How many periscope vents

[MortarThePoint]

There is some inconsistency in the number of vents I'm being told I need:

• My construction drawings have 24 periscope vents, but I don't think much thought was put in to those as at least 3 of them are under doors and another where a drainage lintel sits.
• NHBC guidance says "Voids should be ventilated by openings providing not less than 1500mm2 per metre run of external wall or 500mm2 per m2 of floor area, whichever gives the greater opening area.

  Ventilators should be spaced at not more than 2m centres and within 450mm of each end of any wall. Air bricks should be ducted through cavities and be unobstructed." The house perimeter is 60m so that makes for about 34 if every 2m.

• And this calculator says I need 12 (140m2 and 60m) (Their vents are 7,750 mm2 per unit so 1500mm2/m * 60m / 7750mm2 = 11.6 --> 12)

 

Can I just have 12 vents, 6 on each of two opposing elevations, then?

[Mr Punter]

I think they tend to work better if all around, not just opposite walls.  12 is not many and it would be a shame if BC picked you up on it.

[MortarThePoint]

I'm surprise the calculator was the lowest as it's made by the manufacturer of periscope vents so you'd expect to to come up with the largest number

[nod]

2 hours ago, MortarThePoint said:

There is some inconsistency in the number of vents I'm being told I need:

• My construction drawings have 24 periscope vents, but I don't think much thought was put in to those as at least 3 of them are under doors and another where a drainage lintel sits.
• NHBC guidance says "Voids should be ventilated by openings providing not less than 1500mm2 per metre run of external wall or 500mm2 per m2 of floor area, whichever gives the greater opening area.

  Ventilators should be spaced at not more than 2m centres and within 450mm of each end of any wall. Air bricks should be ducted through cavities and be unobstructed." The house perimeter is 60m so that makes for about 34 if every 2m.

• And this calculator says I need 12 (140m2 and 60m) (Their vents are 7,750 mm2 per unit so 1500mm2/m * 60m / 7750mm2 = 11.6 --> 12)

 

Can I just have 12 vents, 6 on each of two opposing elevations, then?

 

2 hours ago, MortarThePoint said:

There is some inconsistency in the number of vents I'm being told I need:

• My construction drawings have 24 periscope vents, but I don't think much thought was put in to those as at least 3 of them are under doors and another where a drainage lintel sits.
• NHBC guidance says "Voids should be ventilated by openings providing not less than 1500mm2 per metre run of external wall or 500mm2 per m2 of floor area, whichever gives the greater opening area.

  Ventilators should be spaced at not more than 2m centres and within 450mm of each end of any wall. Air bricks should be ducted through cavities and be unobstructed." The house perimeter is 60m so that makes for about 34 if every 2m.

• And this calculator says I need 12 (140m2 and 60m) (Their vents are 7,750 mm2 per unit so 1500mm2/m * 60m / 7750mm2 = 11.6 --> 12)

 

Can I just have 12 vents, 6 on each of two opposing elevations, then?

 

2 hours ago, MortarThePoint said:

There is some inconsistency in the number of vents I'm being told I need:

• My construction drawings have 24 periscope vents, but I don't think much thought was put in to those as at least 3 of them are under doors and another where a drainage lintel sits.
• NHBC guidance says "Voids should be ventilated by openings providing not less than 1500mm2 per metre run of external wall or 500mm2 per m2 of floor area, whichever gives the greater opening area.

  Ventilators should be spaced at not more than 2m centres and within 450mm of each end of any wall. Air bricks should be ducted through cavities and be unobstructed." The house perimeter is 60m so that makes for about 34 if every 2m.

• And this calculator says I need 12 (140m2 and 60m) (Their vents are 7,750 mm2 per unit so 1500mm2/m * 60m / 7750mm2 = 11.6 --> 12)

 

Can I just have 12 vents, 6 on each of two opposing elevations, then?

Normally every 1800

BC will be fine if you have them set out to look ok to the eye say 3 bricks out from the corners 

one either side of patio doors etc 

I used 34 on a 158 m2 slab

You will also need lintels and slots in your barer walls under your block and beam 

[dnb]

I had this very conversation with my BCO just before lockdown. He said he prefers to see vents all round, but is happy enough with vents on two opposing sides providing there is nothing blocking the air path in between. I have 8 vents on each side and 4 on the back wall because I have numerous obstructions (i.e. other walls) in the middle of the house, and having insufficient ventillation on my site would be "a very bad thing" for a whole load of boring practical reasons. Your calculator link says I only need 10 in total, but I don't think the flow on my vents is as good as Rytons. They're cheap enough, so don't worry about a few extras.

