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Are additional airvents required when injecting cavity wall

Postby cornishbiker » Sat Oct 25, 2008 4:27 pm

I am advised that air vents are required to a cavity wall before cavity wall insulation can be applied. We have air bricks and open chimneys. No Gas fires only open solid fuel fires used on an occasional basis.
Are additional airvents rearly required ?.
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Postby Grandad » Sat Jan 03, 2009 5:49 pm

Hi there Cornish Biker. Has everyone ignored you since October 25th?
I don't come on here very often, but I can perhaps help you with your query.
Starting from scratch. Over the years cavity walls have been sealed, vented, sealed, and so on, but if you have no visible vents (air bricks) in the outside of your cavity wall (I presume it is a cavity wall and not solid), then all you have to worry about is the need for your open fires, however occasional, to get sufficisnt air to burn properly. What goes up the chimney must come in from somewhere.
Cavity wall insulating contractors have recently told my neighbour they MUST drill a hole of about 7.5ins diameter and install a permanently open vent to ensure his open fire can get sufficient air to burn properly. This is covered under current Building Regulations.
If you DO have air bricks in the wall, they are likely to be to a larder, common just post WW2, or at very low level to vent the air space under a suspended wooden ground floor.
In these cases, the forming of a "Tunnel" across the cavity was a bit casual, to say the least, and there could be a risk of injected insulation blocking these vents.

Cheers. Grandad.
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Postby cornishbiker » Sun Jan 04, 2009 10:59 am

Hi Grandad and that you for the information. ur house was built in 1928.
It has cavity walls and air bricks and a void under the floorboards that Is about 1 meter deep. I get a tremendous cold draft from under our floors even with fitted carpet. The local authority offered free insulation to our walls and there contractors surveyor told be we needed additional air vents. In the end however the council ran out of money so it was not done. Personally I see little point in filling walls if one is going to make additional holes. I assume as my house has footings which will not be filled I will still have the problem of a drafty floor. Any way I have put the project on hold. I would glasly pay for the instalation myself but somehow seems pointless. Thanks very much for your input it is greatly appreciated. Happy new year to you.
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Postby thedoctor » Sun Jan 04, 2009 3:36 pm

See our project about filling gaps in floorboards, its an amazing invention!
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Postby the specialist » Fri Jan 09, 2009 8:26 pm

Hi sorry this might get long!

Firstly CWI is excellent at reducing heat loss through the walls.
The airvents are necessary to provide cross ventilation to protect the sub floor timbers. The air vents will need to be sleeved to prevent insulation from blocking them. This in itself will increase the ventilation to the subfloor as before sleeving some of the air was dissipating up the cavity.
The current building regs require 1 9x6 airbrick every 2 linear meters of wall.

If you have well fitted carpets it is unlikely you are getting drafts through the floor.
Warm air rises therefore cold air falls. Also if for example you are sat in your lounge with the door shut and the fire on the room temperature will be higher than that in the hall. When you open the door to leave the room warm air will rush out into the hall and be replaced with much cooler air from the hall thus giving the impression that the draft you feel is air coming from outside or the floor. This is merely a convection process which will stop once temperatures equalise.

So get your cwi installed and if necessary additional air vents. you will find that the house is much more evenly heated. leave doors open to allow free movement of air.

Please also be aware that CWI does not always save you money on heating bills. That depends on the efficeincy of your boiler.
some boilers can't seem to get a house warm prior to CWI but then cope after installation, yet use the same gas. Other boilers especially more modern ones will need to produce less heat and so save you money.

Hope I have clarified this for you.

the specialist
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