Have you chosen to undertake work to insulate your roof and attic? This is a great decision if you want to reduce your energy bill while improving your comfort.
ADEME (Agency for Energy Transition) recommends starting insulation work from the top of your house: this is where 25 to 30% of the energy escapes.
But given the great diversity of insulating materials, which is best suited to your project? I give you all the information on natural, mineral, and synthetic insulation materials.
The essential
Very economical, mineral wool has the advantage of resisting fire, but not humidity, which reduces its thermal performance.
Natural materials (animal or plant) compensate for their lower thermal performance with excellent acoustic insulation.
Synthetic materials (extruded or expanded polystyrene or polyurethane) offer good thermal performance which should not overshadow their flammability.
It is advisable to request several quotes to compare prices and make the choice that best suits your roof insulation project.
Reminder of roof insulation techniques
The first thing to know is that it is possible to insulate the roof from the inside but also from the outside. The choice of one or the other method depends on:
- The condition of the frame of your house
- The condition of the coverage
- The use you want to make of the attic
- The space available for the insulation
Why choose external roof insulation?
In this case, the roofing is completely removed from the frame. These works are large-scale, especially if the surface area is large. It should also be remembered that this very effective solution is the most expensive.
In other words, if your roof needs to be redone, you might as well take advantage of it to create efficient insulation.
The roofer, a professional in his sector, can offer you the sarking technique which consists of installing rigid insulation between the frame and the roof.
Read also: How to Insulate a House Against Noise?
Why prefer insulation from the inside of your roof?
First of all, this solution is much cheaper than exterior insulation because you do not change the roofing of your house. Implementation is also simpler and completion faster.
This insulation technique consists of applying a layer of insulation directly into the roof structure: between or under the rafters.
To improve the performance and resistance of your insulation, it is recommended to install a vapor barrier membrane. This may not have been anticipated when the framework was created. If it exists, it may need to be changed.
Before even talking about the choice of insulation, the question of the installation technique arises depending on the type of attic and its usefulness.
Are you planning to make a bedroom for your teenager? Or you can’t fit them out because the height is too low to make them habitable.
- The lost attic
What we call lost attics is a space that cannot be converted, often because the height under the frame is insufficient. The insulation is then done by spreading/blowing: the insulation is poured manually into flakes and distributed with a rake.
- The convertible attic
If your attic can be converted, you must then turn to insulation in rolls (the insulation is rolled out on the floor) or in panels (a layer of insulating panels is placed between the rafters). While taking care not to underestimate the thickness of the insulation for it to be effective.
Presentation of insulation performance indices
To help you choose the most suitable insulation for your home renovation project, you need to know some additional information. Indeed, there are different performance indices for roof insulation.
The R-value is the thermal resistance index
It measures the insulation’s ability to resist heat transfer. The higher the R-value, the better the insulation is at retaining heat inside the building. R-values are usually expressed in meters squared kelvin per watt (m²·K/W).
| Insulating | Thickness in cm | Thermal resistance in m² K/W |
|---|---|---|
| Glass wool | 10 | 2.5 to 3 |
| Rockwool | 10 | 2.3 to 2.9 |
| Vegetable wool (hemp) | 10 | 2.4 |
| Animal wool (sheep) | 10 | 2.5 |
| Extruded polystyrene | 10 | 2.8 |
| Polyurethan foam | 10 | 4.2 |
The U value is the thermal transmittance coefficient
It represents the amount of heat that passes through a material per unit of time and area, in watts per square meter per kelvin (W/m²·K). For insulation, a low U-value indicates good performance, because it means that the insulation limits heat transfer.
The lambda value (λ) is the thermal conductivity of a material, measured in watts per meter-kelvin (W/m·K).
The lower the lambda value, the less heat the material conducts and the better its insulating power. Here are some figures in W/mK that show the fairly similar qualities of all these types of insulation products:
- Glass wool: 0.030
- Rock wool: 0.032
- Duck feather wool: 0.035
- Wood wool: 0.036
- Hemp: 0.039
- Cellulose wadding: 0.039
- Cotton: 0.039
- Cork: 0.040
- Sheep wool: 0.040
Harmonized evaluation system used for insulation
This label certifies that the insulation complies with European quality and safety standards.
Different certifications and labels can be awarded to insulation to guarantee their quality and performance. Common certifications include:
- NF certification (French Standard).
- the ACERMI label (Association for the Certification of Insulating Materials).
- the CSTBat label (Scientific and Technical Building Center), etc.
Mineral wool for insulating your roof
Don’t know about mineral wool? But yes: these are the very popular glass wool or rock wool, the best-known insulators for roofs but also partitions.
Long considered as unecological insulators, they are now produced in an eco-responsible way.
The advantages of mineral wool
- They offer excellent value for money
- They are easily cut and placed in rolls
- They offer very good thermal insulation
- Rock wool offers better acoustic performance than glass wool.
The disadvantages of mineral wool
- They require height under the frame to be installed properly.
- When installing the insulation, glass wool is irritating to the hands and eyes: it is recommended that you wear gloves and glasses.
When to choose mineral wool?
If your roofing is in good condition and you have enough space under the frame (between 15 and 25 cm), glass or rock wool is particularly suitable. You can also choose them in the form of flakes to insulate non-convertible attics.
