Garden Room - Insulated plasterboard, yay or nay?

[Amelia Winters]

So I'm building myself a garden room. In winter. Yes my timing sucks a little for that but it's keeping me busy.

 

So far I'm at the following;

Walls are timber (95x45 @ 400c), sheeted on the outside with 11mm OSB3 and Tyvek wrap and battoned for the eventual cladding that'll go on there.

Roof is 47x175 @ 450c, 18mm T&G OSB3, Polythene VCL, 100mm PIR, 18mm T&G OSB3 - warm room construction (I've duct work to fit in the joist spaces, hence that choice).

 

Walls are to be packed in the voids with 100mm Knauf Acoustic Roll insulation but I'm pondering my choice of straight 12.5mm Plasterboard, or 37.5mm PIR backed board for the ceiling and outer walls (there is an inner cupboard which I'll just use regular PB on, don't see the point in PIR backed for that).

 

I'm swaying towards PIR backed. Yes it'll only be 25mm PIR, but figure that's roughly another 50mm of equivalent in the wool insulation, and gives me a little more depth to use 40mm boxes for sockets, data outlets and fibre stuff.

 

I'm considering interstitial condensation, as I'm guessing I'd effectively be doing some sort of hybrid insulation style - particularly if I use that to board the ceiling?

 

What's your thoughts - is it worth doing, even though it will cost more in materials? If so, outer walls, ceiling or both and should I still use a polythene VCL between it and the timber/wool layer?

 

Apologies if this is a dumb question - I'm reading lots of different opinions on this and getting myself in a bit of a muddle over it!

[Russell griffiths]

I’m not as clued up as some on here with insulation values, but I would say your 100mm acoustic roll is very inadequate.

If you are set on that, then I would add 50mm pir on the inside then a timber batten for a service void then plasterboard.  

[oliwoodings]

Yeah is there a reason you're going with acoustic insulation in the external walls? The thermal performance of that is going to be shocking. And the acoustic performance won't be great either because afaik you're meant to leave an air gap for the acoustic insulation for it to work best.

 

Save yourself some time, money and space. Use 70mm PIR in-between the studs, which leaves you with a service void for cables etc. Then just regular PB over the top.

 

If you want more insulation, you can get 90mm PIR and then batten out the studs to create your service void.

[Amelia Winters]

Ok doke, thanks for the feedback.

I wasn't set on Acoustic wool fully, my thought process was simply that with it being outdoors and next to neighbours that a wool product would provide somewhat of an acoustic layer to somewhat isolate outside noise - more than rigid PIR would anyway.

 

I worked under the assumption (maybe wrongly) that rock/mineral wool is roughly 50% as efficient as PIR so would result in a similar  performance to 50mm PIR. As I say though, very much anecdotal from what I'd read online and possibly incorrect.

 

I will look into using 70mm PIR instead. Whilst I completely hate cutting the stuff it sounds like it would give me much better performance at the cost of acoustic transmission.

Not building a recording studio or anything like that - but just something to block out the neighbours when they want to listen to music outdoors in the summer!

Edited January 16 by Amelia Winters
Typo

[Russell griffiths]

You could look at knauf Omni fit  and fully fill between the studs, then 50mm pir over the top on the inside. 

Really depends what you want the building for and what months of the year you will use it. 

 

Give us a better idea of what hat you are trying to achieve. 

[Amelia Winters]

Looking ideally to use it as a home office really, need an extra room that ideally isn't in the house for the kit I need for work to sit - so pretty much all year round use, but mostly in daytime hours. 

Not too concerned about noise, but would like it to not be fully acoustically transparent - mainly from outside noise sources as opposed to inside.

 

Happy to compromise a little on thermal efficiency, it's going to be heat-pump heated as luckily the neighbour can install and commission them

 

Thank you all for the advice

[oliwoodings]

I have just completed a garden studio for my wife's massage business. Almost identical wall lay up to yours, with 100mm PIR, 45mm service batten and regular PB. Inside it is quieter than the house - and that's even with one wall being mostly glass. For a home office, 70mm PIR will do you absolutely fine both thermally and acoustically.

