Affordable wall cavity insulation?

[ashthekid]

I have a wall layer setup planned like the photo below. 
My question is on the 100mm fibre insulation going inside the unusually large cavity gap which is approx 300mm wide in total.

This is an addon that I have to justify cost wise so I’d like to know what your thoughts are on what is the best form of insulation for heat and acoustic protection that is cost effective? The contractor has suggested putting rigid PIR 100mm in instead and that it would be easier to fix to the inside stud frame but this can work out quite expensive. 
What are my options? What’s the cheapest but most effective heat/acoustic wise?

It’s approx 60sqm.

Is this 100mm really going to make a big difference to U-values?

 

 

 

[Marvin]

Thermally:  Celotex is about 0.022 W/mK  Wood fibre insulation is about 0.038 W/mk over 60m2 outside temprature -8C, inside temprature about 23C  you would use about 35 watts per hour more to keep the inside temperature at 23 using Wood fibre or 0.84 of a kW per day. I think is about the sound.... 

 

Acoustically I think these are 2 different types of product with different purposes. I would check the airborne sound properties of each and the impact sound properties of each. I think its unlikely to need much impact sound resistance but if the echo from banging on the neighbours wall crosses the air gap will the rigid insulation absorb it?

 

If it was sound could you double the soundblock?  But lots of things to be considered like the dew point calcs....

 

https://www.greenspec.co.uk/building-design/woodfibre-insulation-intro/

 

Best of luck

 

Marvin

 

 

[ashthekid]

Thank you Marvin, that’s really useful info. 

So having any insulation would make enough of a difference by the sounds of it and celotex would be better from a thermal point of view which is what I assumed.

Whether that number of extra kW per day would equate to a noticeable sum per year I don’t know. We would be running UFH from an Air Source Heat Pump and everything is on electric as there is no gas in the village.

 

It’s just a shame celotex is a lot more expensive than fibre insulation.

Edited April 30, 2021 by ashthekid

[AliG]

It looks like you have 100mm PIR between the studs already on the inside of the frame.

 

What is the thin layer of insulation on the inside of the studs. Is that another 40/50mm of PIR. I assume it is but it is not labeled.

 

If you already have 140/150mm of PIR you already have a good U-value and there would not be much practical difference between adding a 100mm layer of PIR or fibre insulation. It would give a U-value of 0.12 with fibre and 0.09 with PIR, this would make very little difference to heating costs.  I would recommend fibre as not only is it cheaper, but it will improve the sound insulation. I would not be worried re sound transfer with that build up.

 

Also that wall is between two buildings, not an outside wall, so the U-value will be less important again.

 

 

[ashthekid]

Both side walls are outside walls, only a stretch of about 5m is adjoining the neighbouring property directly. 
 

Yes there is 100mm PIR between the studs and then 50mm on the inner side of that stud wall so there is already 150mm total of PIR. Then the battening and soundbloc plasterboard. 

Anything inside the cavity will be extra and there is a layer of plywood that go in between all of the above and any potential cavity insulation. So in theory I wouldn’t been for massively for more PIR but for the cost I would benefit from 100mm fibre insulation?

[AliG]

Without the extra insulation your U-value would be 0.16.

 

I suspect that strictly calculated the extra insulation would not pay for itself but you would see some benefit in both sound and U-value

 

Normally a timber frame would have a permeable membrane on the outside. I am not sure how adding insulation to this affects it.

 

I just looked this up and in this case the membrane would have to be installed on the outside of the insulation which seems awkward.

 

I ate that no vapour control layer or membrane is shown on the wall drawings which seems odd.

 

Edited April 30, 2021 by AliG

[ashthekid]

Is it regulation to have some sort of membrane layer? I must admit I have seen it before when looking at other similar sketches of wall layers like this. 
 

Without a membrane layer am I best to make sure we have one installed? Or can I just get away with putting a fibre layer of insulation in and that will suffice?

[Marvin]

The different elements are for different reasons. As AliG pointed out the fibre insulation in not needed to meet good insulation level but more likely sound deadening and yes the fact that no membrane layer is identified is odd.

The purpose of the membrane is to stop moisture getting into the plywood sheathing, and using breathable means that moisture can get out. I would check with designer about missing membrane.  Surprised there's no vapor barrier on the inside too. I have to coat under my plasterboard with sheet plastic to stop the moisture vapor?  Well I'm sure someone will correct me.

 

Good luck

 

Marvin

[A_L]

10 hours ago, ashthekid said:

Is it regulation to have some sort of membrane layer?

 

If a condensation risk analysis to BS5250 is O.K. then not required.

[ashthekid]

That hasn’t been mentioned or identified as being required by building control assuming it’s them who would request it?

[SimonD]

When I look over this detail I'm most troubled by the box gutter arrangement and cavity. I don't think that woodfibre is suited to this situation.

 

With the box gutter sitting on top of the woodfibre as it is here, there's too much risk of moisture damage as it needs at least 2 or three layers of protection.For example, flashing and membrane/aluminium tape/epdm tape on top of the woodfibre. As far as I'm aware, woodfibre is also not suitable for use in wall cavities, only as external or internal wall insulation, or behind a ventilated rain screen. There doesn't seem to be any ventilation provision here which would need to be in place for the woodfibre. You also don't have easy access if there is a problem at any point. I think in this instance, you'd be better off looking at a mineral wool like Rockwool, for example. This would be cheaper than the woodfibre.

 

HTH

[ashthekid]

Ok so let’s forget the wood fibre option as those points @SimonD has mentioned are very valid & I totally agree with. 
Therefore I’m left with the PIR rigid form or perhaps a moisture proof Rockwool option which would be cheaper by the sounds of it. 
 

I’m told the box gutter will be lined with a resin with the brick edge header course having lead flashing that will flap over the resin. I have a photo from today in site showing the box gutter being formed. 
 

 

 

Edited May 1, 2021 by ashthekid
Image wrong way up