Hello

Have read your various posts in recent years which I have found really useful, informative and insightful.

I am building a tricky extension, two storeys with flat/green roof/parapet (due to angled relationship to existing house, which is a 1976 bungalow with undercroft garage, like a bank barn). Would build pitched roof otherwise. Dimensions ground floor 7.5m x 6m, first floor 5.5m x 6m. So quite large.

Extension will have lounge/utility downstairs. Bedroom with ensuite upstairs. Intending to retrofit the house with external insulation and vertical timber cladding + MVHR. It is presently a standard 250mm cavity wall, no cavity wall insulation (which I will do before I fit EWI), harling to outside. Well built, dry cavities but ugly and walls that currently feel like a refrigerator!

Intending to insulate the new extension well and have sizeable full width sliding doors to south elevation giving access to the garden. Render proposed to ground floor walls, timber cladding above. Tight budget.

I was originally going for timber frame but I am paranoid about leaking flat roofs in our wet location (Cumbria!) and water finding it's way into the structural parts without me knowing! Plus more experience of laying blocks and stone - I am an "experienced" DIYer having previously built a cavity wall outbuilding in block and stone at my previous house. Plus done up two Victorian houses. Still working full time unfortunately but retiring this time next year.

My question is which way to go in terms of construction. Was planning to do 100mm cavity wall with 425mm teplo ties (which I bought cheap on EBay), cavity 275mm full fill. But now having second thoughts.

Cavity walls are such a fiddle! And what's the point if I am covering with timber cladding? I also have to keep that cavity insulation dry during what is certain to be a slow build. It pours down here at most times of the year, except some springs.

Looking at all your previous posts, and "Understanding Passivhaus" by Emma Walshaw, I am, instead, attracted to building a single block work wall, 215mm thick with blocks on their flat side over 100mm thick Marmox thermoblocks at the base. The external side would have 300mm EPS/XPS at the base, with 300mm wide Larsen trusses above dpc filled with mineral wool. All cross battened and clad.

I can do all this work in a nice linear process - get the walls up, roof on, make the trusses inside, hopefully over winter, then insulate/clad in the following spring/summer. Much easier for a DIYer. Seems a lot simpler than cavity wall - ie. cavity trays, weep holes, cavity ties, keeping cavity clean, full-fill insulation all as we go along etc. Then having to protect wall tops and insulation as winter rain/storms arrive. In fact, this weather can arrive at any time of the year so it's a constant issue.

I am also hoping my SE says I can avoid a structural goalpost frame for the big slider at ground floor with this thicker wall, if I allow a single block for a nib. Again the frame makes cavity construction more or a phaff.

The questions I have are:

1. Cavity wall v single leaf/larsen truss? Which way is easier/better? Building Control are a bit old school so not sure what they will say about single leaf and Larsen truss (still awaiting their response but anticipating "computer says no" or for them to make things difficult). Cavity wall is definitely cheaper but much trickier to build. And are you aware of any issue with mortgage lenders with this type of non-standard construction?

2. Structurally, can I cantilever a Larsen truss/OSB/cladding/insulation off a two storey high block wall 215mm thick laid on flat? The external structure would not be that heavy but approximately 384mm total cantilever might exert pressure in the blockwork at the base of the wall. That said, the mass of a 215mm thick masonry wall must considerably outweigh this. Perhaps one for my engineer.

3. Against that, would the XPS/EPS insulation provide a sufficient base for the truss to reduce the cantilever? (perhaps another question for my SE). They sometimes do this with timber frame but such cantilever loading is less of a problem for timber construction. But I reckon my structural engineer will probably discount any structural role of the XPS/EPS base. Alternatively, do I need to consider a low masonry outer leaf to support the Larsen truss at the base? This would be annoying as it would eat into my insulation thickness - probably could then only manage a 200mm cavity insulation below DPC. Would 200mm be sufficient?

