Best wall insulation construction for garden room. PIR vs Mineral Wool vs Wood fibre or a combination

[Robhan]

Hi 

We are building a large garden room 12m x 5m and around 2.5m interior height. 

 

We have decided to go with a timber frame build rather than concrete block or SIPS.

 

I'm not sure whether to put mineral wool or PIR between the timbers. I know the u-values of PIR are better but I've also read that you can get gaps and also shrinkage which will negate the benefit. I'd also like a continuous second layer of insulation to decrease thermal bridging and improve overall u values and decrement of delay (keeping the building cool on sunny days). So either a 50m layer of PIR, insulated plasterboard or wood fibre (wood fibre is better for helping with the decrement of delay). 

 

Also I've read that insulating the outside of the wall with the 2nd layer - so for example putting the 50mm of wood fibre on the outside of the wall is better for condensation. Is that correct? If so why do people insulate on the inside of a new build? 

 

I want to keep costs reasonable and ideally if the wall could be around 30cm width or less - but if a little more that is OK as we have plenty of room. I also need to consider wiring. 

 

I'm thinking 100mm of PIR and 50mm wood fibre or 140mm mineral wool and 50mm wood fibre or PIR. I'm not sure about the floor or roof. 

Does anyone have any opinions/insights they could help me with please? What thickness of each insulation should I use and what is the optimum construction to have a good u value and keep cool on hot days? 

 

thanks Rob

[JohnMo]

We did 140mm (I think) mineral wool. With a small building you are getting diminishing returns quite quickly. We only need a kW to heat even when minus lots outside. Adding more wall insulation wouldn't have changed that much.

[Robhan]

Thanks. It's quite a big room though at 60 sqm. Do you get any overheating issues on hot days as I know mineral wool has a low decrement delay?

 

A builder friend in the village said he used 140mm SIP + 50 extra PIR on his own recent extension but I'm wondering if that is overkill? By the way we are going to use battened cladding on the outside.

[JohnMo]

Ours is 70mm thick wood, then the 90mm Frametherm 32 mineral wood (not 140mm) in the walls and plasterboard, 200mm of EPS70 and YBS SuperQuilt in roof and I think 200mm of EPS70 in the floor.

 

We have a fan coil for heating and cooling. But yes it can get hot, because it's angled for the views and the glazed end gets the full sun.