Insulate your loft with blown fibre (video)

Insulating your loft has obvious benefits, reduced heating bills in the winter, cooler in the summer, fewer drafts etc. The method you choose to do the job is not so obvious. There are a multitude of products available from the more common glass wool and extruded polystyrene to the more eco friendly cotton or wood fibre batts and plenty in between. They all have their pros and cons and dependent on the application, budget and insulation requirements, they each have their place.

In my case I needed to insulate a loft with awkward access but no restriction on the thickness of the insulation. Some insulation materials have better insulative qualities and can provide similar levels whilst being thinner. The downside is that they are more expensive pro rata. So if you have the space and you are working to a budget you don't need to think thin.

In this case due to the access restriction I chose to use dry blown cellulose, it's a great insulator and as you can see from the video, it's a breeze to install.

Insulating a 40 square meter space at a depth of 38 cms including the cost of rental of the equipment but not delivery (I collected it myself) came to just over 20 euros per square meter.

Using fibre glass rolls (probably the cheapest option) to achieve the same u value (see below) would have been around 17 euros per sq meter.

The benefit of the blown fibres is mainly the ease of installation and the fact that there are no gaps or thermal bridges. Two of us were able to easily complete the work in half a day and it's a much cleaner and more pleasant operation that crawling in to all those hard to reach areas with large fibre glass rolls.

If you have any questions on the subject please feel free to ask in the comments below and I'll do my best to answer them for you!

Keep warm



A ‘u’ value is a measurement of heat loss

Low ‘u’ values mean less heat loss

Each element of a wall has its own ‘u’ value, to calculate the overall ‘u’ value, we use ‘R’ values.

‘R’ values work in the inverse and can be added together

A ‘u’ value of 0.5 is equivalent to an ‘R’ value of 2 (1/0.5=2)

Cold or Thermal Bridges – a point for heat to escape and where condensation may form. Found where there is a break in the insulation such as un-insulated joists. Also found at corners, around doors and windows, where metal pipes cross insulation and where there is a disruption of a wall cavity.

Eliminate cold bridges by careful insulation – ensure loft insulation covers joists, insulate around pipes and add extra insulation at corners and the bases of external walls.


We have just prepared our attic for cellulose insulation by laying paper-backed fibre glass. The reason for this was twofold. On the wooden parquet ceilings (some are plasterboard others are varnish face down parquet flooring wood) we put a layer of fibre glass with paper backing to avoid any chance of the cellulose creeping through the cracks as dust. We also banked it along the eaves so that it has created a wall for the cellulose to blow against and prevent cellulose being moved by draught and still allows air to enter into the roof space. This seems a lot of extra work but we are happy it will work.

The simplified tax credit is now a 30% flat rate but you need to use an RGE approved tradesperson from 1st Jan 2015. There is also the prime energetique for 1350 euro if you meet the qualifying criteria.

I could have saved some money then if I had waited but probably worth doing aswe plan to be in France for ever.

The cost of loft insulation for anyone not doing it themselves and thinking of getting a devis, for us worked out at 10 euros per square metre of floor area.

Thanks Terry, was just going to say the same. Since James put this up my OH has been enquiring and that is exactly what she has found out. Roll on spring and devis!

Gordon, fyi the rules have changed and you no longer need to have two projects to qualify for tax credit, one is enough. And it's gone up!

We had our loft insulated with blown fibre in May as part of a "package" including swapping our old oil fired boiler for a pompe a chaleur air to water heat pump. In order to get the Credit Impot you have to take 2 energy saving measures.

Obviously too soon to know if our heating bills will be drastically reduced but we did notice the house (bungalow) was cooler in the really hot weather due to the roof void being insulated.

Thanks John, thought was good but my closest branch, Trellisac Périgueux, part of the Bricodepot site simply does not work! Staff are actually fine there - ironically.

Brian I know staff can sometimes be un helpful in these places but if this helps at all.

I feel slightly "urban" here in the Hautes-Alpes, even though we're in the middle of the Ecrins National Park :)

I've got a Brico-marché just over the road behind the mairie, a Bricoland, a SAMSE and a Matériaux twenty minutes away northwards and the same southwards. Easy :)

Yes, La Peyre and Brico Depot are about 1.5 hours away and I use the latter anyway fairly often. However, last trip I looked in both. No joy. Leroy Merlin is two hours plus and have the Greenline at least, local Chausson don't do and can't get anything useful... SuperM who used to be VM don't even know any of the products! It looks like I'll do the two hours to Bordeaux and be opportunistic. I already bought a palette of Fermacell wall boards and have a €50 voucher from Chausson as a 'present' for what we have spent there already over five years, which is a good chunk of €350 worth of Greenline, despite two hours each way. I think I am in a bit of a no man's land here, it drives me nuts.

Yes understood, travel distances kill so many things in France. I know Wedi is stocked in La Peyre and Leroy Merlin and a very similar product in Brico Depot if that is any help

John, with you on the boards and yes in principle Wedi boards or similar would be good. Our problem here is distances. To get either I have a hell of a long trip and Fermacell Greenline is available closest, works out cheaper when all is taken into account and on line ordering has no actual advantages given that I would go out and buy other things at the same time. It is, I find anyway, a problem with on line buying building materials that no place ever has all different things one wants, then some have minimum delivery charges that are exorbitant or only deliver out of local branches who don't have what I want.

Sorry James you are quite correct, it's me. It was a similar vein on the construction and renovation group.

No editing on my part John

Looks like there has been some editing of this thread?

Terry, it was me that mentioned the blowing around and in the same paragraph I also said spraying water to form a crust will help prevent this. The demo I witnessed was showing the path that warm air can make through various materials, rockwool, glass fibre and cellulose and with a slight draft the insulation can slightly expand and allow warm air through. As I also said the extra thickness should reduce this issue. The demo was setup by a rigid insulation manufacturer, which of course means if theirs isnt fitted well it's not much good either. My neighbour has 120mm of kingspan fitted by the daftest couple I have met and they had no more snow sitting on their roof than me with negligible insulation.

Brian, I do like Fermacell but in a thin area like you mention, wouldn't foam board still produce better insulation properties? Something like Wedi board etc.

I also don't agree with not insulating the floor, yes heat rises but your feet/calves will feel chilled without it and that makes your body feel cooler. In BIL's house all tiles, you have to put your feet up on the sofas to feel comfortable as the floor reduces the heat at the low level to make it uncomfortable to sit in the chairs for any length of time.

Hmm, our friend did ours, not sure if he did the crust thing though, will have to check that out. Your internal temperature sounds exciting James :) I'm aaaaalmost looking forward to it being cold to test it all out! Also I think we will have to get a dog now to help with the heating bills. Great idea Kent ;-)

Sounds like a good idea. The crust, not the dog farts.

Bit late but here goes. Exactly what I have, James. Gets into all the nooks and crannies. Someone mentioned it blowing about. It doesn't, at least chez moi, because the people who put it in sprayed water over it to create a crust. If you have to stir it up to get at a power cable, for example, a light spray after you've put the insulation back in place restores the crust according to the installer. As for Veronique's comment re lack of beasties that's right. Something in the cellulose (fire retardant or whatever) deters things like lerots from nesting in what would otherwise be a prime location.

Light those dog farts; don't waste the heat potential.