PIR plus soundproofing fibreglass too much?

[BerryNewbie]

Hi guys,

I’m building my own garden room for all year round purpose. I wanted the insulate with PIR and add soundproofing on top. But I’m actually starting to doubt whether I need PIR at all! Without the PIR, the wall layers would be (from outside in):
- 40 mm tongue and groove timber
- 2 layers of r14 (3,5 inch) fibreglass
- one or two layers of 15mm acoustic plaster board.

Do you reckon I might as well leave out the PIR? It’s meant to be an office all year round!

Thanks!

 

[bigsegs]

leave out the fibreglass, keep the pir, the more the better..
I built a summerhouse in lockdown, 3x2 cls with 60mm pir. Inside of that was 18mm ply for stiffness, plasterboard and skim. dpm on floor joists (6x2 @ 400 ctrs, suspended) followed by 25mm pir and 18mm floorboard then vinyl. roof was 3x2 cls pitched with 60mm pir, 18mm stirling board external, breather felt then felt shingles. outside walls were breather felt on thin battens for airflow followed by featheredge (used as shiplap, a mistake, it curls up on the side that gets the most sun) use shiplap if you can afford it...
we had christmas dinner in it that year (lockdown), it took a 1000w oil heater on low and the thing is toasty in the dead of winter, and retains the heat.
The fibreglass will usually sag, (cavity walls use cavity batts which are sat on the wall ties), and fibreglass is about 25% as efficient at retaining heat as pir. jablite or expanded polystyrene sheets/batts are about 50% as good as pir..
pir is your friend...
shop around, prices vary massively, dont buy little 2x4 sheets, buy 8x4 sheets and get them delivered..
seal the joints around the pir with foil vapour tape, its moisture from the inside (condensation, you breathe it out) that gets in the fabric, cant evaporate and rots your subframe....
if soundproofing is super important (kids with decks, drums etc) then youre better off building a box within a box as it were and introducing your soundproofing to that 'skin'.. i.e. green dense fibreglass battens, 2 layers of blue soundblock plasterboard and whatever else you need for the corners for the bass etc..
youll thank me in a few years..