Cavity closers and d.p.c.

[P.J.]

Hi All ,

I'd like some help on 2 issues please:

1. I can't find cavity closers the right size in my area. I've created a new external doorway in a brick/block cavity wall. Can I just cut some polystyrene bats to size and slot them into the cavity, then wire mesh round the reveal and render/plaster on top ?

2. What d.p.c. do I have to fit up and round the new doorway and how ? Does anyone have any section views to show where the d.p.c. sits up and round the frame (I have the usual d.p.c. on both leaves at the base of the frame.

Many thanks,
P.J.

 

[JerryD]

1. I can't find cavity closers the right size in my area.

What size do you need? Every builders merchants stocks pretty much every size nowadays. I hope you haven't been looking in B&Q!

If you use a proprietory cavity closer you do not need vertical dpc as the reveals are not closed by block or brick.. Cavity closers are plastic so you don't need to protect them from damp.

 

[P.J.]

I've had real trouble finding them in the builder's merchants (not B&Q!). Only one has them and they stock 2", 3" and 4". My cavity is 70/65mm so I expect the 2" to be too small and the 3" to be too big.

Good point about not needing a dpc, but if I was stuck, could I cut polystyrene to size and insert, then mesh and render over the top ? In this case, would I just use a dpc to isolate the outer leaf from the door frame and the polystyrene round the reveals ?

Thanks for the advice,

P.J.

 

[JerryD]

65mm is a very popular size, try a better merchant or just order them (most merchants get stuff within 3 days).

Even if you do your polystyrene method, you will still not need vertical dpc. Vertical dpc was only ever needed where the two skins of cavity wall masonry touched together, effectively keeping apart the cold wet outer skin from the warm dry inner skin.

This masonry cavity closing idea is now generally banned and insulated plastic cavity closers are used instead so there is no need for vertical dpc.

Making a decent job of meshing over this polystyrene may prove difficult, a better way is to 'Gripfill' plasterboard over the entire reveal (after polystyrening the cavity). Gripfill however will 'eat' the polystrene so be careful where you put it or use a water based gripfill equivalent.

We often plasterboard reveals even after using proper cavity closers, the plasterboard gets glued in position after the frame is fully fixed and finished.

One point, the back of the new frame must be set far enough back in the brickwork opening to cover the join between the cavity closer and the outer brickwork. This ensures that there can be no bridging of plaster onto any of the outer brickwork.

 

[P.J.]

Thanks, JerryD, much obliged.

P.J.

 

[Malc]

When did cavity closures become a must then?

On a related topic does anyone else have a problem with blinds. They seem to need huge force to wind up and down with the crap (cheap) mechanisms they have these days. Hence you need solid fixings. But drill up and you hit the lintel. Drill sideways and you hit the plastic cavity closure so you are hanging off small bit of plasterboard.

I hate blinds (well fitting them anyway).

 

[JerryD]

Cavity closers have been required for a few years now, maybe 5 or 6 years? Can't quite remember, it could be longer that that! They are there to prevent 'cold-bridging' where two extremes of temperature meet as this could cause condensation inside the window reveal and mould would grow. They are one of the better new rules of recent years!

Regarding window blinds, yes, always a problem. On the sides of the reveal you will always have the first 115mm of the reveal to fix to as this is the block end plus the plaster thickness. However getting closer to the frame and you will be fixing into weak plastic/polystyrene and or the cavity in the lintel (depending on lintel type).

 

[Malc]

Can't be 5/6 years, well perhaps just. I had an extension done in 1999 and the cavity was closed in the old way by in the internal block wall turn through 90 to meet the external brick wall with DPC in between. That gives more strength for blinds although low density blocks are not exactly the strongest thing around

The trouble with cavity closures is that near the window is usually just where you do want to fix the blind.

 

[P.J.]

Closers installed - cavity was well variable in width, down to about 60mm at the base from 75mm elsewhere. Closer squashed fairly readily into place.
Dabbed plasterboard on top and finished reveal with a corner trim before skimming.

Sorted - thanks for your help.

P.J.

 

[Tony6]

Jerry

with regards to cavity closers im fresh out of college nvq level 3 ,2years ago, and we were taught to use the so called old method? the use of plastic closers were for window openings? it cavity walls

Would you get the same structure strenght in a cavity wall at the door opening using plastic cavity closers or bonded with the same material?

Would the door jamb fixing be as rigid

Tony

 

[JerryD]

The strength in the wall at an opening is maintained by incresing the wall ties at the opening to one every 3 courses of bricks instead of every 6 courses.

A frame fixing only relies on the wall it is fixed to so a door frame fixed to the outer skin of the cavity wall does not rely on the strength of the blockwork inner skin at all. For example, take a half brick thick garage wall, this can still have a pedestrian doorway in it yet there is no inner skin and no reveal. It is the strength of the brickwork that the frame is fixed to that matters.

The 'old method' cannot be used in my area but may still be allowed in other parts of the UK?

 

[Tony6]

Thanks jerry

it makes a nice change to have a debate without trying to score points on this forum.

when you say every 3 courses are you reffering to brick. as i work with block every two courses and 600 apart. and every course on corners about 150 in ,which would be 3 courses of brick

Thanks Tony

 

[JerryD]

Yes, I'm talking courses of bricks. So when I say 3 courses of bricks it's 1 course of blocks.

Reveal ties I put in about 100mm from the reveal, this is the optimum position.

So on reveals with plastic cavity closers put wall ties every 3 courses of bricks (or every course of blocks!) and put them 100mm in from the reveal.