[nod]

The number I used was based on anything other than measuring the ones used on sites and assuming that mass built houses will use the minimum allowed 

Thing is they are cheap to buy Quick to instal I think I spent about five minutes on each 

More importantly they can’t be put in retrospectively if you find the airflow isn’t good enough once your house is built 

[MortarThePoint]

Thanks, great advice. It will be a bit more involved for us I think as we are to use a precast slab (Thermabeam) which has particular contact points. I am checking with the manufacturer but I think the vents will have to be lintelled over as shown in the diagram below. The dotted lines are the concrete pillars at the end of the beam and the honeycomb is EPS insulation.

 

I can't see any reasonable way we can hit bond for the outer leaf which is a shame. Complicated a little by Flemish bond, but would be difficult either way.

 

[MortarThePoint]

Something tricky here is that all this needs to get established before brick bond is established, so having the vent nicely in bond on the outer leaf isn't practical. If we had a wider slot in the inner leaf blockwork it would allow movement, but that would mean using narrower than header bricks either side of the vent. The vents look to be 205 wide. To allow 112.5 mm of movement hit bond(*) would need ((440-112.5-205)/2) = 41mm wide bricks either side of the vent on the inner leaf.

 

(*) the semi hitting bond here is being accurate to within half a brick in Flemish which would allow some monk bond like fudges.

 

Could take out two full blocks and use a 600mm lintel with more bricks but that will add quite a lot of work/expense.

 

Can you get narrow periscope vents or periscope vents that allow sideways adjustment. Using some tilt I expect you could tweak about 10mm but not much more with the standard vent type shown here (cavity is 100mm).

 

 

 

Ideal:

 

Acceptable fudge:

Edited May 18, 2020 by MortarThePoint

[JFDIY]

I worked out all my bond and brick positioning before putting the first drop of mortar down, for both inner and outer skins. If you want a particular layout you'll have to get right into the detail, I suspect the brickys will just put them to drawing, or whatever is easiest for them.

[MortarThePoint]

Good point, I do have a bonded out drawing I'd forgotten about, I'll take a look.

 

A half brick periscope vent would be good though. Can you get them? Full brick vents have an area of 7000mm2. Based on those vents, NHBC's maximum 2m c/c is massively over the 12 needed by area (and therefore void volume) and the 13 needed by perimeter length. A half brick vent might have 3000mm2 and that would make for 30 vents needed which ties in well with the 2m c/c.

[MortarThePoint]

This image made me think of a possible solution. Whenever feeling the need to hide something it's worth thinking whether it can be turned in to a feature. I could get some metal grates 335mm wide and recess them in to the wall with the vent behind it.

 

[PeterW]

14 hours ago, MortarThePoint said:

Thanks, great advice. It will be a bit more involved for us I think as we are to use a precast slab (Thermabeam) which has particular contact points. I am checking with the manufacturer but I think the vents will have to be lintelled over as shown in the diagram below. The dotted lines are the concrete pillars at the end of the beam and the honeycomb is EPS insulation.

 

I can't see any reasonable way we can hit bond for the outer leaf which is a shame. Complicated a little by Flemish bond, but would be difficult either way.

 

If you are using concrete beams why are you needing a lot of ventilation ..?? It’s mainly there to stop damp getting into timber beams, you don’t have that issue ..??

 

 

[MortarThePoint]

Just now, PeterW said:

If you are using concrete beams why are you needing a lot of ventilation ..?? It’s mainly there to stop damp getting into timber beams, you don’t have that issue ..??

 

As there was made ground on site the Site Investigation said there could be Ground Gas. Testing for Ground Gas looked as expensive as installing a Ground Gas membrane, so we are just doing the membrane instead. That needs a ventilated void below floor.

[MortarThePoint]

The NHBC stipulation of "within 450mm of each end of any wall" is quite restrictive as if that is referring to the outer leaf (which is where it is ultimately visible from) you have to take wall thickness away from that to see where it ends up on the inner leaf. I have some 390mm thick walls so that puts the vents right at the corner. Can that be right, looks pretty extreme.

 

[MortarThePoint]

It's basically impossible to hit Flemish bond whilst satisfying the NHBC "within 450mm of each end of any wall" requirement if you have cavity walls over 338mm which is the vast majority of mine at sub floor level. 496mm is the first point on bond at which it doesn't clash with the inner leaf of the perpendicular wall. There is only one Stretcher with any part of it within 450mm of the quoin and the edge quoin side of that would cause a clash. I'm presuming the NHBC requirement is any part of the vent within 450mm of end of wall.

 

 

I don't know what this adapted (no Queen Closers, uses 3/4 batts) form of Flemish bond is called but it is possible to get that to work:

 

 

[ADLIan]

Hi all. Just a few comments on this thread.

• The manufacturer is unlikely to overspec or ask for too many vents as the numbers can easily be checked. Possible loss of credibility & loss of sales.
• BR Appr Docs (& NHBC) require sub floor vent in both timber and concrete suspended floors.
• NHBC Standards are more prescriptive with the 2m max spacing. This is a good thing as fewer vents, further apart on one wall, is likely to result in pockets of still air between the vents.
• Could it be argued that the 450mm from each corner be measured from the internal faces of the wall. As above to avoid pockets of still air at this point if, for example, the nearest vents were 2m away.

