We are just starting the build of a modest single storey kitchen extension. Two of the walls on this build are specified as stud work, partly because I can build these myself, actually mainly because I can build them myself. I am keen to achieve a good level of insulation in these walls; both walls have a door and window amounting to about 40% of the wall area.

I have been reading this forum for some months and have consulted other sources such as info on the Findhorn build and the articles by Joseph Little architects, but still at a bit of a loss to know what to do for the best.

The architectural technician has specified these walls as 140mm stud work at 600 centres, with 140mm flexi rockwool between studs, inner face with VCL followed by 25mm battens and 12.5mm plasterboard and skim, outer side of studs having breathable membrane then 25mm studs finished with 25mm vertical timber cladding, the U value specified is 0.28W/m2K.

I like the idea of adding an insulating sheathing to the exterior of the studs, and there seem to be a range of products such as Pavatherm, Steico therm, Gutex, Lion Fibreboard, and even OSB. My research suggests that this sheathing should be breathable but weatherproof. My reasons are to increase overall insulation levels, to minimise the thermal bridging of the studs and hopefully to use the sheathing as a sort of skin to brace the studwork and save me having to put in diagonal braces within the studs.

My questions are:
What experiences to people have of suitable products used in this way?
What thickness should I go for?
Do any of these insulation boards have structural strength such that they can brace the studwork?
Do they still need to be supported by a breathable membrane?
Are there other things that I should be thinking of such as different forms of between stud insulation or internal insulation, or foil backed plasterboard?
Am I barking up the wrong tree?

I suspect the 140 stud was specified in order to get the required level of insulation rather than a structural requirement. If you are going to use some form of EWI and between the stud insulation as suggested by DaryIP (sounds like a plan!) then I suggest you ask the architectural technician what size studs are needed for structural purposes.
Peter

Cobbles,

2 storey houses are built from 89mm stud work, so strength should not be an issue.
If you sheath the o/s with OSB, then insulate on the outside of that, you will keep the studwork at room temps, so no condensation issues should occur...

Cheers...

Posted By: DarylP "you will keep the studwork at room temps, so no condensation issues should occur..."

Not strictly true as it will depend on the internal RH and temperature gradient across the wall.

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=7914&page=1#Item_1

ST.... Yes you are right, I knew as soon as I hit Enter that someone would come back at me! :)
Perhaps running a condensation risk analysis would give a more predictable result...?

Cheers...

ST,

I have done a back-of-fagpacket risk anaylsis, 80mm Kingspan K12 on the o/s of the OSB sheathing mitigates any conensation risk, Feb being the worst. This is assuming high occupancy, kitchen.

Cheers...

Posted By: cobblesRH

Relative Humidity

PIR/PUR are the most expensive of the ones you list & will give the best U values, but are foil faced & so not vapour open. This means you need to make sure you have a good vapour barrier on the inside. Plywood with taped joints might be good enough. It would be safer to use polyethylene.

XPS is slightly cheaper than PIR/PUR & has slightly higher U values, but is closed cell & so not vapour open. I prefer it to PIR/PUR because its a homogenous/uniform product which can be layered without adding unnecessary foil layers, but you still need to take care with the vapour barrier. This is my preferred insulation for use below ground because its less sensitive to ground moisture.

EPS is significantly cheaper than XPS, has slightly higher U values than XPS & has a small degree of vapour permeability. The consensus seems to be that as long as you get the air barrier right then you don't need to worry about its vapour permeability. A graphite loaded type of EPS, often called Platinum EPS, offers similar U values to XPS at a price somewhere between the two. This is my preferred material for external wall insulation or filling existing cavity walls.

Between studs or rafters, or when fully filling new build cavity walls, I would use Knauf Rafter Roll 32 or Knauf DriTherm 32 glass mineral wool batts. The U values are similar to Platinum EPS, but they are totally breathable & they are flexible enough to adapt to timber as it twists & bends during drying out.

So how about k=0.032 glass mineral wool between 140mm studs & platinum EPS to the outside of the studs?

David

Cobbles,

email me at daryl at dkpservices dot co dot uk with the exact spec, and I will give you a U-value calc this pm, no worries.

Cheers...

Its partly windproofing; with the OSB inside the insulation you need a wind barrier on the outside to reduce thermal bypass & wind washing effects.

However, it's mainly weatherproofing to keep wind drive rain out of the insulation zone. It has a very low vapour resistance, while keeping liquid water out, so overall the insulation should be a lot dryer.

David

Posted By: cobblesI was slightly disappointed as my original wall spec put together by my architect (not an eco specialist) had a value of 0.28W/m2k, so after all my research I have saved 0.02W/m2k - which may or may not make a big difference, the numbers don't mean much to me - why can't we have a measurement system, such as "shorts and t-shirt", or "extra jumper", or "jumper, scarf and wooly hat" ? would be a lot more meaningful.

So, this is final call for suggestions to improve on my formula - without a radical change to thickness or basic method - I have already got the 100mm studs for example. Is there something I can use to replace the plasterboard? I promise to update this thread with photos and comment on what I finally build. The build is already in progress, foundations and slab in and block work started.

The numbers are just a matter of getting used to them. Unhelpfully, better insulation has smaller numbers. 0.1 is very good - passivhaus level; 0.3 is the worst - minimum building regs (IIRC). Things go in proportion. To achieve 0.1 takes three times as much insulation as to achieve 0.3. So 0.26 is a bit better than legally required, but nothing to write home about.

The actual thickness needed depends on the type of insulation used, and the 'figure of merit' is called lambda. Again, smaller is better. Generally, 'natural' insulations tend to be a bit poorer in performance than 'man-made' ones. Your Gutex has a lambda of 0.039 whereas EPS tends to be in the low 0.03+; closed-cell insulations are better and are somewhere in the 0.02+ region. Aerogel is below 0.02.

So how to improve your walls? The most obvious improvement is to insulate your service cavity. As well as all the pipes and wires, you can fill that cavity with rockwool or suchlike to make a small improvement. Beyond that you either need to switch to materials that have a lower lambda, or add more thickness.