4. Do you have any other ideas you have for external insulation and hanging timber cladding? I had hoped there would be some nifty GRP cladding bracket by now to use with EPS (has anyone ever tried to fabricate one or seen one you can buy?). But the Larsen truss is at least very easy for a DIYer, cheap with readily available materials, and I can do the existing house in the same way after (the house is easier as can attach the trusses to the rafters to relieve the cantilever). I find the lack of experience/knowledge about Larsen trusses over here is unnerving (though Beattie passive houses uses a similar system). They 2x2 trusses look a bit "waffy", not very robust somehow but presumably good enough for Canada’s harsh climate. Increasingly their size, or using a 4x2 outer stud as modified Larsen systems do, only adds to the weight.

5. In the proposed lounge downstairs, should I go for insulation above or below concrete slab, bearing in mind its wide south facing sliding door? Ideally I would put a solid floor in for thermal mass but I wanted to run an oak floor through from an oak floor in the next room (a bedroom created from the old downstairs utility in the existing house). I have already bought the oak floor but could put it upstairs in the bedroom. Had originally intended putting insulation above the slab and floating the oak floor and installing under floor heating (Wunda system or similar).

6. Any ideas/preferences for the green roof? Is EDPM the way to go or liquid membrane? Was looking at Flexitec which looks like a really good system and easier to do with a parapet than EDPM. Again slightly paranoid about leaks, especially under a green roof. Easier to find a leak in EDPM.

7. Any other useful advice welcome.

Thanks in advance and sorry for so very many questions! I'm probably overthinking, over researching and definitely indecisive!

David

Thanks Djh, really helpful.

To be fair, our local authority building control are really good but increasingly under resourced. So non-standard proposals probably take longer than they should and probably more evidencing, which is no bad thing.

The idea of block pads and a metal post are not ideas I’d thought of, so thanks. If I do have to build a block work plinth wall, however, I wonder, can you use lightweight 100mm blocks below dpc - or is there an issue with frost?. I’d probably tank it anyway.

Was using vertical cladding but the idea of horizontal with repair option I like. I am always trying to make my DIY reversible to deal with future changes or mistakes! Has anyone used Millboard and would you recommend? So wet here that concern about natural timber cladding staining. Looked at that SiOO.X timber treatment to use on larch but it’s incredibly expensive. Other products require regular treatment it seems.

Yes, I prefer the EDPM option with a green roof tbh. Parapet detail more fiddly, though, and may need a coping. Aluminium copings are very expensive (quoted £2,900 by Guttercrest). That said, Liquid membranes can be carried over the parapet to a neat drip detail. Why do proprietary EDPM edge details always look horribly clunky?

Just some observations, which are probably not helpful...

In Scotland, which is no stranger to at least as much rain as Cumbria, the vast majority of houses are timber frame. Added to that, you're essentially proposing a timber frame outside the block wall. If you can't keep a standard timber frame watertight, why would keeping the larsen trusses water tight be any easier. Perhaps consider putting the effort of building two walls and complex support mechanisms, into building a standard timber frame really well. It's tried and tested millions of times, rather than a one-off that you're having to learn as you go along. One off's are much higher risk of getting it wrong.

For timber frame, think lighter foundations, faster build (doesn't matter if it's rain/frost/hot sun) and can have wewather tight in a few days, which in wet areas is a big bonus, minimal carbon versus blocks on flat and mortar with concrete/steel lintels. The frame is a void ready to take insulation in it's full thickness. Cladding externally automatically gives you a cavity, and using cement render boards with render over, or cement base cladding, you get long life and fire resistance. If you're building near to a boundary, there are restrictions on timber cladding versus fire spread to/from neighbours.

Solid block walls are fine for building control, specific dimensions given in Approved Doc C, section 5.8. As long as you're complying with what's in that doc (and all the others) "old school" doesn't come into it. The BCO must ensure what you're proposing complies with those regs. If you do something that's NOT covered in the regs, then it's on you to prove that it can meet the same standard. To be fair, the regs leave a lot of flexibility to change up the materials etc, but you need evidence from data sheets etc that they basically do the same job.

If you want a fast and flexible build using timber frame, build it yourself! We had a timber engineer do the overall structural loading calcs but the detail of wall/windows/doors he provided was all generic. Had a few packs of 6x2, ply sheets and buckets of nails delivered and got stuck in. The kids loved building their own bedrooms!!

If you an use a tape measure, chop saw, hammer and circular saw, you're good to go.

