Newby needing advise on insulating

[OldRobin]

I have a timber framed house - half way between a pre-fab and a Scandinavian property. Wall are 95 mm wood stud filled with mineral wool with plasterboard inside and 19 mm Cedar cladding outside under concrete tile.
To improve the U value of the walls (presently 0.4 about) I was planning to dry-line external walls on the inside with foam-backed plasterboard over the existing (adhesive bond). Using 90mm foam and 12 mm plasterboard the U value becomes reasonably up-to-date (U=0.16) by my calcs.
Is there anything stopping me legally and should I get a structural engineer to advise if I am achieving a particular level of U value for when and if I ever sell? I am just wanting to make the place more comfy and less costly to heat for me for the next few years, but I could easily find it unsellable when the time comes.
Anything to worry about? Advice much appreciated.
I am a mechanical engineer, but not a building structural engineer or lawyer!
OldRobin...

 

[jeds]

Technically speaking you should apply for building regulations. The likelihood of running into a problem if you don't is small but if you really want to get it right you should apply.

 

[OldRobin]

John,
Many thanks. Just the sort of advice I was needing. I will troll off to the Planning Dept and see if they want me to make a formal application or not. Cheers OldRobin

 

[wessex101]

One word of caution.

If you have insulated timber stud walls and you add insulation internally you could move the dew point to somewhere within the existing insulated stud wall creating interstitial condensation and potential rapid decay of the timber frame.

I haven't done a dew point calculation since I was at college a very very long time ago so cannot help on that score but if you speak to some of the insulation manufacturers they may be able to do one for free. They should also be able to provide the U-value calculation for you.

 

[OldRobin]

Thanks for the comment. I will do some research. It seemed to me that the structure of the wall will be warmer than the outside air so the only issue is when there is 100% humidity (i.e. raining) No?
I will see where the vapour barrier is, but when I built the walls on an extension seem to recall tar-paper behind the cedar cladding and a membrane under the plasterboard...
Cheers for the thought
Robin

 

[jeds]

Timber frame structures must always include a vapour barrier on the warm side of the insulation. Typically you would have (from internal to external) plasterboard, vapour barrier, continuous insulation, insulation between studs, sheathing, membrane, battens, weathering.

 

[cjard]

You might find it cheaper to get insulation boards separately from plasterboard as they're more commonly used as separate than combined. Planetinsulation.com does some good deals on massive orders (over 2 grand)

On my structure I'm fitting Tyvek airguard reflective, and I'm battening out the inside to give a void for cables and also an air gap so the reflective surface of the membrane can reflect heat. Ultimately my wall construction will be.. Skim, plasterboard, service void, airguard, osb, insulation, studs with insulation, osb, Tyvek reflex breather membrane, cavity, wall

The osb inner provides a surface to mount the membrane on and something to screw the battens to. I plan to join the membrane laps with a proprietary glue and screw a batten on top to clamp the glue. Overall the aim is to reduce draughts which increases the comfort level allowing for lower room temps and heating bills..

You may get as good a result with no loss of room space if you replace your mineral wool with PIR. Thermalcalconline.com has calculators for u value and even has an example timber frame programmed in for you to alter. You need to get less than .3 for current regs. You might well also find if you install an air tightness barrier (and go round the house sealing up construction gaps), make a minor improvement to your u value and fit a mechanical ventilation and heat recovery system that you end up with a warmer house, lower running costs and cleaner air (no more farty bedrooms ) than just putting 90mm of PIR on the inside. Might also end up cheaper to throw money at air tightness rather than insulation.

Of course, one of the nuisances is that your studs bridge the insulation and your lining proposal does remove that problem. See how much difference it makes on the calcs, by going for a thinner (or no) insulation skin internally. Also, for perspective, a passivhaus wall is .15 or better, and needs to be .1 before the central heating is potentially no longer required and the ventilation system can be relied on to distribute heat. And lastly, don't forget to factor in an appreciation of u value costs and paybacks.. For my house, every 0.01 u I bring the walls down by saves me something like a tenner a year in heating bills so there's definitely a sensible point to stop on insulating and throw money at something else. Current regs are pretty good for thermal, shame that they're shocking for air tightness, and all the heat just disappears out of cracks, gaps and deliberate vents on a to-regs 10 cube per square metre per hour leak

 

[OldRobin]

cjard. Many thanks. Very helpful comments. Old_Robin

 

[cjard]

cjard. Many thanks. Very helpful comments. Old_Robin

Feel free to hit the Thanks button in the top right of the post

 

[Parrotface]

Timber frame structures must always include a vapour barrier on the warm side of the insulation. Typically you would have (from internal to external) plasterboard, vapour barrier, continuous insulation, insulation between studs, sheathing, membrane, battens, weathering.

Is the above information the final word from the planning department?

How would this apply to wooden doors in a timber framed building? Do they need insulation too? (Sorry if that sounds extremely thick I'm very new at this!)

 

[OldRobin]

Timber frame structures must always include a vapour barrier on the warm side of the insulation. Typically you would have (from internal to external) plasterboard, vapour barrier, continuous insulation, insulation between studs, sheathing, membrane, battens, weathering.

Is the above information the final word from the planning department?

How would this apply to wooden doors in a timber framed building? Do they need insulation too? (Sorry if that sounds extremely thick I'm very new at this!)

I had a couple of doors made for me and the carpenter insulated between two layers of boarding in the big panels and this seems to have made a big difference. The membrane requirement probably only applies to the structure of the main wall and not to the openings (doors and !windows).
Thanks for the reply though.