Ventilation requirements for insulated concrete wall.

[Jelly]

Can anyone point me to rules of thumb or design calculations for the ventilation required for a concrete/masonry wall with an insulated stud wall on the inside.

I've tried to copy the "Mike Way" as closely as possible when lining my sectional garage, and have deliberately left a gap inside the studwork (which is all treated) for air circulation but the individual sections obviously don't have any ventilation holes at the bottom, which is desirable to get a flow of air over the outer walls and maximise evaporation; something the current ventilation at the eaves and ridge oy won't deliver.

I'm not adverse to renting a diamond drill to add ventilation in the outer walls, but if I can get away with small holes would prefer to just use my SDS and get it done.

 

[RichardG]

There was a thread which will probably answer all your questions?

Concrete sectional garage workshop refurb - Work has begun.

 

[Jelly]

There was a thread which will probably answer all your questions?

Concrete sectional garage workshop refurb - Work has begun.

Yes and no...

What I'm doing is very similar to the build there (almost unsettlingly so, but I guess there's only a handful of good solutions with these garages), so it's nice to see it works.

But there's nothing on actually working out the ventilation requirements, I guess I could assume that drilling 6 nonstandard size holes and inserting 3D printed gratings would work, but it seems like a bit of a random extrapolation from that success.

 

[RichardG]

If I understand correctly, the other thread had left a bigger cavity over the whole wall whereas you have lots of smaller cavities at each section panel?

From the experience of a new lean-to roof I built over an extension the building inspector insisted on one round soffit vent for each joist void so I would do the same in your case i.e. one per section, but I’m no expert! An alternative would be to drill two 20mm ish holes with your SDS and then use a grinder to join them into a slot? You could probably get away with just pushing a bit of mesh in to keep the insects out?

 

[Jelly]

If I understand correctly, the other thread had left a bigger cavity over the whole wall whereas you have lots of smaller cavities at each section panel?

I have also spaced the wall to give one large cavity behind the stud-work.

What I'm trying to work out is the minimum sensible amount of ventilation I will need spread across the whole cavity, as I'd like to achieve a balance between evaporation and insulation.

If I have too little airflow, then there will be excessive condensation, but by the same token, if I allow more airflow than strictly necessary, the heat transfer regime at the outer surface of the insulation (i.e. the inside surface of the cavity) changes to be much less favourable, and the effective U-Value of the wall will skyrocket...

 

[ManowarDave]

Hi Jelly,

That is my build referenced above.

Despite my seemingly random approach to putting the vents in, I did have one reference document I used to double check it would be enough.
NHBC Standards 2006

In part D4 section (c) of the above it recommends the equivalent of one perpend every 1.5m. One perpend being a brick end on. I took the standard UK brick size of 102.5 x 65.

For my walls that equated to about 3 perpends per wall using "gateposts" ie n-1 for my 1.5 m spacing. My round holes were 117 diameter so that gave me the equivalent of about two round holes per wall.

Dave

 

[Jelly]

Hi Jelly,

That is my build referenced above.

Despite my seemingly random approach to putting the vents in, I did have one reference document I used to double check it would be enough.
NHBC Standards 2006

In part D4 section (c) of the above it recommends the equivalent of one perpend every 1.5m. One perpend being a brick end on. I took the standard UK brick size of 102.5 x 65.

For my walls that equated to about 3 perpends per wall using "gateposts" ie n-1 for my 1.5 m spacing. My round holes were 117 diameter so that gave me the equivalent of about two round holes per wall.

Dave

Thanks very much!

I assumed there would be a reason for your vent selection, but without knowing what it was, didn't want to copy it blindly.

That suggests I could use a 26mm hole every 500mm, as that's equivalent to one perpend per 1500mm replace the perpends.

I now have least 30 holes to drill, but small ones thankfully.

 

[ManowarDave]

Hi Jelly,

A 26mm diameter hole is only going to give you approx 530mm2 area per hole. At 500mm spacing that's only 1590mm2 per 1.5m. One perpend is about 6600mm2.

You'll either need more holes or bigger holes.

Dave

Edit: Also, if your sectional panels are like mine, they will be reinforced with steel. A diamond hole saw is the way to go, otherwise if you hit the reinforcement with a masonry bit you'll come to a stop. The blowout at the back is also less with the hole saw.

 

[Jelly]

Hi Jelly,

A 26mm diameter hole is only going to give you approx 530mm2 area per hole. At 500mm spacing that's only 1590mm2 per 1.5m. One perpend is about 6600mm2.

You'll either need more holes or bigger holes.

Dave

Edit: Also, if your sectional panels are like mine, they will be reinforced with steel. A diamond hole saw is the way to go, otherwise if you hit the reinforcement with a masonry bit you'll come to a stop. The blowout at the back is also less with the hole saw.

Right maths, wrong dimension... 26mm radius!

I have ordered a 50mm hole saw+arbour and will work from the outside to allow me to hide the inevitable spalling,