New house build inside old house - timber stud or SIP?

[Hastings]

The project is an internally insulated airtight/MVHR refurb of an old rectangular stone house on two levels.

 

A standard way to do an internal insulation refurb of an old solid stone walled house in Scotland is to build a new internal timber stud wall, set 25mm back from the outer wall. In my case the wall is proposed as 11mm OSB, 95mm timber stud infilled with insulation, 20mm insulation board, plasterboard and skim.

 

This seems a sensible option if the existing old upper floor(s) structures are to remain. But what if, as in my case, the upper floor is condemned due to woodworm, wet and dry rot and has to be entirely replaced with new floor? Basically the whole house is now stripped out and new roof fitted, leaving a 'barn' like single interior space.  In this case, is there not an opportunity for a thermally more efficient SIP (structural insulated panels) type inner rebuild?

 

In the timber stud wall design I have for my project the engineer has specified a new upper floor structure made up as a stressed deck (plywood screwed to 445x45 joists) that spans the entire open plan ground floor 10mx5.5m kitchen/living space, supported only around the 4 wall edges.

 

What I don’t like about this is that the floor joists extend all the way across the top of the ground floor stud walls, in the form of a double joist ring-beam going around the entire perimeter, thus interrupting the thermal continuity between ground floor walls and upper floor walls. I would much prefer that the whole floor deck hangs off the inside face of the new timber wall without obstructing the thermal envelope, as I understand to be the case with SIP construction.

 

SIP construction of course is in its element when it comes to new build but is there any reason why the design is not also very suitable for putting together from scratch on site in a major internal refurb of an old house?

 

[ProDave]

The 2 things to beware of, are do the existing (now worm infested) joists perform any structural function helping to tie the front and back walls together?  I would want SE input on this.  With a typical 1.5 storey croft house build, the roof may be prone to spreading with nothing tying the walls (though often the sheer bulk and thickness will prevent that)

 

Ant the trap that 99% fall into is to build a skinny frame and insulate it with 25mm of kingspan thinking they have done a good job, but they usually leave the cavity between the stone house and the inner frame open to the cold roof space.

 

It makes it damned handy as a sparky for running new cables but not much good for energy efficiency.

[scottishjohn]

lookin at same sort of thing 

looking at building TF close to inside of wall then  spraying inside of wall +behind TF with  of closed cell foam to water proof  it and fix outer wall with  the closed cell foam 

then  --fill it all  with open cell foam (half the price of closed cell) and carry on as normal TF

vcl -batons service void ,then PB 

wall at this time is 700mm thick  so idea is to remove inside rubble wall leaving the outer cut granite wall + bond to TF --that will stop the rooms getting too small 

 other idea is remove internal rubble wall ,still leave outer skin --then build 100mm block wall and again bond the 2 together with closed cell foam ,or maybe fill with concrete with water proof additive  for the water proofing  --then same TF with lots of insulation.

but with this way you could make a ring beam  with rebar at wall head to tie it all even better 

 comments welcome

 

[Hastings]

This is more farmhouse than croft so two full storeys and the roof space will not be used for living space. The new roof trusses tie the wall heads together. I hope the old rotten mid floor was no longer providing any structural help because it was removed a few weeks ago in preparation for the new internal concrete floor - footings for the timber frame build.

 

I will not use any foams or foam, spray or board based, when it can be avoided as I can't imagine how it will be disposed of at end of life. I've also seen how boards like Celotex can distort and shrink after storing a leftover sheet loose in my garage. I will be using wood fibre insulation.

 

The air gap between the new stud frame and the old stone wall has to be lightly ventilated with small holes in the stone wall 300mm above ground level every 1.5m and open at the top to the roof space. I would rather not ventilate that much but Building control insisted.

Edited July 25, 2019 by Hastings

[Hobbiniho]

you say that the roof has already been fitted? how do you propose to get the SIP sections into place? i presume the plan is to make the timber kit on site within the building so they can just be stood up in place? you cant do that with SIPS

[Hastings]

On 25/07/2019 at 19:04, Hobbiniho said:

you say that the roof has already been fitted? how do you propose to get the SIP sections into place? i presume the plan is to make the timber kit on site within the building so they can just be stood up in place? you cant do that with SIPS

Yes roof is fitted. I would assemble the panels inside the house, in sections and stand them up into position, yes, the same as the with a stud frame design.

It sounds as if I need to understand more what a SIP actually consists of.

