Your thoughts on this design?

[Iceverge]

Beware ply. It can be very variable and expensive l. 

 

Nothing against EPS or OSB for me in there place. 

 

Maybe specifically it's the oil based insulation plus the oak. Either way it's rubbish for decrement delay and in a lightweight structure can contribute significantly to summer overheating. PIR I'm not a fan of really. 

 

How about.

 

Oak frame erected on site on insulated raft foundation. 

 

15mm X 100mm strapping on the outside of the 250mm posts to allow 15mm plasterboard to be slipped behind them later simplifying that joint and reducing cracking. 

 

Then  22* 70 mm strapping to create a service cavity. 

 

The an airtight membrane applied from the outside and sealed. 

 

The a lightweight I joist or Larsen truss wrap around frame. Doesn't need to be structural, just strong enough to take the windows, insulation (blown cellulose preferably) and cladding/roofing. 

 

Then a taped OSB layer or woodfiber for Sarking/racking and windtightness.

 

Then breather membrane

 

Battens 

 

Counter battens 

 

Cladding.

 

 

300mm of insulation should give a genuine U value of about 0.13 excellent decrement delay, top airtightness, no waste or mess of insulation. 

 

 

 

[Papillon]

Douglas Fir is fine for post and beam. Glulam is not as nice looking.

 

I would be doing something like this:

 

https://ibb.co/5TLVKt7

 

 

Edited March 7, 2023 by Papillon

[Alan Ambrose]

@Iceverge - yeah, I'm not a great fan of Celotex etc either, so something else would be better. I haven't given it a great deal of thought or done much investigation yet - I should leaf through my passivehouse books again. Blown cellulose in Larsen trusses sounds a bit like wrapping the building in cotton wool . My inclination was to cover the frame in ply to make a big super-strong and fairly air-tight box and then apply the rest of the lay-up to either side. My understanding, on our current barn conversion, is that to avoid letting the old oak frame become unstable, the builder stripped off the existing cladding boards in sections and replaced them with some sort of wood sheet. Then he worked both inwards and outwards.

 

@Papillon - that does look pretty. I did consider an inset balcony for the top floor but it seemed to take up quite a lot of space. If the planners were not so awkward and hung up on footprint, ridge height, building line etc, it would be easier. For Georgian terraces in town, fine, I get it. But for rural hamlets where everything was built before 1947 and exactly how they fancied at the time - not so much.

[Iceverge]

1 hour ago, Alan Ambrose said:

My inclination was to cover the frame in ply to make a big super-strong and fairly air-tight box

Make it totally airtight! 

 

I being Scrooge came in today for lunch and was about to grumble to myself that my wife had been using the heating (a single resistive rad). I know, how dare she have the audacity to be comfortable in her own house that she's paying for! 

 

However turned out it had been off and the fact we had a couple of hours of sunshine earlier made the house warm up and being super insulated and airtight the heat just stayed put. 

 

I went and ate my brown bread sandwiches feeling a mix of idiotic and smug. Wife none the wiser. 

 

 

 

[GaryChaplin]

I've not read all the replies,  but I would highly reccomend ensuring the design accounts for the 'rule set' of oak  framing before you submit for planning. Having windows and doors where there should be key structural elements could create engineering challenges. 

 

Also bear in mind very large spans can be an issue. A general rule of thumb is to keep any clear span less than 5m. 

 

 

[GaryChaplin]

On 07/03/2023 at 07:47, Papillon said:

oak - very expensive, other wood is fine and looks the same

 

 

There are other timbers that look similar to oak, i.e. Sweet Chestnut. however the issue is structural characteristics when it comes to engineering calculations. It's hard enough finding an engineer that will provide reasonable calcs for oak, let alone other species. 

[Alan Ambrose]

>>> ensuring the design accounts for the 'rule set' of oak  framing before you submit for planning

>>> A general rule of thumb is to keep any clear span less than 5m. 

 

Yeah, I think I've got that. It has 7 regularly spaced trusses 2.4m apart and the max unsupported span is 5m. Our existing barn conversion has 6 trusses, 6m open span, the spacing varies unevenly between 2-3m - probably depending what oak sticks they had around at the time. Some complaints in this thread about the regularity of the windows might not have appreciated the oak frame restrictions, compared to say block.

 

>>> however the issue is structural characteristics when it comes to engineering calculations. It's hard enough finding an engineer that will provide reasonable calcs for oak, let alone other species. 

 

Yeah, I don't understand why though. Fusion will do the structural calcs for you fine. Maybe structural guys are bothered by the potential variation in wood compared to say, steel and/or how the framing joints should be modelled and/or anisotropy/grain. I have not run the Fusion calcs part yet but I'm going to guess that we'll be well over 3x safety margin with 25x25cm beams. There won't have been anyone doing 'structural calcs' at the time of building our current barn (c1850), and it's been there for 150 years . I will, of course, have to run some official calcs around the BC loop though.

  

[Iceverge]

I would say an experienced a oak timber framer would be worth a consultation before committing to a design. 

 

One of the projects I linked had £6000/m2 cost and that's pre 2020's inflation. Oak etc get proportionally much more expensive as it gets thicker and longer. It could easily save you a 5 figure sum to have some pre planning advice from someone in the know. 

[Alan Ambrose]

Yeah, I kind of agree. FYI here's the overall frame for planning application purposes i.e. a structure to hang the look and feel on. That is, before detail frame design.

[GaryChaplin]

It's hard to tell from the CAD drawing, but it looks like you don't have any tie beams at wall plate level? The collars in the trusses simply will not be able to cope at that low pitch of a roof.

 

I would try and introduce some tie beams and full height centre posts in the intermediate walls that will be in-filled, also the gable ends. 

Edited March 13, 2023 by GaryChaplin

[Alan Ambrose]

Yeah hard to tell with all those sticks .

 

There are tie beams all through at wall plate height and 1st floor level. There are meant to be full height posts in the gables but not shown there yet - but good point about the intermediate walls.

 

Re the collars, they could be a bit lower, ceiling height on the 1st floor is 2.7m right now.

 

The truss is there in the design atm mostly, of course, to stop door/window openings from being in dumb/impossible places.

 

Ahhhhh - I've just realised who you are - and I was planning to get in touch! Our existing little barn is in Witnesham. I'll ping you my details.