Building a porch, few questions please

[JimJim]

Hello all.

I am about to start building a porch and had a few questions I am unsure about.

I have read through the forum but haven't found a concrete answer to my questions.

 

My topic could also be posted in a few sections.

But as it's DIY I thought it should go here.

 

I have been upgrading my property and been doing most work myself.(I live in South Wales)

I built a 6m by 4m out building, and was signed off by building regs when I completed it.

I have done paths and patios and fencing etc.

 

I'm just about to finish a garden wall, 8m long double skinned with facing bricks.

I am then moving onto starting my porch. But I had several questions I was hoping someone could help with.

 

 I spoke with planning and building regs, and have it in writing that I do not fall under either, due to the existing internal door is staying in place and and it will be more than 2m from a highway, less than 3m tall and the floor area will be under 3m sq.(Measure from outside skin to outside skin)

 

So I can build to my liking (within reason)

 

I also already poured the foundations prior to the recent change in part L requirements.

 

For foundations I hand dug down to (600mm) and found the top of house foundation. (House foundation is only 200mm!! 1950 built house)

From there I dug a further 500mm.

 

So my foundations are 1.1m down and 700mm wide and I filled with around 400mm of concrete. I will build up to dpc in block or engineering. (The ground is that super super tough hard clay , very very very hard to dig)

 

My questions are as follows:

 

1) partial fill cavity or full fill? Cavity will be 100mm.

 

To me it seems senseless to build a cavity to then full fill it. Isn't that defeating the objective of a cavity. Surely by filling it full fill I'm asking for damp to be bridged through?

 

With partial fill, it says I need a 50mm air gap? Which only leaves 50mm of insulation.

But I have sound some pir batts that are BBA approved and says they can have a 10mm air gap.

90mm board 10mm air gap.

These are more expensive but also have tounge and grove.

Please see attached picture of these bba approved batts compared to standard PIR batts. Are they worth the extra money?

The cheaper ones don't say anything about bba approved? So guess they need a 50mm air gap compared to the other.

Or is it better to go for full fill. I can't decide.

 

2) I will be connecting both skins of the porch to the existing house (which is a rough cast render finish.

Am I right to say I hack off the render 1st then attach my wall starter packs to the brick?

Do I need a vertical damp behind the wall starter pack?

 

3) I assume I will need to cut a vertical cut inside the cavity and insert damp proof course into this to stop and bridging inside the porch cavity to the house.

How deep should the cut be?

How much damp should extend into the porch cavity? ( I assume a 100mm cut and slide a 150mm dpc into, leaving 50mm exposed into the porch cavity ?)

And where should this cut be made ? I assume where the outer skin of the porch will be I make a cut right behind this?

 

4) once both skins are up to wall plate height, do I need to close the cavity off with cavity closures?

 

5) do I need and air bricks to vent the cavity?

 

6) when installing the lintel for the door of the porch, I assume I need a cavity tray.

But I have seen some builders use no cavity tray and just use a concrete lintel as they said as it's so close to the roof it's not needed, but if they was going two up story's they would. Any clarification on this?

 

7) from bottom of cavity I understand you fill this will concrete slopped at a 45 degree angle facing the outer skin. And must have minimum of 225mm gap from this to start of insulation. But where should the cavity wall insulation sit/start in relation to the floor insulation? (Concrete floor)

Does the cavity wall insulation start in line/in relation to the floor insulation? Or should it go further below that. (If that makes sense)

 

Apologies for all the questions, as I say I have done a fair amount of work my self but this project will have some new elements I am unsure of, and what to give it my best.

 

Apologies for such a long winded post!

 

What's your thoughts on this please.

Thanks 

Kind regards in advance.

 

 

Edited May 22 by JimJim

[joe90]

Regarding full fill cavity, this is what I did with my build (200mm) in rockwall batts, as you say they have a bba cert for full fill. Also as a “porch” which is outside the thermal envelope do you need to insulate that well?. I had a porch on my build, only 50mm cavity and full fill but it’s only an “air lock” to stop loss of heat when the front door is open.