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Garden Room build - SIP vs timber frame vs blockwork - advice please

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Hi I've just had permission to build a large garden room 12m x 5m which will form a gym and entertainment room. It needs to be built to building regs due to size. It'll have a small kitchen/bathroom. 

I've spoken to numerous builders (some I know and some getting quotes) and each seems to suggest the best way to build is either SIPs, Blockwork or a timber frame. I'm not a builder so don't know which to choose.

Can anyone advise on the benefits/negatives of each and which might be most cost-effective option please?

Other questions:

Would you go for an insulated concrete base? and with insulation under the concrete or over it?
How do I make sure the floor is strong enough?
If it was timber frame or blockwork what insulation would you use - PIR or other?
Would you go for a warm roof or cold?

Any other major considerations to think of?

Thanks Rob

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Posted (edited)

I did mine from a kit, it's 70mm thick wood, easy and quick to build. Then built a standalone from inside for additional insulation, same with floor and roof.


Things to consider

Lightweight building equal really hot when sun comes out. But quicker to heat up in winter, but cools soon after switching the heat off.


Heavyweight more stable temperature but takes longer to heat up in winter, but slowly cools.


Timber frame and dense insulation. Yes, PIR no.


Block work, mineral wool batts in deep cavity yes, PIR no.


SIP no

Edited by JohnMo
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9 minutes ago, Robhan said:

Thanks. Why don't you like SIPs or PIR? 

Quite a small building will suffer massive overheating if your not careful. Ours with 70mm wooden walls with 140mm frametherm mineral wood inside and about 200mm in the roof got to 27 yesterday afternoon.  A sip would heat up quicker.


You need a high decrement decay buildup, hence not liking sip or PIR.  Floor PIR is fine.

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