[MortarThePoint]

4 hours ago, ADLIan said:

• Could it be argued that the 450mm from each corner be measured from the internal faces of the wall. As above to avoid pockets of still air at this point if, for example, the nearest vents were 2m away.

 

That would be good as 450mm is too tight really otherwise.

 

It would still be good to find a 4" x 3" air brick though. I'm tempted to 3D print an adapter but that would take quite a lot of time and plastic

 

Edit: Here is something that could possibly do the trick, stepping a brick wide vent down to half brick wide

Edited May 18, 2020 by MortarThePoint

[Mr Punter]

It can sometimes be a bit tricky to find room for these, especially with full height windows, doors, steel columns, wind posts and services.  With your brick bond make sure you leave lots of space with a bigger lintel on the inner leaf.

[MortarThePoint]

4 hours ago, Mr Punter said:

It can sometimes be a bit tricky to find room for these, especially with full height windows, doors, steel columns, wind posts and services.  With your brick bond make sure you leave lots of space with a bigger lintel on the inner leaf.

 

That's what I'm concluding too. I think 600mm lintels for these with a 400mm opening would make it easy. That leaves a big opening on the inside of the cavity for critters to get in to the cavity though. 

[JFDIY]

9 hours ago, MortarThePoint said:

That leaves a big opening on the inside of the cavity for critters to get in to the cavity though

Wouldn't you just fill the holes with blocks once the vent is finalised on the outer skin? Or does the floor go in before the outer skin is established?

 

And I suspect the 450mm from a corner requirement is in the space to be ventilated, so inside skin. Might be worth a chat/email with building control, it's nice to build exactly to regs, but if you can work with BC to make it easier for yourself then why struggle.

[MortarThePoint]

Just now, JFDIY said:

Wouldn't you just fill the holes with blocks once the vent is finalised on the outer skin? Or does the floor go in before the outer skin is established?

 

And I suspect the 450mm from a corner requirement is in the space to be ventilated, so inside skin. Might be worth a chat/email with building control, it's nice to build exactly to regs, but if you can work with BC to make it easier for yourself then why struggle.

 

Floor goes down before outer skin is established unfortunately.

 

The BCO said "what does your warranty provider require" and the warranty provider said "what does your BCO require" so that's handy. Obviously NHBC aren't regulations, but they are a standard worth sticking to if possible.

[MortarThePoint]

Well it pays to be looking at the most up to date documents. NHBC had changed their wording since 2011 (I've added the highlight):

"Void ventilation should be provided to whichever gives the greater opening area:

         1500mm2 per metre run of external wall          500mm2 per m2 of floor area.
In the case of timber floors, ventilators should be spaced at no more that 2m centres and within 450mm of the end of any wall."

Edited May 21, 2020 by MortarThePoint

[MortarThePoint]

Building Regulations Part C says that for "SUSPENDED CONCRETE GROUND FLOORS MOISTURE FROM THE GROUND"

 

 

But sets no c/c requirement

[Russell griffiths]

On 18/05/2020 at 09:06, MortarThePoint said:

Something tricky here is that all this needs to get established before brick bond is established, so having the vent nicely in bond on the outer leaf isn't practical. If we had a wider slot in the inner leaf blockwork it would allow movement, but that would mean using narrower than header bricks either side of the vent. The vents look to be 205 wide. To allow 112.5 mm of movement hit bond(*) would need ((440-112.5-205)/2) = 41mm wide bricks either side of the vent on the inner leaf.

 

(*) the semi hitting bond here is being accurate to within half a brick in Flemish which would allow some monk bond like fudges.

 

Could take out two full blocks and use a 600mm lintel with more bricks but that will add quite a lot of work/expense.

 

Can you get narrow periscope vents or periscope vents that allow sideways adjustment. Using some tilt I expect you could tweak about 10mm but not much more with the standard vent type shown here (cavity is 100mm).

 

 

 

Ideal:

 

Acceptable fudge:

You can get a vent that has an adapter on the back to connect to 100mm Flexi ducting, you could install this and leave all floppy until you do your brick work. 

 

Or or I would forget the pre made periscope and make your own 

or fit two periscope side by side, then fit the air brick between the two cutting out the centre bit of plastic and glue in a couple of little blanking bits. 

[MortarThePoint]

4 minutes ago, Russell griffiths said:

You can get a vent that has an adapter on the back to connect to 100mm Flexi ducting, you could install this and leave all floppy until you do your brick work. 

 

Or or I would forget the pre made periscope and make your own 

or fit two periscope side by side, then fit the air brick between the two cutting out the centre bit of plastic and glue in a couple of little blanking bits. 

 

Nice ideas. I think I'm trying to get them on bond and putting them in a 350 or 400mm slot to allow adjustment.

 

 

Edited May 21, 2020 by MortarThePoint