 

If you want to save some cash, check out https://www.secondsandco.co.uk/ . They buy up insulation 'seconds' and resell them. Their high quality seconds are more than adequate for a garden building, I saved a good £700 going with them. 

[Russell griffiths]

You could probably do with saying how big this is, and what the floor is made up of. 

My wife has a garden office that is poorly insulated in my view, but stays warm enough for her to work very comfortable all year round, with only a small oil filled electric heater, I think this is due to its small size. 

I think if you doubled its size the cost to heat it would be painful. 

You only buy the insulation once, you will pay to heat it forever. 

[Amelia Winters]

Perfect, thank you! I'll bin off the Acoustic roll and go with 70mm PIR and bust out the foam gun to seal it all up well. There will actually be two full walls that I don't need a service void in at all, so I'll go 90mm for them. That'll give me enough of a service void to avoid needing to batten the walls out where I need them. Would I be correct in assuming if I go PIR and seal it all with foam I can forgo the need for a vapour barrier sheet behind the PB, or is that still preferable?

 

Size wise it's 3.2m x 4.6m, 2.4m to rafters.

Floor is 120x47? joists, voids filled with 100mm pir & foamed, seams foil taped & 18mm marine ply over the top.

Roof is warm roof, 18mm OSB3, Poly barrier sheet, 100mm PIR, 18mm OSB3

Walls are 45x95 on 400 centres, 11mm OSB3 sheeting outside, Tyvek wrap & counter battening for eventual cladding.

One Window, One Door - Roughly 2.8sqm Glass area

Edited January 16 by Amelia Winters
Question about VB

[Russell griffiths]

Tape all the pir with foil tape, over the studs and any fixing holes. 

You might need a bit of ventilation in there. 

Maybe one of the new fangled positive input things, not my thing so ask on here for some guidance. 

[Iceverge]

I'll do a mock up shortly. But I would counter the idea that PIR between the studs is the way forward. 

 

Too dear, too much waste, to awkward to fit. Timber and PIR will expand and shrink differently and the boards will come loose. 

 

 

 

 

[Iceverge]

[Iceverge]

One of the last two options would be my choice. 

[Amelia Winters]

5 hours ago, Iceverge said:

One of the last two options would be my choice. 

Thanks for all that - makes for some interesting results, some not what I expected but it's all good to know. My choice is probably the latter of them as its the simplest for me to do given how far finished the outside surface is already.

[Iceverge]

You could jump to 50mm PIR on the wall without services or whatever you can fit. 

 

The 400cc studs really have a hit on the performance of the wall at that thickness, the calculator underestimates the true % of timber too. 

 

I would foam the boards and tape them with foil tape. Tape them to the windows, doors, floor and ceiling and you'll have a good airtightness layer too. 

 

 

 

 

Edited January 17 by Iceverge

[Amelia Winters]

Yeah. Im sorta thinking 95mm on the walls with no service voids and a combo of 95mm and 70mm on those that do - the 95 being in stud voids with no services, the 70mm with so at least the majority of the stud voids (all bar 2, maybe 3 of the voids on the external walls) would be 95mm PIR

 

All of course cut slightly undersize and foamed into place and foil taped at seams / over studs & noggins. Sod trying to get PIR to an exact fit - tried that once. Nightmare. 

Someone told me "Cut your best and foam the rest" - so I guess that applies here too!

[Iceverge]

For my 2p worth I was thinking about 50mm PIR Inboard of the studs with mineral wool in between. 

 

 

 

[Amelia Winters]

Do you mean 50mm pir against the osb outer, then mineral wool to fill the rest of the void between pir and plasterboard?

[Iceverge]

From the inside out. 

 

Skim,

20*70mm battens for service cavity. 

50mm PIR on top of the studs. Taped and foamed. 

44x95mm studs infilled with mineral wool. 

OSB sheathing.

Breather membrane. 

 

 

 

[Iceverge]

There's often debate about this on here but I'm certainly in the camp of avoiding Rigid insulation between studs. Pl

 

Perhaps you could do a wall of each if you're curious and report back.