145mm timber frame on 100mm blocks

[Carrerahill]

Hi guys, I am converting a sun-room into a kitchen, I am currently waiting on planning and warrant.

 

To put you in the picture the existing sunroom is constructed of a cavity wall to about 600mm AFFL but externally about 1500mm above ground level, so the wall is actually about a 2000mm wall from the foundations, it has a 100mm cavity and is built in 100mm concrete block - see photo attached, I removed some PB to do an inspection. The top of this wall is then closed, and UPVC windows are fitted around 3 walls. The proposal is to convert it into a proper room which the kitchen shall move out to.

 

So the questions begin!

 

I had a look through the building warrant drawing and this is what it says:

 

Cavity wall construction consisting of 19mm render on 100mm block with 50mm clear cavity and 145mm internal timber frame construction consisting of Tyvek Reflex insulating breather membrane to BS 4016 on 10mm plywood sheathing on 47 x 145mm SC4 pressure impregnated timber studs at 600mm centres. Walls finished internally with 12.5mm Foiled Back plasterboard on 70mm insulation on perimeter walls internal areas as standard plasterboard and 12.5mm moisture resistant plasterboard to en-suites, bathrooms and utility room, all joints taped and filled. 140mm thick Celotex XR4000 or equal fixed between timber studs with Visqueen vapor barrier stapled to internal face of studs in Bathrooms. Within 125mm studs allowance of 25mm given to run services within timber framing.

 

This is clearly my architects standard paragraph, but to adhere to it as closely as possible my thinking is that I shall need to sit a 145mm sole plate on the top of the inner concrete block, but what about the 45mm difference, the construction method above says a 50mm clear cavity, so my thought is that if acceptable I can allow the timber to overhang 45mm into the cavity, but with 10mm ply (local merchant does 9mm or 12mm ply and OSB... so I was going to use 9) but that then means the clear cavity is only 46mm - I suspect I may get away with this or do I need to overlap the frame 5/6mm into the inside of the room. 

 

Anyway, I am sort of thinking aloud there, what is acceptable - 145mm frame on a 100mm block wall? 

 

The next thing I note is it says, "Walls finished internally with 12.5mm Foiled Back plasterboard on 70mm insulation on perimeter walls" so does that mean that between the studs in my extension I need to put 70mm (seems a bit thin by today's standards) insulation, then attach foil-backed PB to the studs - job done wall built. But I then read the sentence about the 140mm Celotex as saying I need to put 140mm insulation between the studs and a Visqueen VB if a bathroom, OR does the whole sentence apply only to bathrooms? Badly written sentence frankly, 2 interpretations for sure. If it does only apply to bathrooms it should say, " Within bathrooms 140mm thick Celotex XR4000...".

 

What are your thoughts here?

 

Thanks

 

 

 

 

 

[ProDave]

To meet building regs these days, you will need a 145mm filled timber frame, AND some sheet insulation over the inside of the frame before plasterboard.

 

Personally I would take down the inner concrete block skin and fit a timber frame from floor level inside. 

 

P.S Do I take it you are another from Scotland?

[Carrerahill]

14 minutes ago, ProDave said:

To meet building regs these days, you will need a 145mm filled timber frame, AND some sheet insulation over the inside of the frame before plasterboard.

 

Personally I would take down the inner concrete block skin and fit a timber frame from floor level inside. 

 

P.S Do I take it you are another from Scotland?

 

Hi Dave, so basically the full wall construction method must be followed, it is not well written. So in a round about way it is basically saying 145mm studs filled with insulation then 70mm insulation over that lot then PB. Fine I can deal with that.

 

I would however prefer not to have to take the wall down to floor level since slab level is about 1400mm below FFL. I am beginning to wonder if I should just change it over to a block cavity wall, fill the 100mm cavity with insulation. At that what would would be required?

 

Yes I am in Scotland, what gave it away?

[ProDave]

The giveaway is you don't hear other parts of the UK referring to a "building warrant" 

 

And the trouble with a building warrant, is you must build what the plans show. If you want to change the design e.g for insulated cavity block that will need re submitted drawings.  Didn't whoever did the drawings address what to do and how to detail it at the transition where the original wall stops?

[Carrerahill]

27 minutes ago, ProDave said:

The giveaway is you don't hear other parts of the UK referring to a "building warrant" 

 

And the trouble with a building warrant, is you must build what the plans show. If you want to change the design e.g for insulated cavity block that will need re submitted drawings.  Didn't whoever did the drawings address what to do and how to detail it at the transition where the original wall stops?

Of course, building control down south.

 

No the warrant drawings do no show a detail for the connection of the new build to the existing sadly, to be honest it looks like a fairly standard plan he uses, my extension is shown in the middle with standard details written for all the different services and construction. If they approve what is shown, and it comes to the bit I will just need to demo the inner skin and build the timber frame up from the deck. I have also asked this question of my architect so I will see what he says. I will be pretty annoyed if demo has to take place as I told him I wanted to build up from what was existing. I wish I had just submitted a notice now and then let the inspector advise as to what he/she wants to see. 

 

It would be a shame thought as it would mean lifting the floor, lifting the chipboard, removing the insulation and removing all the joists take the wall down to the slab then starting again in timber.

 

[ProDave]

I was just thinking demolish the inner skin to floor level, not slab level, and build up from there?

 

I have seen a 145 timber frame built up from 100mm block foundations but am sure that was just a mistake.

[Carrerahill]

1 minute ago, ProDave said:

I was just thinking demolish the inner skin to floor level, not slab level, and build up from there?

 

I have seen a 145 timber frame built up from 100mm block foundations but am sure that was just a mistake.

 

Ah right OK, so you would take the skin down to the floor. I guess if the top of a block was AFFL then I could take it down to the next and then build it back up with bricks and mortar etc, then just sit the wall plate on the floor/top of the block work. That would give me a 100mm cavity mind you - that is 50mm more than the BW drawings. So I would still be tempted to overlap it about 40mm into the inside of the cavity so with the ply on it would be 49 leaving the as prescribed 50mm clear cavity and I don't loose the full 50mm in my room.