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Attic insulation: methods and prices

Attic insulation is the most cost-effective way to save energy. Heat goes up, as a result of which no less than 25% of the energy is lost through the roof. In the following article you can read how you insulate your attic and how much it could cost you.

Do I insulate the attic or attic floor?

When you insulate the top part of your home, you can either take on the roof or the attic floor. Do you use your attic solely as storage space? Then you’d do well to insulate your attic floor. If on the other hand your attic is going to be equipped as living space, then you obviously insulate your roof. You can, of course, opt for maximum insulation and simply insulate both your roof and your attic floor.

Here you find a complete account of the methods and possibilities.

First possibility: insulate your attic floor

If you want to save on fuel costs, insulating the attic floor is the most obvious way to go. That way heat won’t be leaving your living areas anymore, and you can heat your home a little less. Floor insulation has the added advantage of being cheaper than roof insulation.

The way of installing the attic floor insulation depends on the structure and purpose of the floor. A different method is used when you have a concrete support structure or one made of wooden chevrons.

Attic insulation with a wooden support structure

With non-massive attic floors, you fill all the spaces between the floor joists with insulant. Most commonly used are the semi-hard or soft insulation boards made of mineral wool. It’s best to get an insulation package with thickness 15 to 23 centimeters as these have the best cost-saving ratio.

Just like with roof insulation, you should always use a damp-proof membrane or a vapour barrier. This has to be applied to the bottom of the support structure, turned to the warm side. If this proves to be quite difficult to achieve, you can always opt for isolation boards with this vapour barrier already included.

Aside from mineral wool, you can also use blow-in or spray foam insulation. The remaining finishing touches depend on whether or not your floor should still be walkable. Seeing as one doesn’t walk around the attic that often, one can simply use cheap wooden panels like OSB or fibre boards.

Attic insulation with a solid support structure

When it comes to a solid floor, you will of course have to apply the insulant on top of that floor. The first thing that needs to be done is applying a vapour barrier to the existing floor. This can be applied loosely, but the joints should always be taped off well! The same goes for the edges of the film close to the wall. On this film you then apply the pressure-resistant insulation boards, which you preferably lay down in 2 perpendicular layers (with a total thickness of 20 centimeters).

Not a fan of hard isolation boards? Then you could always opt for mineral wool. Though you should take into account that you need to construct a wooden framework on the floor first, between which you then put the insulant. This could of course drive up the costs quite a bit.

Second possibility: Insulate the roof from the inside

By insulating the roof you can secure a nice climate and make sure that the space in question is less susceptible to fluctuations in temperature.

The easiest way to insulate an inclined roof is to use mineral wool. In case of roof insulation with glass or rock wool the spaces between the chevrons can be completely filled with insulant.

It’s important to note, however, that the insulant is applied sufficiently thick. It’s best to maintain a layer of 15 to 20 centimeters, otherwise the insulant won’t be that effective.

Usually, the existing chevron isn’t broad enough to fit in this type of insulant, but you can easily solve this by applying new chevrons transverse to the old ones.

Make sure to cut out the mineral wool a bit bigger than the distance between the chevrons. One extra centimeter should be enough. Glass and rock wool are compressible and stay nicely in place while putting it between the chevrons. On top of this material a vapour barrier made of plastic sheet should be applied.

There’s also mineral wool available that’s already covered with a vapour barrier, the so-called fibre glass blanket insulation. These are provided on one side with an aluminium foil that functions as a vapour barrier. They are available from 6 to 22 centimeters in thickness and 35 to 60 centimeters in width.

These are especially suitable when the distance between every chevron is the same. You just staple these fibre glass blankets to the front of the chevrons. The joints are finished with aluminium tape.

Third possibility: Insulate the roof from the outside

When the roofing requires replacement, or when your roof structure needs to be replaced, you can also choose to insulate from the outside. Different kinds of insulation boards are available to do this, some of which are even equipped with battens or laths. It goes without saying that outside insulation is the most drastic action you can take, but when you’re planning on renovating the roof anyway, you can always consider this option. Just remember that insulating from the inside is always the best way to go.

The cost of insulating your attic

Insulating your attic is not that difficult, but should be done properly in order to avoid thermal bridges and heat loss. Because of the high insulation premiums and subsidies, loads of people opt to let the insulating be done for them, instead of doing it themselves. Add to that the lower heat consumption and you’ll be wondering why on earth you didn’t insulate that attic sooner.

For a tailor-made price offer, we gladly refer you to our quotation service. Receive 3 noncommittal price offers from certified insulation experts in your area.

A vapour barrier is indispensable

When the insulant itself is not equipped with a damp-proof membrane, you should still apply this separately. Often used here is a plastic film of polyethylene. This film should prevent the moisture in that room from condensing against the cold roof.

The drops of water that are being formed, will accumulate here, which will over time inevitably cause problems with mould, wood rot an other complications. So you can see why a vapour barrier is of the utmost importance, and why you should apply the finishing touches with great care.

With damp-proof tape you can neatly seal the attachment points of the film (staples), as well as any overlap or seams. There are also the so-called intelligent vapour barriers that expel moisture when it’s hot. Prices for these types of barriers start around £ 3 per square meter.

Ecological insulation for the attic floor

Should you wish to live more ecologically, you can always choose ecological insulant. There’s flax, hemp wool, sheep wool, cork or paper insulation and many more. Of course with these types of insulation you can expect to pay quite a bit more than for your traditional insulants. More information can be found in the article “insulation material”.

Things to consider when insulating

When you use different kinds of insulants, most of the damp-proof materials should be applied to the bottom to prevent moisture problems. More damp-friendly material can be applied on top of that. Apart from that it’s also important to have a good connection between the attic floor insulation and the wall insulation to make sure that no thermal bridges can be formed.


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