Insulating eaves rooms

24 Aug 2009
Reaction score
United Kingdom
We live in a chalet style dormer house, and therefore the first floor rooms are cut into the eaves. We also have a loft, which I've fully insulated.

The first floor rooms are rather cold though in this freezing weather, and the heating (in the dormer window) in the main bedroom isn't particularly effective at heating the room. I think I need to better insulate the eaves, as they are probably larger than the attic space in total.

The stud / plasterboard walls of the rooms are insulated with a thin layer of polystyrene, however the eaves floor / ground floor ceiling isn't particularly well insulated - in some places none, in other places maybe 150mm of rockwool. I can't actually get to the floor easily at all in some areas as its been boarded over.

The rafters aren't insulated at all (they are felted), although some areas have a thin layer of fibreboard pinned to the rafters. We've had a new bathroom installed upstairs, and the rafters in this room have been insulated with 140mm celotex to comply with building regs (although I don't actually think the walls have been insualted with anything... I don't remember the chippie having enough to do the walls as well as the rafters).

I was thinking it would be more cost effective (and easier) to insulate the rafters in the eaves, rather than both the floor and wall. However I believe Celotex isn't cheap.... Granted I'm not looking to do it to building regs standards yet, but we don't have much (any) money left after all the other work we've had done.

Or could I use space blanket / rockwool in the eaves?

or would I be as well to do the walls / floor rather than the rafters? If so, why?

Is that polystyrene board (of varying thickness) you buy from B&Q of any use at all for walls / rafters, or would it really be better getting something more specialist like the Celotex?
Sponsored Links
My god, I cannot believe the price of Celotex or even just polystyrene compared to rockwool / space blanket! :eek: I accept that roll insulation is being subsidised at the moment, but why not other types of insulation? Any cheaper options?? Should I rule space blanket / rockwool out??

This is a drawing of what I'm working with.... I'm hoping this might prompt some responses! :D

As I mentioned, the loft has been fully insulated, with the 100-200mm that was there originally, and 200mm space blanket on top laid at right angles. There is apparently sufficient ventilation in the loft as there is no condensation, although I did note certainly 1 gap running down the rafter line behind the angled plasterboard in the upstairs bedrooms into the first floor eaves - I'm sure there are others too. There was a warm draft blowing up through this gap, which does make me wonder whether my insulation is compromised...?

Within the eaves rooms / eaves themselves, most (but not all) of the room walls have about 20mm polystyrene backing, and between 100mm and 200mm of rockwool insulation on the floor, but only on the south side of the house. The north eaves are boarded and used for storage, but have no insulation (beyond vermiculite) under the flooring. Typically, these have the main living areas of the house below!

In the boarded eaves, I feel I have no choice but to use celotex, polystyrene or some kind of board insulation in the rafters (leaving a 50mm gap) however as its so much more expensive than space blanket / wool, I haven't discounted using wool (unless I get advice to the contrary). That would mean I don't need to lift the floorboarding to insulate under. However should I block the air gaps into the loft where they exist? The gap was perhaps 10cm wide? Should I try to shove insulation up or down this gap on the wall side?

In the unboarded eaves, how would you look to best - and most importantly cost efficiently - insulate? bolster the insulation on the floor and walls, start the walls from scratch, or add in between the rafters?

Hope you can help! :)
I've seen that rafter roll on a different thread:

Which looks good, but is around £48 a roll - for the 85mm stuff, which is about 5.5m^2 and has a U value of 2.65. To be honest though, whats the difference between that and space blanket, which is on 2 for 1 at about £6 a roll, comes in 100, 150, and 200mm, and has a U value of over 3?? Its almost half the price on a m by m basis!

Am I really missing something here?? Why is rafter insulation so expensive in comparison to roll insulation?
Sponsored Links
I'm going to try to ping this back up again to see if I can get any response... :confused:

I'm still in the same position as last year, and this house is bloody freezing - I'm sure its colder than it was last winter.

I've double insulated the floor, and most of the walls of the south east facing eaves with space blanket / glass fibre (stapled in between the uprights and held in place with cardboard.

I still need to deal with the entire north west side of the house, below which is most of the living space. I'm still toying with stapling space blanket into the eaves, although this would potentially block the attic ventilation on this side of the house. Having said that, I did see polystyrene eave blockers for sale in Homebase..... surely homebase wouldn't sell something bad... :p

Anyway, any thoughts at all (pretty please!!) on how best to tackle this??
Celotex and the like are much more expensive because they are much better insulators ie you only need say 120mm Celotex compared with 300mm of Rockwool or the blanket stuff as you put it. Rockwool is not really practical to be used vertically in a wall so that leaves little choice. Whether you add all the insulation at rafter level and none in the walls is up to you although you're then effectively heating the void between the dwarf walls and the roof. Look around and online for more competitive prices for Celotex or Kingspan or similar, the shed's and many merchants will have silly prices for the stuff.

