We are putting a single storey extension on the rear of a property and as a result the existing dining room now has no windows or doors that open to external air, they open onto the new extension instead.
Therefore, I was wondering what ventilation needs to be specificed to comply with building Regulations without the need for a mechanical extract fan with 15 minute over run. And where is it best to put this ventilation.
It is possible to do as you describe without the need for mechanical ventilation. The Scottish Building regs allow this if it is not possible to insert mechanical ventilation, i.e. there is no external wall. In this instance as long as you have an opening between the existing and proposed rooms which is the area of at least 1/15th of the total floor area of both rooms.
you will have to check what the reuqirements are for English regs. but i'd imagine they'd be in the same region.
I am sure you will be obtaining a Building Warrant permission as part of your extension plans and this question may have already arisen out of this issue.
The Technical Standards Scotland state;
[i]3.14.7 Conservatories and extensions built over existing windows
Constructing a conservatory or extension over an existing window, or
ventilator, will effectively result in an internal room, restrict air movement and
could significantly reduce natural ventilation to that room. Reference should
be made to clause 3.16.2 relating to natural lighting, and to the guidance to
standards 3.21 and 3.22 on the ventilation of combustion appliances, as this
also may be relevant. There are other recommendations in Section 2: Fire
relating to escape from inner rooms.
A conservatory may be constructed over a ventilator serving a room in a
dwelling provided that the ventilation of the conservatory is to the outside air
and has an opening area of at least 1/30th of the total combined floor area of
the internal room so formed and the conservatory. The ventilator to the
internal room should have an opening area of at least 1/30th of the floor area
of the room.
An extension may also be built over a ventilator but a new ventilator should
be provided to the room. Where this is not practicable, e.g. where there is no
external wall, the new extension should be treated as part of the existing
room rather than the creation of a separate internal room because the
extension will be more airtight than a conservatory and therefore the rate of
air change will be compromised. The opening area between the 2 parts of
the room should be not less than 1/15th of the total combined area of the
existing room and the extension.
If the conservatory or extension is constructed over an area that generates moisture, such as a kitchen, bathroom, shower room or utility room, mechanical extract, via a duct if necessary, or a passive stack ventilation system should be provided direct to the outside air. Any existing system disadvantaged by the work may require to be altered to ensure supply and extracted air is still to the outside air.[/i][/i]
The English Approved Documents Part F state similar and can be seen on pages 21 and 22 on attached e-mail site below however this then relates back (as Scottish Technical Standards) to floor area as diagrem 2 on page 23.
If you havent already got your Warrant Approval then please make sure you obtain this before starting work.
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