Non continuation of cavity when building extension walls

[garyo]

I'm not breaking into the cavity of the existing house to build my single storey front extension. Instead I'm taking both skins straight off the existing house outer wall skin, using wall starters.

LABC seem happy, and I'm using extra thick under floor insulation to compensate. Am I being particularly lazy/stupid about not wanting to do this? From a damp bridge point of view, the extension is south facing, but I've not planned any measures to stop a damp bridge.

Gary

 

[freddiemercurystwin]

You will have a masonry cold bridge no mistake. it is possible you will get spots of damp or mould occurring or worse, you will not know until it is all finished though. Insulated cavity barriers were introduced to stop this occurring. Only you can decide if it is worth the risk. One thing is for certain though, it will be a lot easier to do now than dealing with a damp problem in the future.

 

[garyo]

I think I'm coming round to your way of thinking. Especially the point about correcting it after it's built.

If I cut the 85mm slit and plug it with a cavity closer, so I still need to stitch in the new brick work or would I be okay to use walll starters?

Cheers

Gary

 

[freddiemercurystwin]

I have only ever specified wall starters when joining new to old. Fix the starters first before you cut to make sure you don't dislodge/split any masonry when you drill into the edges.

 

[ajrobb]

I'm slightly confused. Originally you said you were NOT breaking into the cavity. Will the 85mm slit now break into the cavity? Why do you need to close it? Is it just a way of getting insulation into the slit or is it a regulation? I'm just interested 'cos I'm thinking of an extension too.

 

[freddiemercurystwin]

Yes he's saying take a vertical slice out of the outer skin to remove the cold bridge and insert a closer as you would for any any opening in a new cavity wall.

 

[garyo]

I'm slightly confused. Originally you said you were NOT breaking into the cavity. Will the 85mm slit now break into the cavity? Why do you need to close it?

I understand the confusion AJ. My thinking has done a u-turn during the course of the thread! Initially I didn't want to break in because the cavity is full of polystyrene balls, and based my my experience so far I'll lose a lot of them if I was required to stitch in the brick work too. If I can use wall starters then I should be able to plug the slit immediately after cutting, and everything should be fine.

Do I need a fire rated cavity closer at this point, or can I use normal window/door closers?

Thanks

Gary

 

[alittlerespect]

Hi

garyo - you say your going to cut an 85mm slit in the existing wall - all ok
then what happens' I assume on one side of the slit you have the new external brickwork, but what are you going to be doing on the other side? I assume taking down the brickwork to form your opening so whats the point in tying into the existing brickwork if your going to be knocking it out?

If you go back a step you will have problems with thermal bridging and also differential movement between the section of brickwork and the internal blockwork which could lead to cracks occuring in the internal finished plaster.

With respect to the existing insulation, you could drill a series of small holes in the mortar joints where the new brickwork is going to abut the wall and get hold of a few cans of 'crazy foam' and do some DIY cavity fill which will hold the beads in place! Hole size - well slightly larger than the nozzle to the cans of crazy foam! How many holes - well purely finger in the air stuff, but you may get away with every other brick or if you want to play safe do every course which I reckon will make around 36 holes at say 5 minutes per hole (using sds drill) gives around 3hrs for half the job assuming there is another side + set up and finish, cups of tea, late start, early lunch etc. say a days working time!

Regards