Before I could start work on the foundations, I needed to finalise the construction details for the shed base. If you were to do a timber frame build, with lengths of C24 treated timber, you probably have some more control over the exact layout of the shed base members. Two great resources for that kind of build are the Oakwood Garden Rooms YouTube channel and Ali Dymock’s YouTube channel. There is an alternative to timber frame building though, in the form of Structural Insulated Panels (SIPs).

Two SIP panels joined by a spline

Image from Big Sky R-Control SIPs.

Broadly, SIPs are two sheets of material (typically OSB) with a foam core. The tight bond between the foam core and sheets makes a load-bearing panel that can be used for walls, floors and roofs. The panels are cut to order and are delivered to your site ready to be installed. In theory, SIPs should be quick to assemble and provide a well-insulated, airtight building.

Quotes

I reached out to a few SIPs manufacturers for quotes, sending them the planning drawings and the 3D SketchUp model. The quotes I received were:

  • Around £9,100 + VAT from SuperSIPs.
  • Around £6,600 + VAT from SIPS UK, with an installation cost of £2,700 + VAT.
  • Around £9,400 + VAT from SipsEcoPanels, with an offer to install for £1,125.

I also considered buying the panels and cutting them to size myself but, in retrospect, I think this would have been slow and messy.

I chose SIPS UK because they were the cheapest and had OK reviews online. The quote was for 170 mm wall and roof panels and 100 mm thick floor panels, with nominal U values of 0.15 W/m²K and 0.28 W/m²K respectively. These are better than the current wall requirements for new builds in England, which are 0.18 W/m²K, but would not meet the roof and floor requirements of 0.11 W/m²K and 0.13 W/m²K respectively. See U Value document at https://www.sips.uk.com/downloads/technical for more.

Process

Once I accepted the quote from SIPS UK, I was put in contact with their technical designer, who prepared a detailed set of drawing, including all panels and splines.

They also got a structural engineer to do calculations for things such as lintel sizes and roof beam sizes, given wind and snow loads in my area.

Lastly, they provided an unofficial recommendation to use 15 pads (in three rows of five) if I wasn’t using solid raft foundation. They felt that the floor panels (relatively thin at 100 mm) wouldn’t comfortably span the 2.5 m front-to-back, and thus 10 pads (in two rows of five) wouldn’t be enough.

Preparing for Assembly

Unless you’re using the thinnest SIP panels, and perhaps even then, you’ll need help assembling the building. Having watched @NewcastleMatt’s videos on YouTube, I thought it might be possible to assemble by myself with a DIY crane, some ratchet straps, a bit of wooden scaffolding, and help on the weekends from friends. I was talked out of that so posted a job on mybuilder.com.

After a couple of email exchanges with a few building teams, Jamie Micallef-Creates Building Solutions looked like the most promising. Jamie came by to take a look at the site, and we agreed to work on a day rate basis, i.e. I could have him and his team of (2-3 people in total) for as many days as I needed at around £250 per person per day.

Note that the installation quotes seem reasonable and that’s a valid alternative to hiring a general contractor.

Speaking to Regi, from SIPS UK, designer allowed me to work out what I would need to provide for assembly:

  • 4.5 x 50 mm exterior screws (x500)
  • 4.0 x 40 mm interior screws (x500)
  • Soudal Expanding Foam B3 (x2)

The screw sizes aren’t critical since they are simply used to secure the panels along their edges (e.g. attaching panels to the floor plate or to splines). I’ve seen other people use nail guns, which makes sense if you have one, but they are less generally useful for the kind of projects I do.

The SIPs come with some kind of unbranded expanding PU glue so you could get by without the expanding spray foam, but I wanted to be certain of air tightness.

Thoughts

On one hand, SIPs have a noticeably higher cost than buying lumber and Rockwool. It could also be argued that SIPs are environmentally unfriendly due to the foam core. On the other hand, SIPs have less thermal bridging and are quicker to assemble. I expect &shed could have been built to a similar standard with more traditional methods, but I was curious about SIPs and knew that they would help me to reach my U-value and air-tightness targets.

Wall construction with staggered studs

If I was building with traditional framing, I would have used staggered studs to avoid cold bridging. I haven’t seen this used on any garden room builds, but it seems to be the standard for timber-framed Passivhaus construction.

Paying a day rate to have 1-3 people help out worked well, as we’ll see in a later post. Not only did the team have the muscle to handle the panels, some of which were a three-man lift, but they also had tools and experience to make the job run smoothly.

What I’d do Differently

I’d have used the great materials pricing calculator at https://www.diysheds.co.uk/wizard to speed up costing a conventional build, to compare with SIPs. It’s free, easy to use and seems to be impartial.