This is a standard loft set up:

Cheers for that! :)

The problem is that there is some storage in the eaves on the north side of the house, and in fact the boiler is in there too, so even if I did insulate the floor and wall, I'd still be heating the eaves from the heat of the boiler. Hence not having much choice about insulating the roof rather than walls and floor. I suspect I'll have no choice but to use Celotex here.

Perhaps a little bit further into the eaves I could consider insulating the walls and floor and blocking off access, although as its boarded (if unused) I'd have to insulate over the boarding.

I did insulate one wall in the eaves on the south side of the house with rockwool and secure it in place with board. I'm assuming this is still going to be fairly effective - certainly more than the thin layer of polystyrene that was there (25mm)? The intention was to do the same with some of the other walls in the eaves on this side of the house, where there is no storage and I have already insulated the floor.

The only other problem is accessing a section of the eaves that is completely blocked off - I may have to take a wall down, but given how cold it is, and how much colder the room below it is than the other rooms on the ground floor, I can't see I have any choice!

I'm surprised to see you being so factually foolish.

you only need say 120mm Celotex compared with 300mm of Rockwool or the blanket stuff as you put it.

PU has an R value of 4.0 per 100 mm
Fibre-wool has an R value of 2.5 per 100 mm

so 120 mm Celotex = 200 mm fibre-wool ( approx) at time of installation.

I have nailed/stapled fibre-wool roll in the vertical in order to save money ( which is the main point of insulation) and don't see why there is any problem with this.
So you're gonna stick a staple through 200mm of rockwool, sounds like a real professional instal there MW! I've not had a Building Regs application returned on a lofty with more than about one condition in years so must be doing something right eh! How about yourself MW, how many Building Regs packages have you put together?
So you're gonna stick a staple through 200mm of rockwool

No. Just detach a little bit of wool from the backing-paper and staple through the paper. Now that was easy, wasn't it ?

I've not had a Building Regs application returned on a lofty with more than about one condition in years so must be doing something right eh

Well, of course not if you are specifying almost twice the necessary depth of insulation. ,might be a bit disconcerting for the customer though , eh ?

How about yourself MW, how many Building Regs packages have you put together?

Well, my goodness, someone is feeling very defensive today, aren't they ?

The answer is none. why would I wish to intrude into such a highly-specialised, interesting field of work ( does it pay well ? ) ?

Ii do have no trouble however, in remembering and correctly quoting U and R values. How about you ?

To take a leaf out of someone else's book, Freddy, don't hesitate to thank me if you found this post useful, informative and career-enhancing :evil:
MW, I have never checked or contacted Rockwool to do a U value check for using Rockwoll in a vertical situation, it’s a bit of a bodge and have never seen it done or specified it on a drawing and have never had a client nor builder ask for Rockwool to be used in such a situation. It’s not really designed to be used in such a situation, that’s why you’ll not find that construction method referred to on the Rockwool website and why they’d inevitably raise an eyebrow if you ever contacted them for such a U value check and as a professional I should not be specifying something for which it is not designed. It’s a typical DIY’ers solution! If used vertically Rockwool is designed to be supported on both sides within a cavity or partition or as a vertical fire break where it’s suspended at the top and clamped at the bottom. If I had a client who asked that Rockwool be used in such a situation I would duly contact Rockwool to seek clarification.

I over-specify insulation in everything I do and in anycase the minimum compliant U value they must achieve is always stated too, I do not consider skimping on a few mm of insulation to be economical. That way when the builder does a shoddy job elsewhere or changes stuff and swaps 50mm of Kingspan for 50mm or rubbish he’s picked up cheap from his mate down the merchants or just wants to change the construction or whatever I’m in the clear and transfer responsibility to the builder for compliance. On 99% of my domestic jobs clients or builders have no further need of my services hence why responsibility must be transferred to the builder.

I do not understand the reference to what I earn has any relevance to this thread? Actually looking at what other trades charge for mainly manual albeit skilled labour I think I’m pretty good value actually!
OK, thread dredge time! Just a sanity check on what I'm planning...!

I think I'm about to settle on installing 50mm Recticel (PUR) into the eaves between the joists. This will allow me to add more on top of the joists later if I desire. We're looking to install some eaves cupboards at some point as well - hence installing into the joists rather than into the walls / floor.

Jewsons are selling it at £15.45+VAT, which is cheaper (or as cheap as) anything I've seen elsewhere, including eBay and - I'm probably looking at needing 20 sheets, hence trying to save some costs!

I understand that the main difference between PUR and PIR is that PIR is more fire retardant. Insulation properties are about the same.

So... is anyone seeing any flaws in my plan???

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links