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SIP Technical Build Guides

A plain-English look at how a proper SIP garden room is actually put together - walls, roofs, membranes, airflow and the little details that keep water out and heat in.

This isn't a full engineer's manual - it's the level of detail I wish more homeowners saw before signing off on a build. If another quote doesn't tell you what's happening in these layers, you're being asked to take a lot on trust.

Guide 1 - Walls

Typical SIP wall build-up

  • SIP wall panel sized and specified for the span and height.
  • Warm-side vapour control layer where required, joints taped properly.
  • Service zone / battens where needed so we're not chasing loads of wiring into the panel face.
  • Breathable membrane on the cold side as the secondary line of defence.
  • Ventilated cavity (if the cladding system requires it) so any moisture can drain and dry.
  • External cladding fixed with the right fixings and spacing for that material.
Guide 2 - Roofs

Flat / low-pitch SIP roof approach

  • SIP roof panel designed for spanning, with correct fall built in or created by firrings.
  • Attention to joints and perimeter, so the roof behaves as one piece.
  • Deck or board as required for the roof covering system.
  • Roof membrane or finish installed to manufacturer guidance - not "near enough".
  • Proper edge trims and drip details so water goes where we want it.
  • Thought given to future access for maintenance and checks.
Guide 3 - Membranes & airflow

Keeping moisture moving the right way

  • Warm-side vapour control layer continuous where it needs to be, especially at junctions.
  • Cold-side breathable membrane outside the SIPs where the system requires it.
  • Ventilated cavities at cladding and roof finishes that rely on air movement to stay dry.
  • Clear paths for any water that does get in to drain out, not sit against the structure.
Guide 4 - Openings & junctions

Doors, windows & corners

  • Openings planned around SIP joints, not cut randomly through panels.
  • Sub-sills, trays and flashings detailed so they can actually work in heavy rain.
  • Reveal details that protect panel edges and keep the vapour and weather lines continuous.
  • Corners and junctions that tie the box together structurally, not just cosmetically.

If another company can't show you a basic wall and roof build-up in writing, it doesn't automatically mean they're doing it wrong - but it does mean you're having to trust a lot. I'm happy to talk through what I do in as much or as little detail as you'd like.