Green Building Forum - Insulated Slab and Timber Frame - Best Practices Sun, 19 Apr 2026 10:52:15 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Insulated Slab and Timber Frame - Best Practices https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310203#Comment_310203 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310203#Comment_310203 Sat, 14 Feb 2026 16:01:38 +0000 Victorianeco I’m looking for some guidance and shared experience as we move into the early stages of a rear extension project.
We’re proposing a single-storey rear extension, approximately 10m wide x 4m deep (c.40m²). There is a possibility it could become two storeys, but at this stage my wife and I are still undecided. Our intention is to start making progress this year, and from April we’d like to begin with the groundworks so we can at least get something tangible underway.
We are strongly considering a timber frame build, ideally on an insulated slab with underfloor heating within the main slab. At present we’re undecided on the roof design:
Pitched roof (possibly with eaves), or
Flat roof incorporating 1–2 roof lanterns
The rear elevation is likely to have two or possibly three sets of French doors, although we may also consider sliding doors.
We fully expect to engage a structural engineer for calculations, but before getting too far down that road I’m keen to understand best practice from those who have done something similar—particularly from a low-energy / green building perspective.
Some of the specific areas I’d appreciate advice on:
Insulated slab vs traditional strip foundations
Given the footprint (~40m²), would an insulated slab typically be preferable, or are strip foundations still a sensible option in this scenario?
Slab depth, insulation type and thickness
Any guidance on typical build-ups, edge detailing, and thermal performance targets would be very helpful.
Timber frame wall build-ups
From reading previous posts on this forum, a timber frame with OSB sheathing for racking strength and an EWI system externally seems to be a robust and efficient approach. I’d be keen to hear views on this and any lessons learned.
Roof build-ups
Whether flat or pitched, are there recommended best-practice details for insulation, airtightness, and moisture control?
Building in masonry is still an option, but one reason we’re leaning towards timber frame is that my 16-year-old son is currently doing a carpentry apprenticeship, and this feels like a great opportunity for him to be involved and develop his skills.
If anyone can point me towards online resources, technical details, drawings, or case studies covering slab, wall, and roof best practice—or share their own experience with a similar project—I’d be extremely grateful.
For roofing finishes, if we go pitched I’d likely consider Eternit / fibre-cement slates, possibly aluminium-clad profile sheets, or other lightweight profiled systems.
Thanks in advance for any advice or pointers—really appreciate the collective knowledge on this forum.]]> Insulated Slab and Timber Frame - Best Practices https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310205#Comment_310205 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310205#Comment_310205 Sat, 14 Feb 2026 16:53:44 +0000 djh Posted By: Victorianecoideally on an insulated slab with underfloor heating within the main slab
Insulated slab vs traditional strip foundations
Slab depth, insulation type and thickness
Any guidance on typical build-ups, edge detailing, and thermal performance targets would be very helpful.
You'll want an insulated slab in any case, especially as you're thinking of UFH. So I think what you're really asking about is the edge design. There's a useful guide at https://www.kore-system.com/wp-content/uploads/2022/02/KORE_Passive_Slab_Design_Guide.pdf that shows how that detail depends on exactly what construction you decide on for your wall. We used their EPS under our slab, which was designed by Hillyard Tanner who is very familiar with the products. Most of ours was grey EPS 90 rather than the white they mention, but the EPS300 is white. Eliminating cold bridges needs most detailing. Detailed design also depends on what loadings you're going to impose on the slab. Ours has extra depth and reinforcement at places for load bearing wall and floor supports.

A timber frame with OSB and EWI is a reasonable choice. EPS is a good choice for the insulation but maybe wood fibre slabs have better environmental credentials. An alternative choice is a double timber frame (Larsen truss) with the space in between filled with Warmcel. We have a few small parts done like that with woodwool boards on the outside, finished with render. There's panelboard on the inside and skimmed plasterboard on battens inside that. We used an Intello membrane for airtightness and vapour control.

With roofs again it's the detailed design of the junctions at eaves and verges that needs care. There's some examples shown at https://www.firstinarchitecture.co.uk/passivhaus-roof-details/ or https://detail-library.co.uk/product-category/roofs/passivhaus-roofs/ or https://mbctimberframe.co.uk/passive-roof/

Oh, and I'd really recommend using PHPP for the thermal modelling.]]> Insulated Slab and Timber Frame - Best Practices https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310207#Comment_310207 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310207#Comment_310207 Sat, 14 Feb 2026 17:00:23 +0000 Nick Parsons Insulated Slab and Timber Frame - Best Practices https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310208#Comment_310208 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310208#Comment_310208 Sat, 14 Feb 2026 17:14:58 +0000 Peter_in_Hungary Posted By: VictorianecoWe’re proposing a single-storey rear extension,At present we’re undecided on the roof design:............
Pitched roof (possibly with eaves), or
Flat roof incorporating 1–2 roof lanterns
I have a bit of difficulty in seeing how a pitched roof would work with single story extension on a 2 story house.

Posted By: VictorianecoThere is a possibility it could become two storeys

If there is a reasonable probability it will be 2 stories then construct the wall to take the second story without alteration and go for a flat roof with roof timbers sufficient to become floor joists when you go up a floor.

