Under Floor Ventilation Grills

[PSE]

We moved into our new home around 4 months ago and we've had a few problems that some are still being sorted out and others are starting to appear.

We had the property professionally snagged and one of the questions that the builder (Bellway) has not yet really answered was the amount of ventilation required under the ground floor.  Each side has either 2 or 3 under floor ventilators except the one which has just had one.  In the snagging report, it asked the builder to confirm that this is correct and that there is enough ventilation for the property. 

Bellway have not really confirmed if there is enough, only verbally saying that the NHBC have passed it so there must be enough. Belway also added a extra ventilator to this side by saying that on another house the same as ours, that there is one in this position. When I looked how this under floor ventilator had been fitted I find it hard to believe it's actually doing anything as they only removed one outer course brick to fit it!
How would it be possible to fit the periscope type ventilator when only removing one external brick?
The other thing I've noticed on closer inspection is all the originally fitted ventilator grills seem to be upside down!
The writing on them is upside down that says " Equivalent Area 6170mm2" and also the way the slots on the grills are angled it would mean driven rain would easily enter the ventilator as opposed to if they were the opposite way round it would be more difficult for driven rain to enter the ventilator.
The ventilator that was recently fitted, the grill is the opposite way round - with the writing the right way up!

Can someone confirm if this is correct as I've 'Googled it' but can't find the answer. 

Thanks

[New Home Expert]

The NHBC standards are quite clear on this and the requirement is very well known. As such I am surprised your snagging inspector "asked Bellway to confirm that is was correct" rather than knowing it was wrong!

NHBC standard 5.2 - D10 states:

Ventilation of underfloor voids
A minimum void of not less than 75mm should be provided below the underside of floor slabs and beams.
On shrinkable soil where heave could take place, allowance should be made for the void to accommodate the following movements according to the shrinkage potential of the soil:
high potential - 150mm - medium potential - 100mm - low potential - 50mm.

In all regions except Scotland, all underfloor voids should be ventilated by openings providing not less than 600mm² of open area per metre run of external wall, equivalent to one 225mm x 75mm clay air brick to BS 493 at 2m centres.

In Scotland, all underfloor voids should be ventilated by openings providing not less than 1500mm² of open area per metre run of external wall, equivalent to one 225mm x 150mm air brick at 1.5m centres.

Ventilation openings should be provided on at least two opposite sides. Where this is not possible, effective cross ventilation from opposite sides should be provided by a combination of openings and air ducts.

Just because the NHBC inspector "passed" the house does not mean that everything conformed to their Standards and the Building Regulations.  You say Bellway has added a ventilator to the elevation that only had one, having originally missed it.  You are correct that this air brick is only ventilating to cavity and is not ducted under the floor as it should be and would have been if it had been fitted during the build.

It gets even worse, as Bellway have even managed to fit all the air bricks upside down.  Even more hard to believe as the airbricks have "TOP" marked on them! 
You are right to be concerned as they will clearly channel water down from the face of the wall under your floor!

Finally, the bottom of the airbrick is normally at least one course or 75mm above finished ground level.
The airbricks should not be located under doors or within 600mm of the external corner.
Any intermediate supporting walls under the suspended ground floor should also have voids to allow cross-flow ventilation.

[PSE]

Thank you very much for the information you posted. 
As I went out yesterday I quickly read your reply that confirmed the vents were the wrong way round. so as I left I informed the site manager.  Upon my return last night I've noticed all the vents around the property except the back garden, have been altered to the correct way up. 

The new vent that was installed last week has also been reinstalled.  It still looks to me though that no other bricks have been removed so still not sure if it's been fitted correctly? Surely though they would still not get it wrong?

Thanks again.

[New Home Expert]

I am impressed that a quick word to the site manager facilitated the air bricks being cut out and re fitted the right way up.
I am very suspicious that this was done without telling you and without you having the opportunity to be there to watch!

To install the periscope duct and more importantly cut a hole in the blockwork under the suspended PCC beam and block floor to fit it, the air brick and at at least 4 courses of brickwork under the air brick, two bricks wide would need to be taken out and replaced. It is quite possible that two courses would be below finished ground so the paving may also need to come out and a small hole excavated (by hand)

Finally yes, "they would still get it wrong"  all housebuilders are master bodgers!

[PSE]

Thanks again for your reply. 
When I see the site foreman next I'll cross reference question him about how it was fitted.
I've measured the total length of all the walls and rounded up it comes to 33 metres.  We have currently installed 9 or 10 ventilators fitted around all four elevations. Each ventilator has "Equivalent Area 6170mm2" wrote on them.
We have none under any doors but I've seen some on the estate under patio doors??

Thanks again

[New Home Expert]

NHBC Standard for this states: "Ventilation openings should be provided on at least two opposite sides."
You have air bricks on all four elevations which is good and better. The air area 6170sq.mm is correct too.
What you don't know is if they are periscoped under the floor and/or properly connected to the periscope ducting.

