insulating timber framed walls

[dragonfly]

Well it has only taken 2 years (sloooooow architect + putting hubby in charge of organisation + not nagging = 2 years) but we finally have LBC to get the render replaced on the timber framed part of our house. We have been doing some exploratory work which basically meant taking off the plasterboard that had been used when previous occupants renovated/decorated about 15 years ago. Much not to my surprise said plasterboard was painted then wallpapered and the back of it was foiled, with a lovely thick layer of fibregass loft insulation sandwiched between the board and the original external lath and plaster. In places the laths have crumbled to dust as well as much of the render being very damp to the touch (but drying out nicely now it can breath) so it must just be the cement and sandtex 'repairs' that are holding the render on at the moment. On a more positive note, most of the the timbers themselves are in relatively good condition - a rotten purlin which we knew about from the original structural engineers report, along with minor wood worm holes and a couple of scratty timbers. Having said that we are working top down so GOK what we will find downstairs when we start exposing the sole plate.

Which brings me to my question. We will obviously be replacing the render with a proper job of lath & lime. The timbers are no more than 3 inches depth. Hubby is keen to leave the faces of the timbers exposed as part of the restoration (He loves his wood!), but rebating the internal plasterwork will reduce the gap to the external laths. Obviously will not be stuffing again with fibreglass & BIL the carpenter says we could use a particular type of breathable plasterboard with an insulating backing which would leave a nice breathing gap but keep us warm. IS this an acceptable way forward?

Would welcome comments & suggestions & will attach a photo in a mo to give an idea of the issue.

 

[dragonfly]

This is part of the wall where the timbers and laths are in relatively good condition and only lime to be seen. Happy chappy is my BIL the carpenter!
View attachment 1

This is the poor purlin with dodgy pasterboard still in place on the wall but removed from roof timbers (they are another story!!)

 

[Penners]

breathable plasterboard with an insulating backing which would leave a nice breathing gap but keep us warm.

Given my amateur status, I'm by no means familiar with every product on the market, but the words "plasterboard" and "breathable" don't necessarily sit easily together.

I'm also a bit sceptical about there being enough room in a 3-inch cavity to be able to set the inner lining back (ie: expose the beams), plus leave an air gap plus insulate to any effective degree.

Can your builder give you a bit more information about the exact lining product he is proposing?

 

[Inspector John]

Just a thought
http://www.lime.org.uk/products/boards-and-backgrounds/ty-mawr-wood-wool-board/

 

[dragonfly]

Just a thought
http://www.lime.org.uk/products/boards-and-backgrounds/ty-mawr-wood-wool-board/

Ooooh! 20 products here so which would i use. I am a complete novice with this and have been reading loads of other threads over the last hour to try and make sense and still can't.

The wood/wool board seems a good idea but (unless I am completely thick) seems to have been described used on the outside of the house and rendered - I will have to use lath & plaster as that is what is there now so not an option.

Am thinking you can't stuff the gaps with sheepswool as alternative to the loft insulation (got this from another thread) without some sort of gap due to condensation issues - I don't think I have deep enough timbers to do this.

Similarly can't used foiled membranes as they don't breath.

I think what BIL has in mind is some sort of foam backed board attached to recessed battens such that the front of the board is recessed so that only the front of the beams are seen. The depth of the foam should then leave the required 2 inch gap from back of foam to the external laths. Am I completely barking or is he?? The room was warm enough before with just a sheet of foil backed plaster and a single layer of fibreglass on the sloping ceiling bit (all nicely squished against the roof felt!!) such that none of the recent snow melted off the roof before the weather warmed up, hence me thinking it was doing a good if bad job so now want to an all round good job IYSWIM.

Words of few syllables appropriate to a 3 year old is about my level of understanding but I am trying - it has taken only 2 years but I now realise that purlin, joist and truss are all different things.....

