F.A.O. Noseall

[self builder 123]

Hi Noseall

I was wondering if you could help me ?

You kindly helped me the other day regarding my question about in situ cavity trays.

The building is a garage / workshop. To provide background ventialtion we have 9 x 3 air bricks fitted through the outer wall right through to the timber inner frame complete with plastic telescopic sleeves. The ventilation will be controlled via hit and miss screens.

The cavity is approximately 65mm deep and the building is to be rendered with wet harl with waterproofer in both the scratch coat and the top coat.

In your experience should we fit plastic cavity trays above each of the plastic telescopic vents complete with a weep vent ?

Cheers

 

[noseall]

no. i think c.t's in a lot of cases ate a waste of effort.

there is evidence that in exposed coastal areas cavity trays actually perform a function, but the brickwork and pointing would need to be vulnerable (rubbish) in the first place.

as you are rendering i can not see the point.

 

[self builder 123]

Many thanks Noseall

Could you give me your opinion on how many air vents we should fit ?

I have planned for 9 in total fitted both at both low and high level. I understand that the free flow area of each is 6600 sq.mm.

The garage / workshop measures internally 7.1m x 7.1m x 2.5m Tall.

There are no windows hence no trickle vents.

There are no fuel burning appliances.

Cheers

P.S. I think that I have read that weep vents are not theoretically required in a building which has been rendered / wet harled. Is this correct ?

As we have a timber frame we have fitted weep vents in order to allow air into ventilate the cavity.

 

[noseall]

as you are waterproof rendering the outside then interstitial condensation may be an issue, depends how you intend using the building, so you need to vent and tray accordingly.

are you insulating the cavity?

are you heating the workshop?

it sounds like you will be fitting background ventilation for the building itself, good plan. i guess there will be a large garage door?

the problem is, that the measures taken to keep heat in a building contradict those that deal with condensation issues and countering condensation issues can lead to a heated building becoming inefficient.


this makes good reading.

 

[self builder 123]

The cavity between the timber frame ( complete with breather paper ) and the outer leaf will remain fully open. Weep ventshave been fitted so as to ventilate the cavity.

Our plan is to insulate the walls with a mixture of 150mm Rockwool Roll and 150mm Frametherm Glass Wool. The walls will then have a vapour barried fitted and be finished with WBP plywood.

The ceiling will be insulated with 300mm Frametherm Glass Wool and then be finished with either a vapour barrier and then WBP plywood or foil backed plasterboard followed by WBP plywood so as to protect the plasterboard.

The roof pitch is 10 degrees. Preformed trusses have been clad with sarking board, then roofing felt and then box profile sheeting. The soffits have been ventilated so as to allow cross ventilation and air bricks have been fitted in the gables so as to aid in the cross flow ventilation.

The garage will be heated using 2 in number 3kW electric fans heaters mounted on the walls and controlled via a room stat.

There will be a 2.8m wide x 2.5m tall insulated roller door together with a door for access.

As we will have a vapour barrier, ventilated cavities and a ventilated roof space plus dry electric heat when required, I am hoping to avoid any condensation issues.

Is there anything that you could add to this ?

 

[noseall]

it seems like you are taking adequate measures to alleviate condensation.