Under Floor Ventilation Grills

[PSE]

Hi

I'm trying to get all the information we need printed with regards to the NHBC Standards as proof for when they visit us.  One area that is a bit of a grey area is with regards to the spacing of the underfloor ventilators as depending what you read is a bit contradictory and confusing. 


On the Timloc Underfloor Ventilation information they recommend:

• Building Regulations require a free airflow below suspended ground floors of at least 1500mm2 per metre run of wall. This can be achieved by spacing the ventilators at 4m centres, however, such a wide spacing is not recommended as stagnant air pockets could form in the underfloor void. Timloc recommend a spacing of not more than 2m centres.

• The NHBC recommend that underfloor ventilators are spaced at a maximum of 2m centres, with ventilators also positioned
not more than 450mm from the ends of the wall.
• As a minimum requirement ventilators should be positioned down two opposite sides of the building so as to create a cross
flow ventilation action. It is good practice to position ventilators around the full perimeter of the building, particularly with complex building designs.
• If the underfloor void is separated by dividing walls, openings must be provided to allow a free flow of air around the underfloor void.
• The standard ventilator will fit into a cavity wall with an external leaf of 100-102.5mm, a cavity width of 50mm and an inner leaf of 100mm. External and internal horizontal extension sleeves are available to accommodate thicker walls or wider cavities"

In the NHBC Standards 5.2 D10 it states:
http://www.nhbc.co.uk/Builders/ProductsandServices/Standardsplus2014/#142

And then in the NHBC Standards 5.2 D22 it states:
http://www.nhbc.co.uk/Builders/ProductsandServices/Standardsplus2014/#143


Could you confirm please if the maximum 2m centres of the underfloor ventilators is only applicable to timber suspended ground floors as the NHBC 5.2 D22 states or is it applicable to both pre cast concrete suspended ground floors and timber suspended ground floors??  In the NHBC Standards 5.2 D10 there's no mention of the maximum 2m centres as in NHBC Standards 5.2 D22  but in NHBC Standards 5.2 D10 and D22 they both state:
 
"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."

So I am finding it a bit confusing and contradictory.

Thanks again.

[New Home Expert]

Forget what Timloc state.  It is the NHBC standards that count.
Yes the 2m centres is for all ventilated floor void, including pre cast beam and block.
Standard airbricks give the 1500sqmm of ventilation area required and at 2m centres will comply with NHBC standards.

Just you knowing what the standard is and where it is will be enough to stop (either/or/both) the NHBC inspector and housebuilder from b*llsh*tting you!

[PSE]

Thank you again for your advice.  You've been extremely helpful. 

[New Home Expert]

You may like to record the conversation you have. Easy to do on most smart phones.
Be aware that the air brick may not line up with the periscope sleeve.
Another trick builders use in cases such as yours, is to use just one part of the periscope so it only ventilates the cavity not the floor void.

[PSE]

Hi

I wish I'd have read your message yesterday instead of today


Basically the NHBC were a waste of time and backed up the builder (it makes you wonder if the builder buys them off well before they get there??) . 
As for the DPC level they said DPC and Cavity Trays should be installed to prevent moisture entering the building - which they said it is - as for the 150mm rule they said that that is only a guideline how it may be achieved . 

As for the Underfloor vents they did the 1500mm2/metre run calculation and the 500mm2/m2 calculation and said based on that there should be a minimum of 7 vents as we've got 10.  As for the minimum every 2m rule again they said that is only a guideline and is more for suspended timber floors .

Thanks again

[New Home Expert]

Not good.
I am coming across the 150mm above ground level for dpc being a suggestion quite a lot.
I cannot understand why as the NHBC standard is quite clear:

DAMP-PROOF COURSE
5.1 - S9
Moisture from the ground shall be prevented from reaching the inside of the building
A damp-proof course should be positioned at least 150mm above finished ground or paving level and should link with any ground floor dpm.
The dpc should be of the correct width and fully bedded.
For guidance concerning setting and bedding dpcs, reference should be made to Chapter 6.1 ‘External masonry walls’ (Sitework).

Whilst it is true the Building Regulations do have Approved documents that offer guidance, the NHBC standards state "should be". They will say this is not "must be"

Regarding the vents, if you have sufficient ventilation area and they work properly, I would let this go.
Should you have any issue at all in the future, you will have a written record from the NHBC regarding what the NHBC inspector said and the fact that you reported it within the 2 year period means the warranty for any problems relating to this breach of NHBC standards would be for the entire 10 year warranty period.

It does the NHBC no credit at all to continually "side" with the housebuilders.  It results in suspicion that the NHBC are "in the housebuildr's pockets" which to an extent they are as they pay huge sums in premiums to the NHBC for the warranty policies on the homes they build.  However, I very much doubt any housebuilder would or would be able to bribe an individual NHBC inspector prior to a warranty claim visit.

[PSE]

Thanks for your reply again 

Basically they said the:
"Moisture from the ground shall be prevented from reaching the inside of the building"
is in black and what must happen, and the:
"A damp-proof course should be positioned at least 150mm above finished ground or paving level and should link with any ground floor dpm. The dpc should be of the correct width and fully bedded. For guidance concerning setting and bedding dpcs, reference should be made to Chapter 6.1 ‘External masonry walls’"
bit in light blue is how it may be achieved. 

They said our property satisfied them as no moisture from the ground was reaching the inside of the building so has been built to the standards but not the guidelines. 

I gave them a scenario of a property being built with the DPC being 150mm below the ground and for whatever reason no moisture was entering the inside of the building.  I asked them the question would this property still be built to the standards as no moisture was entering the building??  As you can guess I never really got an answer to the scenario. 

Thanks again 

[New Home Expert]

So what the NHBC are saying is:
Unless there is a breach to their standards or the Building Regulations that IS actually causing a particular defect that is a requirement NOT to happen, nothing is required to be done.

A bit like having bald tyres and saying it is OK because it isn't raining or the car hasn't crashed yet!

The old "should be"   "must be" clarification get out!

[New Home Expert]

PSE, I hope you don't mind but your recent PM has some interesting opinions, that deserve a wider audience -as does my reply.

PSE said:
"Unfortunately the standards are worded in such a way that the NHBC can interpret them how they want to any given situation.  The NHBC Standards 400+ pages should be condensed down to about 2 or 3. I'll wait for the report before my next plan of attack but it is frustrating and time consuming but I'm determined and won't just let this drop.  I am thankful that people like yourself are A LOT more help than the NHBC."

The NHBC Standards exist to give housebuilders, more particularly their site managers a clear easily understood requirements to which they should build the nation's new homes. Unfortunately, the vast majority of site managers are either not up to the job, are poor managers, or just don't give a ***k!

It would be impossible to condense the standards down, in fact there should be more clarification. The "should be" suggestion needs to be changed to a  "must be" requirement.

Whilst attending and NHBC course years ago, a retired NHBC inspector joked that among their inspectors, the NHBC stood for "No Hope Of Bloody Claiming" 
It would appear to still be the case.