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building a timber frame first floor over a cavity wall


Rdddk

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Hi I am building a timber frame first floor over a block cavity wall. I have the outer block one course higher than the inner and a 100mm cavity, I was hoping to have the floor joist crossing the cavity by 90mm so that i could use 195mm timber plate sat on top of the outer block and the joists so that i can build 195mm timber frame and not loose the space in the rooms the first floor is to be cladded and the ground floor rendered.  

 

My building inspector has said my joists cannot hang over the cavity which means i will potentially have to build the wall 300mm thick. Can anyone suggest an alternative option?

 

 

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Overhanging the joists into the cold cavity is definitely not a good idea, as they will rot out as a consequence of interstitial condensation, I suspect, which is why your building inspector won't have it.  You have to keep the joist ends on the warm side, ideally hanging from the inner wall, as that is far and away the lowest risk solution.

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1 minute ago, PeterW said:

Have you considered a beam and block first floor as that can hang over the cavity and give you the arrangement you want. 

 

Still gives a nasty thermal bridge though, as the beam ends will be near enough at outside air temperature.  The heat loss rate around the periphery may well be high enough to cause internal condensation problems around the first floor/wall junction, inside the house.

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You could finish the outer block skin one block lower than the inner block skin. Then the floor joists could safely jetty out as far as you want. Just carry the first floor cladding down low enough to cover the gap and ensure the insulation carries on up from the block wall to the timber frame.

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1 hour ago, JSHarris said:

 

Still gives a nasty thermal bridge though, as the beam ends will be near enough at outside air temperature.  The heat loss rate around the periphery may well be high enough to cause internal condensation problems around the first floor/wall junction, inside the house.

 

Sorry just re-read it - thought the cavity was 195mm..!! 

 

What about losing the top block and basically making it like a standard TF and clad the outside with wood fibre insulation ..? That would allow you a thinner internal frame but keep all of it warm. 

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Just a couple of ideas

Is the cavity staying open?

can you not close the cavity with a block on it’s side with a damp course between the outer block and this block on its side

then put a wall plate on the inner block 50mm high then have some insulation 50mm high in the cavity area, sit your joists on the wall plate, build your new studwork off of the joists. 

 

Just an idea. 

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We're doing something similar building over a full filled cavity wall 400mm thick. Above this is a 245mm i-Beam frame. With the help of the posi suppliers I had planned to run the joists right out across the top to flush with the outer skin and then build off this - much like a timber frame deck construction. Wondering if some of the potential issues might also apply to us so keen to hear other responses.

 

My understanding was that as our cavity is full filled and the space between joists the same then we're limiting any risk - happy to be told otherwise before it's actually built though! Our BCO hasn't flagged anything either at the design stage...

 

 

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A while ago I found the detail(s) shown below. These might help @Rdddk  ? - Does anyone have any views on their pros/cons? I'm not too sure on the detailing for the air-guard as shown which looks like it would be fiddly . I'd had planned to simply wrap the airtight membrane around the joist end before tucking back in to the wet plaster coat below.

Thanks as always

 

 

5a2527722fe08_WallPlateDetailJPG.thumb.jpg.8da51fe5a678ddaacaac5fa57c4c87c3.jpg

Edited by bissoejosh
typo
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  • 2 years later...

Any idea why no one ever replied to this? I have to do something quite like this.......I have a 50mm cavity wall and was going to roll a bit of dpc under the timbers from the inner leaf and stapple it down the side of the top plate and roll it up the front of the osb on the front.....other option was to close the cavity with a normal cavity closer and stick some dpc on top of that and take it out the front. 

 

Thanks for any input.

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hi,

 

the only problem with the pic above is the cold bridge from the inner and outer leaf. it would need some sort of insulation below the kit such as marmox or similar. then there is also the insulation required inbetween the joists, are you going for a suspended timber floor?

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@Simplysimon yeah its 7x2 timber floor joist. which at the moment are over hanging the outer leaf. with a facia and sofit in wood. Here are a couple of drawings of what I have now and what I am sort of looking at doing...but maybe stick some dpc under it....  yeah timber floor with chicken wire and insulation between it....

Cedral click cladding by the way...

ext1.jpg

Extension2.jpg

 

Edited by glenboy
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1 hour ago, glenboy said:

 

@Simplysimon yeah its 7x2 timber floor joist. which at the moment are over hanging the outer leaf. with a facia and sofit in wood. Here are a couple of drawings of what I have now and what I am sort of looking at doing...but maybe stick some dpc under it....  yeah timber floor with chicken wire and insulation between it....

Cedral click cladding by the way...

ext1.jpg

Extension2.jpg

 

that's not going to work, a big cold bridge at floor level. why is there edpm? i don't think it will meet regs for u value to start with. best case scenario would be to take kit to walls still with some insulation under, binder along inner wall and hang joists off that. i take it that there's currently a flat roof single extension and you're going two storey? if so, have you checked to see whether founds are up to it?

 

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1 hour ago, Simplysimon said:

that's not going to work, a big cold bridge at floor level. why is there edpm? i don't think it will meet regs for u value to start with. best case scenario would be to take kit to walls still with some insulation under, binder along inner wall and ha joists off thatng. i take it that there's currently a flat roof single extension and you're going two storey? if so, have you checked to see whether founds are up to it?

 

  Well it is first floor and the cedral click ask for the edpm to run upto the top of the batterns which I think are 38mm deep by 100mm wideand the edpm is fixed to that. 

Yep it is going on the current flat roof extension to 2 storey. Yep the foundations are up to it as I dug them out. I think from memory its 1.5 meters deep there.

Other option I can only think of would be this...... maybe have another block between the joists. Celotex at the front and the knuff wall behind the block. 

An the pink panther insulation is in the 50mm cavity below

 

Extension3.jpg

Edited by glenboy
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image.thumb.jpg.805660e8a7cca293e9ca0e720decf51b.jpg

not to scale, and you will obviously need to check with your structural engineer regarding loading, but I think this will be easier to build, your original ideas will be a pain to air seal and insulate well. 

Cutting the joists back short will allow you to install a rim board that could have a VCL stapled to it so when you build the upper storey you will have something to tape to. 

1. New wall

2. Joist hanger

3. insulation 

just me thinking out loud. 

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10 hours ago, glenboy said:

@Russell griffiths @Simplysimon Thanks for your input.....this is what I have for the side walls.....for the guy who drew the plans... This is looking from the front and with one of the joists....the left wall rests over the cavity as can be seen in this

 

sidewall.jpg

Don’t like that idea 

pain to build, where would you put insulation?

not really understanding how the joist is attached. 

 

One question that  would really affect how you build this would be if you want to leave the down stairs room Undisturbed, with my first version you will end up cutting into the downstairs ceiling, not a problem if your ripping it all out.  

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Thanks for your replies on this. As this has had over 6000 views so far it means that its of interest to a lot of people. I am guessing by looking at the drawings etc....its a case of up to whoever is building it. Both/most ways will work. Inner and outer are to some extent connected by catnic and steels with plate welded on bottom. Some sort of cavity closer/Dpc just in case of any damp from wall below. I guess the dpc if it was bedded on the external leaf would allow a bit of movement. The inner being strapped and with existing top plate will help to fix the new timber spanning both inner and outer leafsidewall.jpg.37600fc155df14f6fd29d3a10ad18ed6.jpg

So put insulation in yellow. The existing timber wall plate in green and along the bit between the joist the box section of timber the odd bit of timber down the run as noggins. 

  The Front wall I would guess you do the same.... as per my other drawings above slightly adapted.

Edited by glenboy
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