Posted By: VictorianecoInsulated slab vs traditional strip foundations

By insulated slab I presume you mean a reinforced concrete raft vs. traditional strip foundations. Your local BCO might make this choice for you !! If you go for a raft then probably raft - insulation - slab containing UFH and timber walls built off the slab with the wall outer sheathing going down to the raft. If there is rebar in the slab then 100 grade EPS for the insulation would probably suffice.]]> Insulated Slab and Timber Frame - Best Practices https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310209#Comment_310209 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310209#Comment_310209 Sat, 14 Feb 2026 17:15:19 +0000 Nick Parsons Insulated Slab and Timber Frame - Best Practices https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310210#Comment_310210 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310210#Comment_310210 Sat, 14 Feb 2026 17:17:10 +0000 Victorianeco
The rest of the house isn't going to be Passive Haus performance levels so realistically is there any benefit over engineering 1/5 of the footprint using PHPP?

Nick: we think a 2nd storey will darken the house too much so probably not even on the radar at this point. The house is fairly large anyway and there's only 3 of us. So realistically I think we'll skip the 2nd storey design. But perhaps the slab should be engineered 'just in case' as a future selling point.

We're also thinking the glazed area should be around 20-25% of the floor area to allow sufficient light in as it's north facing and we'd want to minimise heat losses. We assume this % would include rooflights and windows?

Would such design be best built using SIPS panels including the roof? Or build as we go ie. buying timbers, insulation etc separately?]]> Insulated Slab and Timber Frame - Best Practices https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310211#Comment_310211 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310211#Comment_310211 Sat, 14 Feb 2026 17:19:35 +0000 Peter_in_Hungary Posted By: Nick ParsonsWhy not frame the flat roof ('first floor') with a temporarily-filled-in stairwell?
Unless access to the 2nd story of the extension is through one of the existing rooms of the house (typically through an existing window) which avoids a second staircase in the house.]]> Insulated Slab and Timber Frame - Best Practices https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310212#Comment_310212 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310212#Comment_310212 Sat, 14 Feb 2026 17:20:28 +0000 Victorianeco
I quite like this approach as I don't think there are many local builders near me who about this approach and basic extensions is an area I want to grow my own business (we specialise in intense repair work at present)

If we can master basic single storey extensions using modern energy practices I think this could become a niche sector locally]]> Insulated Slab and Timber Frame - Best Practices https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310213#Comment_310213 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310213#Comment_310213 Sat, 14 Feb 2026 17:36:09 +0000 Nick Parsons
''Unless access to the 2nd story of the extension is through one of the existing rooms of the house (typically through an existing window) which avoids a second staircase in the house.''

Indeed! Typing with my brain switched off! (Though 2 staircases can be fun if you have kids!)]]> Insulated Slab and Timber Frame - Best Practices https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310214#Comment_310214 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310214#Comment_310214 Sat, 14 Feb 2026 19:12:21 +0000 Peter_in_Hungary Posted By: VictorianecoVikinghouse website and the link djh has posted both suggest EPS100 for main slab and EPS300 for load bearing walls.
The 300EPS is where the EPS carries the load of the walls. If the load is on the slab with rebar (typically 15 x 15 x 8 or 10 mesh) then the slab takes and spreads the load and in this case 100EPS should suffice.]]> Insulated Slab and Timber Frame - Best Practices https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310215#Comment_310215 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310215#Comment_310215 Sat, 14 Feb 2026 21:00:54 +0000 djh Posted By: Victorianecois there any benefit over engineering 1/5 of the footprint using PHPP?Using PHPP doesn't commit you to over engineering, but it is a better thermal modelling tool than most.

Rooflights are bad from a thermal perspective, particularly if they're shaded by the house and only see north light. Triple glazed windows might well be worth while if north-facing. Maybe worth designing in some external shutters.

Gareth, if you want your son to gain experience then stick building will allow him to learn more, I'd think.

Hilliard produced quite a complicated looking design for our slab, that varies in thickness over its area. There's rebar throughout to give tensile strength and it's thicker where there is load on top of it, plus in strenghening ribs. As PiH says, the heavily loaded parts use EPS 300 and the rest uses EPS 90 (pretty similar to EPS 100). Your structural engineer will decide what's needed where for you.

Posted By: Peter_in_HungaryI have a bit of difficulty in seeing how a pitched roof would work with single story extension on a 2 story house.
Presumably a monopitch sloping down away from the house.]]> Insulated Slab and Timber Frame - Best Practices https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310216#Comment_310216 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310216#Comment_310216 Sun, 15 Feb 2026 08:54:06 +0000 fostertom Posted By: Victorianecowe think a 2nd storey will darken the house too much
Posted By: Peter_in_Hungary... Unless access to the 2nd story of the extension is through one of the existing rooms of the house
etc etc. Solving such questions, like getting light in, even on the north side, or where at first sight it'll be blocked, finding collateral benefits such as easy access to attic, good 'feel' effects rippling wider through the house, and do you even need so large an extension, or at all, with some internal reorganisation of existing, is where a good architect can synthesise something unexpected that exceeds imagined expectations. Certainly, pitched roof on single storey can work just fine, in fact can be spatial key to beneficial effects.

If you're thinking of
Posted By: Victorianecotimber frame with OSB sheathing for racking strength and an EWI system externally
then simply carry same up and over the roof, same detail just tipped over, seamless together at the eaves.]]> Insulated Slab and Timber Frame - Best Practices https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310220#Comment_310220 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18405&Focus=310220#Comment_310220 Sun, 15 Feb 2026 10:49:02 +0000 Victorianeco
However would there then be a way to make benefit from both morning son from the East and as the sun passes around the property from the west in that case we're to make sense to have a dual pitch roof sloping from left to right as opposed to a single monitor roof sloping from the house toward the garden full stop or would of course the values or roof lights in the single monopitch roof wood fulfill this purpose and anyway

Would a 25% total glazed area in this room makes sense but would also like to bring the garden into the living area so these are strong considerations to take on board.]]>