Rockwool fibre insulation v Cellotex boards

[Jupiter01]

Hi All,

I appreciate that neither Cellotex or Rockwool loft insulation are designed for acoustic purposes; they are thermal insulations.

With that declaration in place, I need to understand which of the two are more effective at blocking noise. I am planning to replace my current loft insulation (a couple of layers of Rockwool loft insulation) with 100mm Cellotex boards between all the roof rafters. On a thermal basis, I am yet to find anyone that thinks that the Rockwool will outperform the 100mm Cellotex. Happy to hear to the contrary.

Returning to my question, I pulled out some of the existing rockwool and am amazed by how much noise it was blocking out as I can now hear the road outside. I am wondering whether the Cellotex will at least be as good at blocking the noise out too.

Hope that makes sense.

 

[geraldthehamster]

Celotex is more thermally efficient than Rockwool. However, it's nowhere near as good at blocking noise.

Cheers
Richard

 

[Jupiter01]

Thanks for the response.
I'm in compromise territory then...unless I fork out for new sound proof plasterboards...

 

[r896neo]

You dont need to go by asking people? Insulation is rigorously tested and so you can check its thermal properties by looking at the specs.

Of course real world applications will never be the same as lab results. In very simplified terms, compared to a layer of rockwool, half the thickness of celotex is needed to get a comparable result. Compressing rockwool will lower its insulation value where as rigid foam is not compressible under normal situations.

The quality of noise and thermal effectiveness when using boards is highly dependant on quality careful installation whereas rockwool can be thrown down by almost anybody and achieve a pretty good result.


As said, Rockwool is far far superior in sound insulation

 

[geraldthehamster]

I appreciate that neither Cellotex or Rockwool loft insulation are designed for acoustic purposes; they are thermal insulations.

With that declaration in place, I need to understand which of the two are more effective at blocking noise. I am planning to replace my current loft insulation (a couple of layers of Rockwool loft insulation) with 100mm Cellotex boards between all the roof rafters.

Firstly: Rockwool acoustic batts specifically are designed for acoustic as well as thermal insulation. Even ordinary Rockwool is promoted as also good for soundproofing.

Secondly: you say "loft insulation" but then mention "roof rafters", Could you clarify whether you are referring to the ceiling joists (horizontal) or to the rafters (sloping)?

Cheers
Richard

 

[Jupiter01]

Sorry, I was referring to ceiling joists. I'm not planning to insulate between rafters.

I think there is a consensus on Celotex being more effective in terms of thermal efficiency but I'm still left with a challenge of noise. Apart from sound proofing boards, I don't see an alternative?

 

[geraldthehamster]

It's certainly more usual to use roll insulation between/over ceiling joists. 300mm was the current standard, last time I looked.

 

[Jupiter01]

Am I compromising the Celotex boards if I roll out rockwol over it at a later date? My thinking here is that, if after installing 100mm Celotex and the sound proof plasterboards, the noise was stil too high, could I resort to this as a last measure?

I don't initially want to go down the rockwool route as a) it's inefficient and b) it will keep me from boarding out the loft above.

Appreciate all the help.

Regards,

 

[theprinceofdarkness]

Celotex is difficult to install, you have to cut it around cables then air seal any gaps, especially for sound proofing. You can't even cut it in-situ, because a full sheet won't go through the loft hatch. So up and down the loft ladder twice per cut section? You will get fit!
Compressing glass fibre reduces its insulation value , but when limited by space constraints, thicker glass fibre compressed is a better insulator then the nominal thickness. I have spent hours on the web trying to find some rules about this. I only ever found one site which basically said 6" compressed to fill a 4" gap had the same insulation value as 5" of glass fibre. So if you only have 4" gap and fancy glass fibre then 6" would give a better value, but not as good as Celotex, if it is installed properly. I have an insulated army hot food carrier, it was insulated with glass fibre which was compressed so hard it was like cardboard. Some post grad fancy doing some research into this?
Frank

 

[Jupiter01]

Thanks for the responses.
I will be removing my ceiling plasterboarrd which will make the installation of Celotex a lot easier. I don't have the cutting and loft hatch constraints described in the earlier response (thankfully!).