I realise there are sort of two threads in parallel (block/timber frame) but I'm sure you can pick out the bits you want.
Lots of ways to do this as a timber frame, but one option might be, based on your comments above;

- block on flat for underbuild 215 wide
- 140/145 kit (structure) on outside edge of block filled with a 0.032 W/mK wool
- 95 studs inside that, staggered to avoid bridging, filled with same wool. Studs will slightly overhang the blockwork, but it's not load bearing, so won't matter, plus that's an opportunity for additional perimeter insulation to blocks.
- VCL with battens over to give service void
- Uvalue of 0.15W/m2K

Kit can go up quickly to weather tight. Then build the stud inner skin in the dry, at your leisure. Insulate etc all in the dry. Roof detailing much easier.

I don't entirely get the 'flat roof will leak' part, and that it will only leak at the walls. It will leak where ever, and run along till it finds a way down to your ceiling plasterboard.

Roof overhangs can be much smaller, and so cold bridging for roof joists can be mittigated if not eliminated.

I personally go for the simplest method, and put effort into detailing, making it even better than "standard", rather than a load of effort to over come self imposed complexities from the outset.

Thanks for your spec GreenPaddy. Really appreciate it.

Putting EPS/XPS outside the block underbuild up to dpc - do we not end up with the insulation forming a plinth jutting out round the edge of the building?

Is there anything to stop you also putting 95mm studs/mineral wool to the outside to improve the insulation further? Add 75mm of the vertical timber cladding. That’s 170mm outside the frame, then add similar amount of EPS/EPS below. The wall would then align.

The only other problem is thermal mass, or lack of it. The new lounge at ground floor would face a garden and will have a large south facing sliding door. Plan was for room to pick up some of the sun’s heat in the colder months and for this to keep the room warm and for some of this warm air to circulate up the adjacent stairs to the main house to an open plan kitchen/dining/ sitting room.

Concerned about overheating, though have another door and window for cross ventilation. Again the solid surfaces were to mitigate this.

I guess with a timber frame, a solid concrete floor is my best option for thermal mass. But if I lay an oak floor over do I lose that?

Thanks for your thoughts on my solid wall option.

Your projecting roof joist suggestion, picking up the trusses at the top, sounds like it could work in taking up some of the load of the Larsen/cladding. My extension has four sides due to an angled relationship between the house and extension and different roof designs. So I would need ladder boards to two sides without joist ends I guess, projecting through the masonry

Needs precise design to get joists and trusses to align unless I attach a horizontal wall plate/ledger to the protruding joist ends perhaps, and attach the trusses to that. My SE might like the idea of wall plates/ledgers fixed to the joists, acting as a structural diaphragm, to restrain the wall top.

The other complication is I was planning open web pozijoists to the roof to enable MVHR pipework. Can I take pozijoists through the masonry to fix to the Larsen. Seems tricky.

Or instead, I could just use timber joists and just box the pipework in around the perimeter of the upstairs bedroom and en-suite. A bit ugly internally but as you rightly say, everything’s a compromise in the end.

Posted By: RubbermuppetThe other complication is I was planning open web pozijoists to the roof to enable MVHR pipework.

Open web joists are very helpful for running MVHR ducts. We ran Ubbink AE35SC ducts through our first floor joists. It might need careful design to run ducts through roof joists because they might compromise the roof insulation.

In relation to the ground floor, I have seen in previous posts (by Tony, Christmas Day 2016) that it is possible to retrofit beam and block with insulation beneath to fully fill the void, subject to BCO approval.

I recently laid a beam and block deck for a large first floor Terrace and found it blissfully simple. Unfortunately, lowering the ground floor of the extension still does not give me enough height to use beam and block with a 150 mm void underneath. However, I was just wondered if anyone has ever laid a DPM over sand blinding, put down some insulation and laid a beam and block across the top, ensuring the void is fully filled, doing this as part of the construction rather than retrofit..

I was going to lay a slab, but it is a big floor area which involves a lot of work and would be much easier to do with beam and block.

I also have read that one of the problems is if the subfloor area floods, how do you get the water out? However, this is not likely to happen where my house is nor is there lots of ground water pressure. And surely if there is a DPM any water should not get in anyway.

Be interested to see any comments on this.

