Loft insulation: cellulose Vs rockwool

[j_s]

I've asked about cellulose before, maybe on the old site. I had a quote to do my 100m squared loft with 400mm of cellulose and it was pretty damn high. 

 

I'm right in the middle of working to seal absolutely everything in my loft to make it airtight. I'm taping all the top of the stud walls, foamed all gaps and taped over the top, will be running all cables in conduit down into the stud walls and sealing around the conduit with sealant and tape and foaming any larger gaps etc. Will then use pro clima stoppas in the conduit.  It's a much longer process than I ever thought it would be but it needs to be done.

 

I can get pics if anyone wanted to see how thorough I'm being , or maybe someone might comment it's not thorough enough

 

I could just save a few quid and go for 400mm (or higher if worth it) of rockwool. I was actually going to use the brand Rockwool unless someone else thought a different mineral wool was better?  I'm doing it for sound and heat and I'll be doing it all meticulously. 

 

Any thoughts on which would be best to opt for. It's not a passive house but I am working on airtightness everywhere and improvements where possible

 

Cheers 

James

 

 

Edited November 9, 2017 by j_s

[ProDave]

The Knauff Earthwool products are a lot less nasty to use than many types of glass wool insulation. I don't know which one is recommended for loft insulation.

[gravelld]

Sounds like you're doing a good job James.

 

The things to consider with cellulose are:

 

- Inherently better air tightness of the insulation itself, which leads to...

- Less wind-washing

- Higher decrement delay

 

Assuming it's being pumped in and it's a ventilated roof, how do you deal with maintaining the ventilation gaps?

[j_s]

I've already done the eaves with dritherm 32 insulation and eaves trays are in place now (allows for about 20/25mm gap) which will allow for 400mm I believe up to the tray. I was going to add some more dritherm 32 batts against the existing dritherm 32 in the eaves so there is no chance of the cellulose getting through as the dritherm is very dense.

 

I also have a breatherable roof felt which surprisingly is of a decent quality and fitted properly which is a first for this poor attempt at construction. Not that this will hold anything, just a bit of extra info to show the roof make up.

 

 

[Dudda]

Is it block or timber frame? If timber then cellulose would be better due to the decrement delay but if it's block earthwool or similar is good enough if you're looking to make savings.

[j_s]

It's aerated blockwork. It's more about comfort than just savings in all honesty. 

 

Would it cause issues if I used cellulose in my circumstances?

 

 

[Jeremy Harris]

I'd go for cellulose, for several reasons.  It has a fairly high heat capacity, so increases the decrement delay.  It tends to improve airtightness, and reduce wind-wash, and last, but not least, it's pretty good as an acoustic insulation material too.

[Dudda]

36 minutes ago, j_s said:

It's aerated blockwork. It's more about comfort than just savings in all honesty. 

 

Would it cause issues if I used cellulose in my circumstances?

If money isn't an issue then I'd go with cellulose. It won't cause any issues. 

 

While cellulose is superior than earthwool and a hell of a lot quicker as someone does it for you in a day it's one of the decisions where the additional cost to additional benefit ratio isn't great. I put in 500mm of earthwool instead of 400mm of cellulose which gave a better u-value and still worked out a lot cheaper. It wasn't hard but did take me a while though (three layers).

 

It's a decision that doesn't have a wrong answer if you ask me.

[Nickfromwales]

@j_s

You can't go thicker than 300mm really with wool as it crushes down under its own weight at 400mm. Wool relies on it being uncompressed to achieve its stated values, so after 300mm your just wasting time effort and ? for no additinal reward. ?

Cellulose has far more benefits so I'd go that way TBH. 

[PeterW]

6 minutes ago, Nickfromwales said:

@j_s

You can't go thicker than 300mm really with wool as it crushes down under its own weight at 400mm. Wool relies on it being uncompressed to achieve its stated values, so after 300mm your just wasting time effort and ? for no additinal reward. ?

Cellulose has far more benefits so I'd go that way TBH. 

 

@Nickfromwales that’s the benefit of going with something such as Frametherm against “ordinary” wool as it has a much better structure that resists compression. We stacked up 600mm to see how much it compressed - lost about 15mm over a week. 

[Nickfromwales]

10 minutes ago, PeterW said:

 

@Nickfromwales that’s the benefit of going with something such as Frametherm against “ordinary” wool as it has a much better structure that resists compression. We stacked up 600mm to see how much it compressed - lost about 15mm over a week. 

?. Batts are more rigid again, but rockwool was mentioned in the OP. 

 

Edit : Do you know the cost per M2 for frametherm at 100mm ? 

Edited November 10, 2017 by Nickfromwales

[PeterW]

Let me check - I buy it by the pallet .....

 

looks about £1.80 but my maths may be rusty and I’ve not had enough coffee ..! 

[j_s]

Thanks everyone. 

 

If I were to use cellulose would the combination of a denser wool at the eaves plus eaves trays be a good enough barrier for the cellulose or should I look to add something else to hold it in place? The dritherm in the eaves hasn't budged at all in the high winds we had recently and with an additional piece at 450mm high against the existing wool in the eaves I would have thought that should provide enough of a block/dam for the cellulose.

 

Please do comment if you can think of a better way or even if I'm on the right track. The knowledge you guys have is a massive benefit and I really appreciate your input

 

Thanks

James

[Dudda]

4 hours ago, Nickfromwales said:

@j_s

You can't go thicker than 300mm really with wool as it crushes down under its own weight at 400mm. Wool relies on it being uncompressed to achieve its stated values, so after 300mm your just wasting time effort and ? for no additinal reward. ?

Cellulose has far more benefits so I'd go that way TBH. 

People usually put the first layer between the ceiling joists / trusses which prevent compression of that layer. The next layer rests on the timber as it's laid at 90 degrees. You therefore don't really have it crushing itself providing the first layer is between timber.

[j_s]

Well I'll need to extend the eaves trays as they are 120mm short for 400mm of cellulose. I've already started with 25mm celotex. Need to use it to protect my hands on some of the plaster board overboarding we had done 3 years ago, the builder decided not to use nogins and put the drywall screws in at angles. Ive already punctured my palm and lacerated a finger, that was within 5 minutes of each other!