Steico flex 100mm WF compression tolerances

[sb1202]

Apologies if this is a daft question but I can't find an answer in any tech sheets or on here.

I'm looking to order 100mm steico flex 1190 x 375mm WF insulation and fit it into new stud walls. The stud walls are standard CLS at 38x89mm and will be built to 400 centres, so I'm presuming the 375mm insulation will give a nice tight fit and I can work out for the long side with dwangs.

My question is about the depth. Does this insulation compress into the depth of a 89mm stud against a membrane or do I need to pack out the joists with strips to get to the 100mm? (I couldn't get 95mm cls anywhere and 120mm/140mm deep studs are too deep for my application)  

 

Cheers

[Canski]

Years ago I made the mistake of putting 100 mm rock wool into a 4x2 stud wall. Bear in mind the 4x2 comes out at 97 x 47mm. We squeezed it in and fixed the plasterboard only for the fixings to pop. 

[Conor]

Are these external stud walls, and therefore is this your main insulation layer? Or, are these internal partition walls?

 

If the former, then you'll need to batten out using 22 or 25mm battens. If you compress the insulation, you reduce it's performance.

 

If these are internal partition walls, use 50mm acoustic insulation instead.

 

How are you running services?

[sb1202]

3 hours ago, Conor said:

Are these external stud walls, and therefore is this your main insulation layer?

Yes

 

3 hours ago, Conor said:

you'll need to batten out using 22 or 25mm battens

Not sure what you mean by this. Do you mean to pin a 25mm batten on to the short face of the stud? I can easily rip a 10mm strip for this.

 

3 hours ago, Conor said:

How are you running services?

Electrical only, through the insulation and sealed over the VCL with tape. This is a renovation so I can't afford reduce the room space with a service layer. 

[Russell griffiths]

I’ve only had this product in my hands for a few minutes, and it felt very firm. 
the stuff I messed with might not have been the flex version, have they made one more squishy for this sort of application. 
 

my opinion would be that it won’t compress down to 89mm. 
 

that’s a very expensive insulation option, have you looked at other methods. 

Edited December 21, 2024 by Russell griffiths

[sgt_woulds]

Natural insulations will always cost more in the UK as it is such a small market and buyers have to absorb the increased costs of production, shipping and Brexit red tape.

 

Unlike many countries in the EU, there are no local political forces to drive wider adoption.  In most of Europe, there is price parity due to the scale of sales; in France and Italy for instance it is very hard to specify unnatural insulations due to building regulations.

 

In the UK, the best we can say is that prices are on par with mineral wools and have better availability - rockwool costs a bomb to manufacture as it requires huge amounts of heat, and recently it needs to be ordered weeks in advance to ensure supply.

 

There are many advantages to natural breathable insulations, especially for internal insulation.

 

Regarding the OP's specific question, Steico should be cut roughly 5-10mm oversize between the battens.  I wouldn't compress the surface more than 3-5mm behind plasterboard - any more than this and you will get bowing.

 

400 centres on your battens should work well, but I'd add 10mm strips to the face as you suggest.  Installation info can be found here: 

STEICOflex_handling_instructions_en.pdf

 

It is important to note that STEICO does not generally recommend achieving high U-values with IWI due to the condensation risk to the structure.  There is, generally, a sweet spot between 40-100mm of woodfibre that balances the energy savings, cost, and condensation risk.

 

 

If it’s of interest, their Technical Director took part in a webinar that covers some of these points:    Rethinking IWI with Natural Fibre Insulation

 

Useful advice can also be found in the following links:

 Insulation and retrofit - Finding the sweet spot - The Alliance for Sustainable Building Products (asbp.org.uk)

The-use-of-natural-insulation-materials-in-retrofit.pdf (stbauk.org)

 

If used behind plasterboard, you will need a moisture vapour variable membrane, (e.g. STEICOmulti renova or SIGA Majrex® or PRO CLIMA intello plus).  These ‘smart’ membranes will limit the amount of water vapour entering the fabric but still allow the wall to ‘breathe’ during warmer periods.  Correct installation of the membrane, and sealing connections to all surrounding elements is the critical factor with this approach.

 

The best advice is to have the external walls assessed via hygrothermal software -  such as WUFI - which will take all of the site variables into account.  Internal wall insulation is more complicated than external due to the way it moves the dew point within the construction.  In addition, standard U-value calculations will not correctly account for the sorption properties of wood fibres nor their ability to pass on liquid water through capillary action.  WUFI purely considers moisture issues and how the various elements of the building fabric will deal with the volumes based on site-specific conditions. 

 

STEICO doesn't offer this additional calculation service on a site-by-site basis, but one of their distribution partners, Back to Earth, does.  Chris is a great font of knowledge regarding renovation and upgrading of older properties and is a good first point of contact when specifying IWI.

 

Siga & Pro climba with also provide free WUFI assessments if their V-VCL membranes are specified and used in a batten and board approach.

 

 

[Mike]

With one reservation (on VCLs) I agree with @sgt_woulds advice, and in particular the recommendation to get the proposal modelled in WUFI - or at least to take manufacturer's advice - since there can indeed be some site-specific peculiarities.

 

For example, from my own experimentation with WUFI, when MVHR is present (it keeps internal humidity down), and when an element absorbs little rain + has a sunny orientation, then the rule-of-thumb on choosing a conservative insulation thickness doesn't necessarily apply; that is, more insulation might reduce condensation risk within the structure, as well as retaining more heat.

 

However, if the element does absorb some rain, then the presence of even an 'smart' vapour control layer may cause a catastrophic build-up of moisture within the structure, as WUFI predicted for the North wall on my current project. In contrast, if the VCL was omitted, then there was no risk. A good illustration of why VCLs aren't a magic wand.