Thanks

Posted By: RubbermuppetI was just wondered if anyone has ever laid a DPM over sand blinding, put down some insulation and laid a beam and block across the top

FWIW we have a big hole with a load of hardcore followed by sand, then 400 mm of EPS and finally our slab. I suppose you could use beam and block instead of the slab. edit: The DPM is in the middle of the EPS.

Posted By: Mike1

Posted By: djhSouth-facing windows are easy. Just put up a brise soleil. We have one - a pergola-style construction; the size and height is calculated so it shades in summer but not at all in winter. East and especially west windows are more tricky.

+1. Or use an actual pergola and grow Clematis, or similar, up it - that way the amount of shade is 'biologically controlled' with the season.

Yes, I have a grape vine and a flowering climber growing up ours. They're just about getting to the stage where they might do some good. Up until now I've just laid some hessian on top of wire netting on top of the pergola structure.

Posted By: RubbermuppetA bit of a b#ll ache to lay a slab imo, ie. shovelling in hardcore, level it, wacker down, shovel in sand, wacker it down, dpm, insulation (before or after), mixing loads of concrete, pour concrete with another person tamping it with a long piece of timber. (Sorry you know all that)

Yes, doing everything by hand will be hard work! You could hire a mini dumper for at least the hardcore. Depending on your access you may want to consider bringing in lorries of self-compacting concrete & chuting it through the windows / doors. And if you don't want to cut your insulation, you can get that sprayed in too, though at least it isn't heavy!

Posted By: RubbermuppetSure there are good reasons - but be good to know exactly why not.

I suspect that it could be due to the long-term risk of rebar corrosion due to moisture, but you'd need to ask the technical department of a manufacturer or two. Although they could just be following a standard that requires it...

Posted By: RubbermuppetBTW, how do you all do the blocking text to make quotes?

Select the test to quote & hit the quote button (at the head of the reply that you're quoting). Then, before replying and below the edit box, make sure that the "Format comments as Html" is checked.

Unless your comment will start a new page, in which case this won't work (due to very old software)!

Thanks GreenPaddy

I was looking for a u value of 0.12 or better to compensate for the largish ground floor slider.

Am I correct in my concern about using PIR in timber frame? Concern is breathability and condensation and the problem of vapour migrating into the frame, however well you detail the VCL and the PIR preventing its escape. Similar to the problem of putting timber decks above insulation on warm roofs, which rot a few years later as moisture somehow finds its way past the VCL.

I also read an article from the US, based on research, that found that PIR, ironically, did not perform as well in cold temperatures and had to be kept above 16°C to achieve optimum performance, implying you need to insulate your insulation!

All that said, if the PIR can be kept warm, I presume these problems may go away. Given this is a Green building forum, does anyone use PIR, pragmatically, because of its significantly better thermal performance?

My timber frame company gets u value down to 0.13 (they claim) using 120mm PIR between the studs and 50mm PIR over the top internally. I wasn’t keen on this for the above reasons.

So what is the best way to improve the thermal performance of a timber frame? This why I looked at the Larsen truss, which I was initially going to use with a timber frame, because it creates a wall which breathes both inside and out, keeps the frame warm without thermal bridging, with no fiddly VCL membranes. There is a good video which explains this on YouTube

https://youtu.be/ICa-guS6HOQ?si=zG8bKByiBqijlUch

Also the video is at this link

https://www.ecohome.net/guides/3418/larsen-truss-walls-for-new-homes-energy-efficiency-renovations-of-existing-homes-double-stud-wall/

It seems a very simple system to me, though off piste this side of the Atlantic.


I will definitely do a concrete slab. Take your point about cost and need for screeds etc with B & B. Plus I have an adjacent driveway so can order pre-mix.

This forum has previously favoured insulation above than below. So that was my plan, with an engineered oak floor floated over under floor heating, pipes in a Wunda system or similar inset on pre- channelled PIR. Not so good for thermal mass, though. Would it be better with the concrete on top and the floor floated above a screed. I don’t want to use battens, which I have done before, because I am struggling with height.

Cheers

Thanks Green Paddy. Really appreciate the time you have spent to share your expertise with me. And to others on this forum.

I am still mulling things over but the advice has been invaluable.

David

Thanks djh. Sounds like an economical but highly effective structure.