Cavity Gap when joining timber frame to brickwork?

[Co1]

Evening all. I’m planning a timber framed extension on the back of my brickwork house. Do I need to create a cavity gap in between what will become the internal and external brickwork where the timber frame joins it to prevent thermal bridging?

Thanks
Col

 

[Co1]

Anyone? I know this is a bit specific and probably more at home on a building forum.

 

[HOJ]

What is the detail for your timber frame, single stud or double stud.

 

[Co1]

Thanks HOJ, it’s single stud, 140mm.

 

[Lard]

Yes you should, but more-so for damp penetration. I've heard of similar examples where conservatories have been fitted and then had damp problems. I was a little surprised, and somewhat disappointed, that a 'fitter' that I knew couldn't 'see' the problem and, obviously, hadn't even given it any thought.

Simplest method would be to disc cut and insert a vertical dpc.....consider the detail of a window/door reveal where cavities are 'closed' with dpcs/insulated closures in order to prevent water penetration and, as you are already aware, thermal bridging. It's exactly the same.

 

[Co1]

Thanks Lard. Was thinking of a 30mm gap, DPC to the “outside” and 30mm PIR in the Gap. Is that sensible?

 

[HOJ]

Quick Q are you creating an opening where the timber frame wall will land or is the frame landing on an outside wall and forming a "T" joint, difficult to explain, sketch helps, C or D/E




Most of the builds I have done we open up right through, so can close the cavities:

 

[Co1]

Thanks HOJ. I’m doing sketch C.

 

[Lard]

I think I understand what you mean (which seems fine) but any chance of a simple sketch?

 

[MikeJhn]

Compriband is made for just that sort of joint.

Look up Canadian rain wall for the construction of your extension.

 

[Co1]

Of course, see attached. Hope this helps.

The stud that abuts the wall will have a 3mm Epdm barrier to prevent damp and help create a better seal to the existing brickwork. DPC on “outside” of the thermal break in the existing brickwork, with PIR behind that to fill the cavity.

 

[HOJ]

Your detail is more for masonry walls where you would continue the insulated cavity into the new walls, and tooth in the new brickwork, for stability as well, in this case your timber stud will create its own thermal bridge across your brick work.

There isn't as far as I am aware a predefined solution to this, and I have had a look through my Tech books.

I can see possibly 2 solutions to this, as you have detailed, but centralise the cavity break so you can fix the end stud, with an insulator across it, (I use this product for doors and windows etc: Compacfoam) to both sides of the masonry wall to keep them tied together, (A) my main concern with this is, (unless your geared up for it) firstly, cutting the channel, you'll need a big 12" cutter to get through a brick and leave a clean cut edge, and then the closeness to the brick cut ends for fixing to without the risk of them blowing out.

Alternatively, and in to be honest a lot easier, (B) apply insulated Plasterboard to the masonry wall that will ultimately become an internal wall, but still use an insulating isolater on the end TF stud.

Drawing helps again:




Whats the proposed TF wall make up? I can suggest some methods, and is this work to be done under Building Regs? as you may need some calcs done on the SAP values and other considerations.

 

[Co1]

Brilliant Hoj, thanks a lot. Option would be preferred as it means I do f need to hire a big cutter, and it’s one less thing to do! I was planning on using insulated plasterboard anyway, so that’s great. The TF wall is just standard brickwork.

Pretty sure I won’t need building regs as it’s only going to be single storey and 4m2, but I’m checking with my local branch this week.

Thanks again, it’s hugely appreciated.

 

[Silken]

Brilliant Hoj, thanks a lot. Option would be preferred as it means I do f need to hire a big cutter, and it’s one less thing to do! I was planning on using insulated plasterboard anyway, so that’s great. The TF wall is just standard brickwork.

Pretty sure I won’t need building regs as it’s only going to be single storey and 4m2, but I’m checking with my local branch this week.

Thanks again, it’s hugely appreciated.

All extensions require building regs if it is going to form part of the main dwelling and is more than a simple lean-to building also a timber framed structure extension will have different specifications than brick or block.

 

[Co1]

It’s a front porch. Having read the regs I’m pretty sure it’s exempt, but like I say, I’ve a call with the local office next week to get a steer. Either way, I’ll still be building as if the regs apply, so it won’t make any difference.

 

[HOJ]

You've confused me :

planning a timber framed extension on the back of my brickwork house

It’s a front porch.

The TF wall is just standard brickwork.

 

[Co1]

Ah, I see why!! I confused myself! The front porch it this years job, the extension is for next spring!! Been drawing and prepping the porch all day so it’s on my brain, I need a beer!

The last quote is testament to the fact I need to stop for the day!

 

[Lard]

As per your own sketch I would still cut out a chase and fit a vertical dpc as well as fit the thermal break.

Mind you I'm a little ocd and know it would bug me if I didn't!

 

[Co1]

I must say that I’m the same! If I know I’ve not done anything to prevent the damp, I’ll be constantly paranoid!!

 

[HOJ]

cut out a chase and fit a vertical dpc as well as fit the thermal break

Simplest way would be to use a vertical lead flashing, mastic sealed into a slot cut into the brick, you will have a similar situation arising where the new roof will abutt the existing external masonry walls, unless you want to be cutting out to fit insulation or fitting cavity trays, then your into whole different world structurally.