decree). Insulating a historic floor or joist structure as a rule does not fall under this and therefore usually requires an omgevingsvergunning; the municipality determines whether a floor has heritage value. For municipal listed buildings, the rules are set out in the omgevingsplan (environment plan) or the heritage by-law. So always consult the municipality in advance. What is generally permit-free is explained in When do you need a permit.

What belongs in the quote?

Request two or three quotes based on an on-site inspection, and check that they contain the same elements:

• the outcome of the inspection: height, access, moisture, condition of floor and joists, pipes and any suspect materials;
• the method: floor insulation, ground insulation or a combination, and why;
• the surface area in square metres, and which parts will not be insulated;
• the material with brand and type, the thickness, the reporting code and the minimum insulation value afterwards, as Milieu Centraal recommends;
• how the ventilation openings are kept clear and whether the hatch is insulated;
• insulating the water pipes in the crawl space and sealing gaps around pipes;
• any repairs beforehand, such as rotten joists or a leaking pipe;
• the certification, the warranty and photos of the work;
• an invoice with the details you need for the ISDE, and the split between labour and materials for VAT purposes.

How to compare quotes is explained in Comparing quotes: how to read a building quote.

In short: measure the height of the crawl space, check whether there is standing water, have suspect materials tested and then choose between floor and ground insulation. Keep the ventilation open, insulate the water pipes and keep the invoice and the photos. On VerbouwPro you can post your job free of charge, and contractors and carpenters from your area will respond.