Hello,
I'm building a timber frame house 140mm studwork braced externally by 9mm OSB board, breather membrane, battens and horizontal larch cladding.
I'd welcome advice/opinions on insulation for the external walls.
I'd like to use 140mm sheep wool in the studwork, which I have been told is good in timber frame because of being able to absorb moisture. However, I don't think this is enough to get through Building Regs so I'm considering adding to the inside to improve performance and eliminate thermal bridging, but keep getting different advice.
Here's what I'm considering:
a) 25mm kingspan type rigid insulation - seems to give good performance, but would adding a layer of this detract from the benefits of the sheepswool?
b) 50mm more sheepswool with horizontal cross battens. We like the idea of sticking with sheepswool, but this loses the most space, and still would give an amount of thermal bridging where the battens cross the studwork.
c) Steicotherm rigid boards. Seems to my limited knowledge to complement the properties of the sheepswool, but performance doesn't seem to be as good as Kingspan, so probably have to use 40mm instead of 20mm boards.
Does anyone have any comments to help please?
Also, I was going to do insulation, then polythene vcl, then plasterboard. Someone has suggested using sheepswool in the studwork, then vcl, then steicotherm then plasterboard, so that fixings (sockets etc) are less likely to puncture the vcl. Does that seem sensible? Also to improve the vpl by using a product called Intello, so moisture can be redirected.
Thanks in advance for any help, I seem to be going round in circles with this one!

debbiel,

Even tho' we are in Wales, and I would love to say to use sheep's wool, DON'T!

As above, PIR is your best insulant in terms of cost/thickness/performance.
Fill the studs with 140mm PIR, (snug fit/foamed in place) then consider minimum 50mm PIR on the o/s of the 9mm sheathing, vcl on the inside of the 140mm studs, and if you need it, battened service cavity.
Part L sorted, good air-tightness will pay dividends in the future.

Good luck

Is thickness important? If not I would've thought EPS would give best bang for buck.

Like Tony, I would suggest putting insulation on the outside of the frame rather than the inside. I would also suggest using a regular TF rather than a deep one. You could use rigid insulation on the outside of the timber frame and either rigid or cellulose between the studs. Either EPS or PIR depending whether you have the space for EPS. Or you could build a frame with Larsen trusses on the outside and fill the whole space with cellulose.

If you have the insulation outside the timber frame, then the frame never gets cold enough for condensation or rot to occur, so the VCL is less important (some say not necessary).

As long as you have enough insulation on the outside to keep all the structure above dewpoint, breathability isn't an issue. I would always prefer EPS personally, but if there isn't space then PIR may be a necessity although I would then think carefully about fire risk. I also personally prefer something conformal like cellulose or mineral wool or even sheeps wool in between studs, but others have said that fitting rigid insulation is practicable if done right.

Having recently fitted PIR between stud work, I can confirm it takes an age to measure, cut and fit and is very messy stuff to cut, dust ever here! Any gaps then have to be filled with expanding foam.

Next time I'm going for high density mineral wool or cellulose between the 140mm studs and PIR boards on the outside of the studs, so as to eliminate cold bridging. Just need to sort out which gives the best u value?

Oh, and consider a passive slab for this build type.

Thanks for all your comments. Unfortunately I can't insulate on the outside of the frame as I've already done the cladding. Duh, the things you learn on your first build! So back to internal solutions. Must say I am surprised at the lack of enthusiasm for sheepswool - it's making me reconsider. My reasons for choosing it was because supporting British farmers (if you choose the right brand), the moisture absorbing properties, fire safety and the fact that it's supposed to create a healthy, non toxic environment. But maybe I've just been persuaded by the hype?!

Posted By: debbielMust say I am surprised at the lack of enthusiasm for sheepswool - it's making me reconsider. My reasons for choosing it was because supporting British farmers (if you choose the right brand), the moisture absorbing properties, fire safety and the fact that it's supposed to create a healthy, non toxic environment.

Your reasons are all valid but are very similar to the reasons for using cellulose (recycled newspapers), which to my mind is an equally virtuous but more predictable and less expensive alternative. There's a big difference between the price of raw fleeces and the price of wool insulation that makes me worry.

The problem with internal insulation is that it pushes down the temperature of everything and especially the wooden structure outboard of it, meaning they're more likely to get into troublesome humidity regimes, so careful design is definitely needed. That said, my preference would definitely be for a hygroscopic insulation in such circumstances - wool, cellulose, wood fibre, straw etc.

It might be worth talking to one or more of the firms who specialize in these types of insulation to get a recommended and warranted design.

Hi Debbiel,
Is there not an architect involved who can run dew point calcs? Unclear why the buildup has emerged as a problem at this stage in the Warranted work? There is a lot of empirical knowledge on this site, a lot of wisdom. Be wary, though, of stepping in to building physics relying on hearsay...

The steico (universal board) comes in at either 35mm or 50mm and would maintain breathability. We have had transportation damage with this product, so talk this through with your supplier beforehand- ie if tongues/ grooves are snapped or dinged you will not accept them. If in good nic, you can line out a room rapidly with it.
djh speaks sense