Asking the foreman, he will tell you they are!  He is hardly likely to say anything else even if it is the truth.
The only way you will know for sure is to get a replacement air brick, then smash one out and check yourself.
You should see the top half of the black ducting. You will then need to poke a piece of wire down it and should hit the bottom periscope bend. This will confirm the ducting (to that air brick) has been installed correctly and may give you re assurance the rest have been too.

You need to measure each elevation separately.
If it is 12 metres, 5 vents at 2m centres with the first and last 1m from the end.
If it is 6 metres, 3 vents at 2m centres etc. with the first and last 1m from the end.
Putting a vent under a door means the top of the periscope will be shallow in the floor screed and may cause it to crack, if indeed there is any cover over it at all.

Whilst important, if the air bricks and ducting are not 100% compliant it is not a major issue provided the floor is ventilated to some extent.  What it will tell you ios that your new home has been "bodged" in this respect and that other issues may be lurking to be discovered at a later date that may cause you more serious problems.

I know of one Bellway new homebuyer who had his house demolished because the wrong bricks had been used, the mortar used was too weak and the roof had been bodged during construction and then made worse by the NHBC's contractors attempting rectification.

http://forum.brand-newhomes.co.uk/nhbc-warranty/nhbc-hire-cowboy-builders/
http://forum.snagging.org/nhbc/2077-nhbc-butchered-my-roof.html#post7909

[PSE]

Thank you again for your reply. The link that you posted is a X-rated horror story too.

[PSE]

Just an update. 
The builder got the groundworks company to do a report/survey on the under floor ventilation and the drainage on our property. 

I finally received the report and with regard to the underfloor ventilation it basically says as a rule of thumb they try to add ventilation every 2-3m on all elevations to cover every situation.  Regarding our particular property, the report says it was unable to do this along the drive elevation due to the ground height of the tarmac drive (at the top of the drive there is nearly one brick course height to the cavity tray and at the bottom there is just over two brick course height to the cavity tray). They also say in their opinion the 10 ventilators we have is sufficient enough for our property.

I originally asked the question to the builder/NHBC/local building control should there be at least a two course brick height (150mm) to the damp course & they all said this rule goes out the window when a cavity tray is used?  After receiving the report I've filed a claim with the NHBC with regards to this.

Wish me luck!!

[New Home Expert]

The good news for all new home buyers is that the NHBC standards are now available to anyone FREE online CLICK

So you can check the standards for yourself and speak/write from a position of knowledge.
Cross ventilation is a requirement but ventilation on all elevations is obviously better.
Airbricks must be at 2 metre centres, so again I am not sure why 3m is deemed acceptable either.

I must say that a cavity tray (if indeed one was actually installed) does not necessarily mean that the DPC can be below 150mm above finished ground level. It may indeed give a slightly better protection (depending on how high the tray extends above external DPC level) but it will not prevent water splashback soaking above DPC and upwards through the external brickwork although gravity will help a bit!

[PSE]

Thank you again for your help & advice.

Back in April I asked the builder first about the DPC I was told by the builder that as a cavity tray had been fitted it was OK.
I then emailed the NHBC regarding this and they checked with the inspector who "passed" this property and they said "weep holes are used to drain cavity trays, where these are used the 150mm rule does not apply." 
I then telephone the local building control just for advice on this and they said it was also acceptable?

[New Home Expert]

Weep vents can also be used for ventilating cavities, mostly in timber frame buildings.
Quite why everyone is telling you the 150mm rule does not apply when the dpc is part of a cavity tray I dont know.
Perhaps they don't know what a tray is and what it does.
A dpc stops damp rising.  If rain splashbacks occur above a too low dpc, then the damp can percolate upwards on the brickwork - maybe even above the level of the internal height of the cavity tray.

It's easier to state it is acceptable than to do something about it when it is wrong.

[PSE]

Thank you for your reply. I called the NHBC Technical Department and in their words "it wouldn't be detrimental to our property." As I posted in a earlier, I've now filed a valid claim with the NHBC.

[PSE]

Thank you for your reply. I called the NHBC Technical Department and in their words "it wouldn't be detrimental to our property." As I posted in a earlier, I've now filed a valid claim with the NHBC.

Just to update you that I've got the NHBC calling next week with the builders regarding the above.  I'll get all the information from yourself & the linked NHBC Standards that you posted.  Any other advice with regards to this will be gratefully appreciated.

[New Home Expert]

I would print off the relevant NHBC standards regarding the Air Bricks.
Good luck with the inspecytion visit, I hope you can get a satisfactory resolution.
It is not as if this is an expensive breach/defect to rectify. 

[PSE]

Ok thanks for your help regarding this.