Rockwool fibre insulation v Cellotex boards

[endecotp]

You've already heard the feedback - no-one else would do it that way. But you've already made up your mind.

Have a look at the celotex website. They have documents describing every conceivable use of their products - walls, floors, roofs - can you find anything suggesting using it in this way?

 

[cjard]

Base and TopUp vary only by where the roll is pre-perforated or by thickness. The rolls of base I have are pre-perf at 400 centres, the idea being you can just break the entire 1200 roll into three 400mm wide ones, place a 400mm roll in the 400mm gap between your joists, and then roll it out in a one

TopUp might be perforated elsewhere (e.g. 600 centres) or combi cut (perforations at 400, 600, 800, you choose which to utilise), or not perforated at all. It's also likely to be thicknessed to 170mm (which in combination with the base, gets you to 270mm recommended minimum)

Even if you choose the wrong one, or your joists are at an odd spacing, it's not hard to put a saw through the entire roll before you unwrap it. A Bahco Profcut insulation saw will cut it with relatively low dust. Buy a good dust mask like a 3M 06941

 

[geraldthehamster]

Isn't there another school of thought which goes like this

No, not really.

 

[cjard]

The new model will results in 100mm celotex (it must have some acosutic properties!)

Of course it has acoustic properties - everything does. If you mean sound reduction properties, I think you'll be sorely disappointed; rigid materials generally don't unless they have considerable mass

It feels like you're expecting some massive transformation from using 100mm of PUR instead of wool, but per square metre of ceiling space (and neglecting thermal bypass caused by the rafters), 100mm of board is going to save you at most 0.37 watts of heat per degree of temp difference between the warm side and the cold. If your loft is at 5 degrees and your room at 20, and your room is 4mx4m that's about 88 watts less heat you need to supply to maintain the same temperature in the room or, in financial terms about 1.3 pence for electric heating or half a penny of gas saved, per hour. The law of diminishing returns steps in when using the board in combination with the extra wool, and you're looking at around 6 watts less heat input to maintain the same 15 degree difference over a 16 sqm area, equating to 0.1p of electric or 0.04p of gas per hour

 

[geraldthehamster]

The OP is creating a huge amount of work, trying to do this a non-standard way. Rockwool: roll out, cut, squidge, bish bosh, job done. Celotex: lots of measuring, cutting (it has no tolerance for being compressed or bent to shape), taping, expanding-foaming and general mess everywhere, with no acoustic attenuation at all and only imperceptibly better heat retention. Just saying.

cheers
Richard

 

[cjard]

+1 Mr Hamster, +1

 

[Jupiter01]

I have taken on board all the feedback provided.
I spoke wth Celotex today. I wanted to check on whether there are any issues with throwing rockwool over celotex. This is not an isssue, I have been informed.

I thought there was a consensus that Celotex is twice as effective as the same thickness of rockwool. I'm doubting this from the recent responses posted above?
It's based on this premise that I thought the belt and braces approach would be to install Celotex between joists and then throw over the inexpensive rockwool type insulation on top of it. I was planning to have 200mm where ever I can (excludes the central section where I will be boarding up.

@cjard thanks for clearing up the difference and since its just based on perforation, I will probably select two lots of 100mm base insulation. I am guessing that two lots of 100mm are going to be more effective than a single 200mm insulation.

 

[endecotp]

I have taken on board all the feedback provided.
I spoke wth Celotex today. I wanted to check on whether there are any issues with throwing rockwool over celotex. This is not an isssue, I have been informed.

Well it's true that it isn't going to react with it, or anything like that.

I thought there was a consensus that Celotex is twice as effective as the same thickness of rockwool. I'm doubting this from the recent responses posted above?

Rock wool is easy to fit snugly between joists so you get close to the theoretical performance. Celotex is harder to fit so you get more gaps and less close to the theoretical performance.

It's based on this premise that I thought the belt and braces approach would be to install Celotex between joists and then throw over the inexpensive rockwool type insulation on top of it.

Rockwell works best between joists. Celotex works best laid over the tops of the joists where you can tape the sheets together without gaps, and then put chipboard or plywood over that if you want to store things.

I am guessing that two lots of 100mm are going to be more effective than a single 200mm insulation.

Why would you think that?

But sorry, I'm getting bored with this now. Do what you want, I don't care.

 

[Jupiter01]

Thanks for the responses @endecotp. I thought two layers of 100mm would be more insulating than one which is 200mm based on what I was told about keepng warm and the need to wear multiple layers
Sorry if that doesn't hold true in this regard.

 

[Jupiter01]

This discussion has also got me thinking about how I should sound proof my floor joists (separating downstairs and upstairs) and also the stud walls. I had initially considered the Rockwool Sound Slabs but given how effective the regular loft insulation stuff is and alo the vast difference in cost between these two, I wondered if I could just use the loft insulation stuff between joists and also in the stud walls. Perhaps I could compact it to create greater density. Is this an approach that anyone has adopted or would reccommend?

 

[endecotp]

The acoustic properties of normal vs acoustic mineral wool scale approximately with weight, so if the acoustic material is 4 times the weight it will absorb 4 times the sound energy. You can look up the weights of the various materials. Things like wood fibre insulation batts are also a possibility.

Note that acoustic properties are expressed in dB, which is a logarithmic measure. So '4 times' does not mean that the sound reduction in dB is 4 times better. It will be 10 x log10(4) better, which is about an extra 6 dB.

 

[Jupiter01]

Whilst compressing regular rockwool insulation is inadvaisable from a thermal point of view, does it become more effective from a acoustic perspective by doing this?

 

[r896neo]

Yes it will improve acoustic performance. As said mass is your friend, a heavy acoustic batt is much superior than equivalent loft mineral wool (dimishing returns apply here though). Your issue is the loft timbers meeting a rigid foam board. sounds travels very well through timber and by adding large rigid boards fixed firmly between them you will transfer the sound through the boards. Deadening material like mineral wool will stop this transfer and reverberations.

Have you got a commission with celotex or something that your determined to use?

 

[Jupiter01]

Thanks @r896neo

 

[geraldthehamster]

I thought two layers of 100mm would be more insulating than one which is 200mm based on what I was told about keepng warm and the need to wear multiple layers
Sorry if that doesn't hold true in this regard.

It doesn't. Multiple layers of clothing keep you warmer because of trapped air between the layers. Insulation keeps heat in because of air or gas enclosed within the insulation