Cavity Wall Vent Positioning Question

[tikroh]

Hi,

Background (Single brick wall, 50mm air gap, stud wall with 75mm PIR - Used as a Tropical Species Reptile Room, so highish humidity potentially.)

I am after advice on where to place vents for my cavity wall.

Does it even matter? I have read I need 1 vent per 2msq?

I have built a stud wall that connects itself on 2 walls, a side wall, and a back wall. (Please see image as I am bad at explaining).

Given the walls are connected within the same cavity, can I get away with vents only in the end walls?

The back walls go in to an area which I would absolutely struggle to get down to.

I was thinking 1 vent middle bottom, and 1 top - close to the back wall to create a flow through rather than a single vent, (If I even need 2?)

In the image the green section is one cavity, the blue another, they are not connected, so I would duplicate whatever I do for both sides.

Thank you.

 

[noseall]

Hi,

Background (Single brick wall, 50mm air gap, stud wall with 75mm PIR - Used as a Tropical Species Reptile Room, so highish humidity potentially.)

I am after advice on where to place vents for my cavity wall.

Does it even matter? I have read I need 1 vent per 2msq?

I have built a stud wall that connects itself on 2 walls, a side wall, and a back wall. (Please see image as I am bad at explaining).

Given the walls are connected within the same cavity, can I get away with vents only in the end walls?

The back walls go in to an area which I would absolutely struggle to get down to.

I was thinking 1 vent middle bottom, and 1 top - close to the back wall to create a flow through rather than a single vent, (If I even need 2?)

In the image the green section is one cavity, the blue another, they are not connected, so I would duplicate whatever I do for both sides.View attachment 321097

Thank you.

You don't need to vent the cavity. Rather you need to insulate it. You do however need to design your insulation and VCL detail so that it prevents the migration of moisture. Easily done with foil taped PIR and Duplex boards etc.

 

[tikroh]

Really? But everything I have seen online says having a cavity is best practice.

If you Google typical timber construction, every single result says the same about a vented cavity of approx 50mm.

So this has really confused me now.

 

[noseall]

Really? But everything I have seen online says having a cavity is best practice.

If you Google typical timber construction, every single result says the same about a vented cavity of approx 50mm.

So this has really confused me now.

View attachment 321227

That's a (brick skinned) timber frame structure, that has been built from the inside out.

 

[tikroh]

Right...

Given this am I better off taking down what I have done? I have framed the whole room.

I have just measured the cavities, and they are unfortunately not even. Some are 40mm some 50mm. So I would have to dissemble and rebuild.

I'm not against it if it's the right thing to do.

This is what it currently looks like.

 

[noseall]

Right...

Given this am I better off taking down what I have done? I have framed the whole room.

I have just measured the cavities, and they are unfortunately not even. Some are 40mm some 50mm. So I would have to dissemble and rebuild.

I'm not against it if it's the right thing to do.

This is what it currently looks like.

View attachment 321267

Is that a single brick structure?

 

[tikroh]

Yes it is.

It was originally used as a little workshop by the previous owner. I am converting it to a reptile room, but struggling with just about everything! .. But it's great fun learning.

I just feel stumped as to what to do, take it all down and start again, or something else.

Thanks for the help so far though.

 

[^woody^]

Normally, you would line the back of the outer brick wall with polythene to keep damp out, and then insulate or build an insulated frame tight up to the wall with no cavity. Or have a frame with membrane on the outer face and just a small 10mm or so gap between the frame and the brick wall.

Have a look at some of the garage conversion threads.

The important thing would be the detailing at the bottom where the wall membrane connects with the floor membrane.

It looks like you have a small gap between the studs and the wall. If so, if you have access to the external faces of the walls, you could coat them with a quality water repellent product to replace an internal membrane, fully insulate between the frame and then put a thin insulation layer across the whole inner face before plasterboard.

Also as this is going to be a warm and possibly humid room, a vapour check layer should be fitted to the inner face to stop any moisture getting in the frame. If taping with foil tape, do a thorough job. Or a polythene layer with few fixing holes and seal the perimeter too.

And consider if a suitable extract fan is required - maybe a constant trickle with humidity-controlled boost. But I suppose that will depend on the reptile needs, but basically you may not want to room becoming too humid unless you are confident that the frame and roof are properly protected.

 

[tikroh]

Thanks.
I do not really have full access to the whole outer wall.
It backs on to a neighbours garage with little to no gap.
An extraction fan inside is a brilliant idea, and I can absolutely do this. The vivariums themselves hold enough humidity, normal room humidity levels of 50 - 60% are absolutely fine.
I have aluminium tape to seal up between Insulation boards and buttons. I was also going to add a vapour control barrier over the whole lot before plasterboarding for extra security.

Main issue I have now is how to move forward.
The walls aren't exactly straight either hence another reason I thought an internal studded wall would be best.

Given my updates, should I ventilate my cavities, 1 top 1 bottom (or am I letting moisture in and making things worse?)
Or should I start again? I want the best job so willing to do what I am supposed to.
But yes, I would struggle to do much with outside wall.

Thanks

 

[^woody^]

You do not want cavities vented internally. The concept is to prevent warm air getting into a wall, as then that leads to interstitial condensation with nowhere to drain.

Has that existing wall ever had a penetrating damp issue?

Have you fitted a floor DPM?

 

[tikroh]

Not that I can see. Seems to be fully dry but I have only been checking over the last 4 weeks since I moved in to the property.

I can see a standard damp proof layer 2 bricks from bottom. I presume that's it?

OK so it sounds like I need to take it apart and start again if the cavity is an issue.

Tale down everything I've done, add pir to wall and then put my stud wall back on top with pir between?

Annoying and time consuming but I want it right.

There's no way I can do anything else as efficient? If leave cavity, but 20mm pir over my 50mm will that work? Would it stop warm air getting in?

Thanks

 

[noseall]

Yes it is.

As per Woody - i.e. we would have trapped or foamed or stixall'ed some vertical DPM to those walls prior to studding and allowing the polythene to travel across the floor and ceiling a wee bit.

 

[tikroh]

Thanks.

OK so is this my plan... Just so I have it straight in my head.

Step 1 - Attach some DPM to the inside wall, overlapping a bit of roof and floor. Should I use Liquid type first on wall, and overlay that with pondliner type? Any advantage to both?
Step 2 - Put my stud wall back in place, but this time against the wall. (Or am I better off attaching first layer of PIR straight to wall here?)
Step 3 - Add PIR in between the studwall.
Step 4 - Aluminium tape all gaps.
Step 5 - Add cross battens across.
Step 6 - Add another layer of PIR overtop of the stud wall.
Step 7 - Add a Vapour Control Layer overtop of the Studwall and PIR.
Step 8 - Add plasterboard overtop.

Does this sound right?

Can you recommend any DPM for the interior wall? Paint on Liquid type, or pond liner type?

Also, if I stud wall straight to wall, do I push insulation right to wall, and sit it inside timber so the wall doesn't directly touch but its very close?

Thanks

 

[tikroh]

I have just watched this video...

At 6 minutes he states that you should build a wall 50mm off the wall as it is single skin.

Is this guys simply doing things incorrectly?

I intend on working on it this weekend, but right now, my head is simple exploding.

Those of you who do this professionally, hats off to you. A lot more goes into this stuff that people like to imagine.

 

[^woody^]

Is this guys simply doing things incorrectly?

Differently. And perhaps not optimally.

There are many ways to achieve a similar outcome, and there are OK ways and better ways. That bloke is showing an OK way, but does not fully understand concept.