SIP vs timber frame garden room

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Hi all, we are considering getting an outbuilding to serve as gym/office, about 4m x 3.5m in size. We are not experts at DIYs so it looks like the most economical options are some ready-made timber frame ones or SIP kits. A few questions that would appreciate some comments please:

Roof: The timber frame one (https://dunsterhouse.co.uk/titania-garden-office-w4-3m-x-d3-3m) is using "Insulated IzoPanel", while I read here that the most preferred option is EPDM roof?

Insulation quality: Is there any significant difference between the two options please? We plan to use the outbuilding in all seasons.

Thanks!
 
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How much space will you have around your garden room to clad? If you are building tight to the boundary, SIPS wont really work. If you have access, SIPS are better as effectively you are just bolting it together. It will be light, strong and very insulated. When I built my own, I used the SIPS method to pre-fab each section and then clad and painted after. Personally I used 3 layers of roofing felt for the roof on OSB with between joist insulation and standard plasterboard.
 
How much space will you have around your garden room to clad? If you are building tight to the boundary, SIPS wont really work. If you have access, SIPS are better as effectively you are just bolting it together. It will be light, strong and very insulated. When I built my own, I used the SIPS method to pre-fab each section and then clad and painted after. Personally I used 3 layers of roofing felt for the roof on OSB with between joist insulation and standard plasterboard.
Thank you! That's a good point... The answer is not very much, as we plan to push the outbuilding (<15m2 floor area) quite close to the boundary. Is access less of an issue for timber frame ones? They also need cladding right?

I called Dunster House today and they said that their roof doesn't have felt (but insulated panel) so doesn't need EPDM? Makes any sense?
 
The SIPS installers will normally have the building up in a day, which makes it hard for you to add external cladding etc., after the event, If you don't have access due to boundary fences. It really depends If you can build the walls which are going tight to the boundary, clad and then push in to place. For me, I clad the hidden walls with treated marine ply with mastic on the joints and battens to cover. I could just about get the impact driver down the gap to screw in the screws. I then only clad the visible walls which saved a lot on cost.

These buildings are easy to build btw. Maybe look at getting a joiner to give you a quote.

The other thing to check is the quality of insulation (U-value), any electrics that are going in and of course the cost of the concrete base.

I build mine (15m2) I did it for 4k, but it would probably cost 6k now.

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This is the izo panel roof
 

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I clicked the link in the original post, the roof is fine with 100mm of insulation, the walls and floor aren't. 25mm is no where near enough. You can address the floor by standing it on an insulated concrete slab. (hardcore + sand + 50-100mm of polystyrene then a concrete base.). The U value for the roof will be about 0.2, with the insulated base the floor will be around 0.19. The walls will be about 0.8. To keep it warm in the winter is going to need about 800-1.2kw of heating. I guess they cut down the wall insulation because there is a lot of glass and doors.
 
I clicked the link in the original post, the roof is fine with 100mm of insulation, the walls and floor aren't. 25mm is no where near enough. You can address the floor by standing it on an insulated concrete slab. (hardcore + sand + 50-100mm of polystyrene then a concrete base.). The U value for the roof will be about 0.2, with the insulated base the floor will be around 0.19. The walls will be about 0.8. To keep it warm in the winter is going to need about 800-1.2kw of heating. I guess they cut down the wall insulation because there is a lot of glass and doors.
Thanks again for the very detailed comments!

For the floor, the outbuilding will be sitting on a raised concrete base (around 80cm tall) as the garden is on a sloped site. There was an old concrete base in place already and it's basically level with the highest point of the surrounding ground, so we think planning permission shouldn't be an issue. Plus all neighbours have outbuildings on similar raised platforms. Not sure if it makes a difference in insulation quality, but will definitely ask our builder to explore the polystyrene insulation idea, thanks!

For the wall, think their argument is that Polyisocyanurate is better than other insulating material. I looked it up a bit (https://learnmetrics.com/iso-r-value-chart-thickness-of-polyiso-insulation/), seems like 25mm of it can get an R-value of 5.8 so U-value of around 0.17?
 
Its basically similar to Celotex 25mm boards (celotex is slightly better due to the foil). 25mm get you around 0.91. My guess is the R values in your link are imperial and need to be divided by 5.678 which match my calcs to get a metric U or around 0.9. You'll get a bit from the other wall materials so 0.8,
Celotex CW402520x0.45x1.2 = 10.8m²R=1.10U=0.91

With 80cm - there is plenty of room for 10cm of insulation. That btw is a lot of concrete - around £600 worth. For planning its measured from the highest natural point. it doesn't really matter what was there if you are adding something on top.

For my build I used 100mm celotex insulation (walls and roof and 50mm floor on top of 100mm poly concrete base). That was a bit OTT and 70mm would have been a lot easier as the studs ended up needing spacers for the plaster board as 100mm of wood is apparently its wet measure and its annoying 95mm dry. Live and learn.

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Yes, unfortunately the concrete base we have to pay to get it done anyway, as the existing platform is crumbling away and with some rotten decking on top too, becoming a safety hazard.

I looked up some SIP kit suppliers, and found that to get our size it would cost around 5k just for the SIPs, then adding on top the doors and windows (1k), pressure treated cladding (1k?), EPDM roof, labour cost to put it together etc, it'll be pushing towards 10k anyway so the same as the ready-made timber-frame one which hopefully we can put together by ourselves...
 
Honestly, Its just as easy to build your own from scratch, vs a self assembly kit. Double glazed windows and doors are easy to find on eBay. For the money you save you could probably hire a joiner/builder to help for a few days.

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With a DIY design, you also have the option of making feature of the slope and even designing a basement like storage area:

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