 

[Inspector John]

I could not suggest to you which product, it's still something I have to get my head around
When I was on their lime course last year they were using (apart from lime and sand mortar) hemp and lime for plastering which has certain insulating qualities
Ring or email them. They are usually very helpful

 

[Flyfisher]

Interesting topic. This is something I've been thinking about for a while, but with no really firm conclusions as yet. As you can see, we have basically the same issue except our main timbers are nearer 4 inches deep plus they have been "drylined" with plasterboard on top of a secondary frame.

The render you can see between the laths is, sadly, cement, so it will be replaced in due course, but I wanted to see 'inside' the wall to check for any damp/rot issues and thankfully - in this area anyway - there is none to be seen. However, even with 4-6 inches available I'm not sure how best to insulate while retaining sufficient air-gaps/ventilation. I have considered adding insulation to the exterior of the frame before re-rendering (lime of course) but even then there are issues around the windows.

But something will have to be done because at present the structure is little better, thermodynamically, than a double-skinned garden shed - albeit rather more substantial!

 

[MdB]

One way to choose is to look at the data sheets for each product. In particular you want to look at the Thermal Conductivity (or k value) for each part. This gives you an indication as to how much heat passes through each type of product.

Lets assume you want to bring the wall up to a low level modern performance equivalent to 50mm of Kingspan. (I think the regs are now 80mm but I'm not sure. As the building is listed, meeting the regs isn't as important.)

Kingspan has a k value of about 0.024 W/mK. Higher values indicate greater heat loss. Lower value indicate greater insulative properties.

Calsitherm Klimaplatte has a k value of 0.065 W/mK which means in rough terms you would need 135mm of thickness to match 50mm of Kingspan.

Diffutherm has a k value of 0.044 W/mK so roughly 91mm is required to match 50mm of Kingspan

Gutex Multitop is 0.044 W/mK also so 91mm

Gutex Thermoroom is 0.038 W/mK -> 79mm

Gutex Thermowall is 0.039 W/mK -> 81mm

Gutex Ultratherm is 0.043 W/mK -> 89mm

Pavadentro is 0.043 W/mK -> 89mm

Pavatherm Floor is 0.044 W/mK -> 91mm

Pavatherm Thermal is 0.038 W/mK -> 79mm

Sheep wool is 0.039 W/mK -> 81mm

Rockwool is 0.039 W/mK -> 81mm

So the best performing of the above is Gutex Thermoroom, Pavatherm Thermal and sheep wool. Lets also say that you leave the wood exposed but plaster in between level with the wood. Lets also say that your lime plaster layer is 25mm thick (this figure is likely wrong) leaving you with 50mm of room for insulation. 50mm of Pavatherm will be roughly equivalent to 31mm of Kingspan which isn't all that great. I have over 3x this in my front wall but Kingspan isn't a good choice for you application due to the lack of breathability.

I have assumed that no airgap is required as the whole structure is breathable. I am not sure if this is sensible and should be checked by an expert.

Note: all above figures are approximate and there will be quite large variations depending on where you look for figures. Due to surface effects, you can't do a straight comparison of thickness the way I have done but it does give you an idea. The figures are good enough to tell me to stay clear of the Calsitherm product.

My next port of call would be to find out which products are available in the thickness I would like and then compare costs.

I hope that is useful.

 

[Flyfisher]

Very. Thank you for taking the trouble to enter it all.

My understanding is that works to a significant thermal component of a property (and I assume the walls count as such!) usually involve BC and a requirement to bring things up to current standards.

As the property is listed, how does this work in practice? Does the CO lay down the physical constraints (wall thickness etc) and let the BCO then dictate the insulation material for best possible performance, i.e. closest to the regs as possible?, or would the BCO be satisfied with almost any thermal improvement, which let's face it wouldn't be hard to achieve in these examples!

 

[Penners]

Hmmm - seems we're back to the CO/BI "who trumps whom" question.

I put this question recently to a CO of our mutual acquaintance. His answer was diplomatic (not entirely characteristically!), but gave me the impression that predicting the outcome of such a contest is akin to a black art.