Forget mineral wool, if…
…you have little space under the frame: this type of wool requires a certain thickness to insulate effectively.
Synthetic materials
Made of extruded or expanded polystyrene or polyurethane, these insulators are available in bulk or in semi-rigid panels for attic insulation. They are very efficient thermal insulators, which resist humidity.
Therefore, they are very suitable for insulating roofs, exterior walls, and floors. These materials are more efficient than mineral wool and require less thickness. It is important to note that synthetic materials are not ideal if you live in a hot region.
They have a fairly limited thermal phase shift and therefore low thermal comfort in summer. Finally, on the other hand, they are less efficient acoustically.
Polyurethane (PUR)
Very popular, this synthetic insulation is available in the form of rigid panels or sprayed foam. An excellent thermal insulator, it does not lose its insulating capabilities upon contact with humidity, unlike mineral wool.
Certainly, it is not the most ecological insulation, but it is certainly the best thermally. In panel form, it is easy to cut and in foam form, it can fill all the corners.
In summary, the advantages of polyurethane:
- low price
- good thermal performance
- does not rot
The faults:
- non-renewable material
- non-breathable
- fire resistance
- degradable for rodents
Polystyrene
Polystyrene is made from hydrocarbon expanded with steam. This material is very compact. It is then mainly used for interior insulation.
The strong points of polystyrene:
- low price due to low weight and volume in terms of transport (and therefore ecological);
- good thermal performance;
- non-perishable material.
The weak spots :
- non-breathable material;
- fire resistance ;
- non-renewable resource.
When to choose synthetic materials for roof insulation?
Extruded or expanded polystyrene or polyurethane are good choices if you don’t have a lot of space to insulate your roof and you need insulation with a minimum thickness.
As they are very efficient thermal insulators, they can achieve a good insulation value with a few centimeters of thickness.
On the other hand, if you have enough space, choose mineral wool: it will cost you less, and the sound insulation will be better.
Focus on vegetable wools
Another type of wool is possible for insulating your roof: vegetable wool. These are made from wood, hemp, cotton, recycled paper, or even linen and all have appreciable qualities.
The advantages of vegetable wool
- Wood wool, hemp wool, linen wool, and cellulose wadding come in panel form for ease of use.
- Wood wool is the best-known and most used plant-based insulation: its thermal comfort is particularly appreciable at times of high temperatures because its density limits the installation of heat under your roof.
- Cotton wool is the most environmentally friendly since it is made from recycled fabrics.
- Hemp wool is known for its thermal and sound qualities.
- Linen wool, which is a recyclable product, is particularly suitable for insulating attics and ceilings.
The weak spots
- Some ecological insulations such as hemp wool and linen wool have average acoustic performance.
- The resistance to humidity of biosourced insulation is average, except for wood wool which is very resistant to water.
- Their favorable ecological balance and their very good thermal performance come at a cost!
When should you choose vegetable wool for insulating your roof?
If you are attentive to respecting nature, this type of insulation is particularly suitable. Their environmental footprint is reduced and some, like linen, are recyclable. But you should always be attentive to how this product is manufactured: hemp requires a lot of water and is not always produced in France or Europe.
Don’t rely on vegetable wool
If your roof is not perfectly waterproof, you may encounter problems with mold in the insulation.
Insulating your roof: what about animal wool?
What if you relied on sheep’s wool or duck feathers to protect your home from the cold but also from the heat? This is possible thanks to animal wool.
The strengths of animal wool
Sheep wool or duck feather, insulators can be used in panels or in bulk for attics or slopes.
They can absorb humidity and release it when the air becomes drier: with no danger of mold or condensation on your ceiling.
The disadvantages of animal wool
In general, animal wool is slightly more expensive than other commonly used insulation materials, such as mineral wool or synthetic insulation.
Animal wool can be attacked by moths: they are often chemically treated against these insects. Some give off an odor that can be unpleasant.
Why choose animal wool?
High-quality animal wools can be durable and resistant to sagging. They retain their structure and their insulation effectiveness over the long term, which allows the thermal performance of your roof to be maintained over a prolonged period. It’s a long-term investment for your home.
Tips: your energy renovation work is eligible for several financial aid: this allows you, for example, to choose a more expensive product. To benefit from it, you must call on an RGE craftsman (recognized environmental guarantor).
What is the best insulation for roofs and attics?
The best insulation for your roof and attic is the one that corresponds to your work project, your budget but also your ecological beliefs.
If you are looking for the best performance/price ratio
If your budget is limited and your roof allows it, opt for mineral wool: it is the best choice in terms of quality-price ratio. And if you want to have good sound insulation, rock wool is the best option.
If you want thermal, sound, and ecological insulation
Choose cellulose wadding which combines all these qualities! Without forgetting the thermal comfort that it offers both in summer and winter.
If you are aiming for external insulation
If you insulate your roof from the outside (in this case, we speak of a sarking roof) or it is a flat roof (which must always be insulated from the outside), opt for synthetic insulation. It’s also a good choice if you want a thin layer of insulation.
If your attic cannot be converted
You should not hesitate to favor installation by blowing because the price per m² is lower for an efficient result. Cellulose wadding is particularly popular for insulating unused attics and is installed by blowing insufflation, or wet spraying.
Read also: How to Insulate a Cob Wall?