[PeterW]

12 minutes ago, Hastings said:

It sounds as if I need to understand more what a SIP actually consists of.

 A SIP is a structural insulated panel. It’s basically two sheets of plywood, OSB or other panel board where a polyurethane expanding foam is injected into the gap between them in a factory. These are trimmed and designed to make the whole wall panel, and are joined by special filler sections and by design are heavy as they are big. They need either craning, or lifting into place by a telehandler.  

[ProDave]

A stick built on site timber frame would make more sense for this.

[scottishjohn]

On 25/07/2019 at 18:11, Hastings said:

I will not use any foams or foam, spray or board based, when it can be avoided as I can't imagine how it will be disposed of at end of life

end of life ?? you are refurbishing an old building --so when do you envisage the rebuilt building  being flattened --another 200 years?-- hardly a good reason not to use best waterproofing solution for  an old rubble wall 

[Iceverge]

Hi Hastings,

 

What a lovely project. What is the maximum depth you are prepared to loose from the room? It sounds similar to the Green Building Store retrofit of Stirley farm and I believe MBC have completed a similar project if you look through their facebook photos.  They lost a lot of floor space however. 

 

How sound are the external walls? Might they be able to take a perimeter ring beam bolted to the stone with compacfoam or similar to create a thermal break? The joists could be hung from this and the external walls then internally insulated (carefully) with a breathable insulation like wood-fiber or lime hemp?

 

 

 

 

[Hastings]

10 hours ago, scottishjohn said:

end of life ?? you are refurbishing an old building --so when do you envisage the rebuilt building  being flattened --another 200 years?-- hardly a good reason not to use best waterproofing solution for  an old rubble wall 

I can't imagine any new refit of today not becoming very obsolete within a very short time compared to the overall life of the building (approx 150 yrs to date) - say 30-50 yrs tops?

Never heard of foam as best way to waterproof rubble walls.

[Hastings]

8 hours ago, Iceverge said:

Hi Hastings,

 

What a lovely project. What is the maximum depth you are prepared to loose from the room? It sounds similar to the Green Building Store retrofit of Stirley farm and I believe MBC have completed a similar project if you look through their facebook photos.  They lost a lot of floor space however. 

 

How sound are the external walls? Might they be able to take a perimeter ring beam bolted to the stone with compacfoam or similar to create a thermal break? The joists could be hung from this and the external walls then internally insulated (carefully) with a breathable insulation like wood-fiber or lime hemp?

 

 

 

 

Useful suggestions Thank you. Will look the GBS project up.

Who is MBC?

 

I'm aiming at adding only 150mm to the existing 500-700mm walls so not that much impact on floor space. In fact I look at the reduction as a positive, being a fan of less is more. I would go further but for the not so positive 'tunnel' effect on window openings (even with their angled reveals).

 

The current design has Simpson L brackets fixed to ring beam and to external wall just to tie in walls as well as maintain 25mm air gap and maintain air flow up to roof space. Compacfoam is interesting - will have a look at that. Having the upper floor resting on the stone walls instead of on the stud frame would allow a reduction in the framing and therefore more insulation in place it.

 

 

[Hastings]

On 26/07/2019 at 20:59, PeterW said:

 A SIP is a structural insulated panel. It’s basically two sheets of plywood, OSB or other panel board where a polyurethane expanding foam is injected into the gap between them in a factory. These are trimmed and designed to make the whole wall panel, and are joined by special filler sections and by design are heavy as they are big. They need either craning, or lifting into place by a telehandler.  

Thanks for that very clear explanation.

I now realise that what I was thinking of is not really SIP but perhaps semi-SIP, where two OSB layers are seperated by timber framing and then insulation blown in on site after construction. Posi joists barely penetrate the walls at all. I've noticed this from several Passivhaus new builds (e.g.. @JSHarris's very helpful draw gs)

I will need metal web floor joists to facilitate air ducting.

[JanetE]

MBC... do a search on this site.  Jeremy's, ours, and many others have been build by them. 

[Hastings]

On 26/07/2019 at 21:18, ProDave said:

A stick built on site timber frame would make more sense for this.

Yes, that's the current plan but I'd like to improve on the design details ie. get the upper floor joists to be contained within/inside thermal envelope and sole plate insulated from foundations concrete.

I don't feel like I have the best stick frame design details in place. Is there somewhere online I can refer to for current best practice?

[Hastings]

Thanks for the help and ideas everybody, I've now headed over to the Timber Frame forum here to learn more.