I'm torn between the two choices at the moment and my head tells me to go with celotex as it wil be more efficient in thermal terms, longer lasting and give me the loft space. The only challenge is that it may not block out the noise as well as the rockwool has. Hence the question on whether I can throw over rockwool as a last resort?

Am I overlooking anything in my summary above?

 

[oldbutnotdead]

Celotex sheets aren't going to offer much in the way of soundproofing (go to your local Wickes or wherever. Tap a sheet of Celotex. Then tap a roll of loft insulation. Which one reverberates?)

Celotex has better resistance to thermal conduction per metre thickness than loft insulation BUT that assumes solid sheets and tight taped joints between sheets. You won't get perfect joins to your timbers therefore you won't achieve the perfect thermal resistance quoted. Rockwool can be stuffed into random spaces and still perform pretty much to its spec- it is more efficient when not stuffed but still works well when stuffed.

Theoretically 100mm of Celotex (thermal conductivity 0.022 W/mK) will do the same job as about 200mm of loft insulation ( thermal conductivity 0.044W/mk). 100mm Celotex will cost you about £10 per square metre. 200mm of loft insulation will cost you about £2.70 per square metre (Wickes have an offer on at the moment).

If you have the headroom in your loft to deal with the extra depth (I believe 250mm is the new build requirement for loft insulation) and you really want to store stuff up there then double your joists- put 6" x2" on top of your existing 6" x 2". Cost of treated 6 x 2 around £2/metre so even at 400 centres you're still only at £8.70/sq m with loft insulation and just think how stiff your ceiling will be.

Yes you can chuck rockwool on top of the Celotex but imagine how annoyed you'll be after spanking £10/sq m and all the faff of cutting it to size (remember you'll have some scraps to throw away as well, as least you can use any rockwool scraps).

And as for longevity- 20 year old rockwool that hasn't been trampled, crushed, got wet or full of dust is still pretty effective. Even if it isn't, replacing or supplementing it is quick, cheap and easy.

Can you guess which path I'm in favour of?

 

[endecotp]

If you have the headroom in your loft to deal with the extra depth (I believe 250mm is the new build requirement for loft insulation) and you really want to store stuff up there then double your joists

Or put insulated loft boards over the existing joists.
Knauf have a product.

 

[Jupiter01]

Thanks guys.
I think I can guess what you are favouring @oldbutnotdead
Isn't there another school of thought which goes like this: I will be taking ceiling boards down anyway which makes Celotex easier to put in and we have all recognised how thermally efficient it is. It also gets me the space I need in the loft and if it is good enough on the sound front, bingo, else, I can throw over some inexpensive rockwool. What do you think?

 

[r896neo]

In my loft for the area i needed floored for storage I insulated between joists with 4 inches of rockwool which is fast cheap and easy. Then i selected an area without cables and obstructions and layed 100mm celotex boards taped and foamed together and then floated chipboard sheets over the boards.

This in my mind is the best of both worlds from noise and thermal performance and a lot less work than building a false floor of perpendicular joists etc.

As long as you are careful where your boards terminate (i.e bang on a joist) then the celotex is plenty strong enough to not deform over the joists and not break once the chipboard is on top.

Filling between joists and then boarding over with Celotex will have almost no sound dampening properties i would guess. Noise will reverberate through it if its in contact with all the timber.

 

[Jupiter01]

OK...
I will celotex out the entire loft space in between the joists. I wil use foam filler in the gaps to close them out as much as I posisbly can. Given how inexpensive the rockwool loft insulation is, I will pile that on top too. This will give me even more thermal efficiency and I can then be confident that I will have at least the same level of acoustic protection as I have now. I currently have around 200mm of rockwool and from a sound perspective it is flawless. The new model will results in 100mm celotex (it must have some acosutic properties!) followed by 200mm of rockwool on top.
@r896neo, I have a slightly different situation to yours as I want to board the central section of my loft. My ceiling joists are 70mm deep. I will need to stick some battens in the central section to accommodate 100mm celotex and I will then lay boards on top. I will go slightly higher with the battens and stick in 100mm rockwool under the boards too if I can...

Happy to hear feedback on the above.
Question - Do I need "base" or "top-up" (wickes terminology) insulation in the above scenario and also would I be better with two layers of 100mm or 1 layer of 200mm?

Thanks for all your help.