Basement ventilation

alprod · 26 Aug 2008

I have had some conflicting advice regarding my basement ventilation.

At the moment I have two large air brick vents at either side of large basement (front and back of house) but there is still alot of damp in there so was thinking of putting in an extractor fan over one of the vents.

Someone has advised that i should put an extractor on each vent and draw as much of the damp air out as possible as this will pull the warm air from the house down throught the floorboards and out.

I always thought you should have a crossflow of air in basements, one side in, one side out but this other method seems to make some sense. What do you all think?

Also, my basement is split into two rooms so there is not a diret air flow from the front vent to the back vent.

Thanks

Al

mattylad · 26 Aug 2008

I'd have thought that if you have a fan sucking out, you also need a vent where fresh air can enter the rooms?

alprod · 26 Aug 2008

His theory was that the warm, dry air would get sucked from the house above...

alprod · 26 Aug 2008

Untitled

alprod
26 Aug 2008
1

here is a plan of my basement so to show you what i mean by no direct air flow from the front to the back of the house. Big wall in the way!

^woody^ · 26 Aug 2008

The concept is to draw air across a room and also to draw it from corners if possible.

Natural convection and use of heating helps.

But and extract near to an intake is the worse possible layout

alprod · 26 Aug 2008

Ok, where would be the best place to put an extractor fan? Bottom right corner?

I am looking at tanking the basement and then plasterboarding it so how should should i ventilate the room?

^woody^ · 26 Aug 2008

Generally fans are best placed at 1800mm off the floor and 300mm in from a corner.

Inlet vents should be about 200-300mm off the floor again 300 in from the corners. So you will have to duct the air down from inlets at a higher level.

Another option is a positive pressure fan system. There was a post on basements and venting not long ago, and some of the positive pressure fan manufacturers

JohnD · 27 Aug 2008

sucking warm air from the house is not a good idea, as warm air holds more moisture which will tend to condense in the cold cellar.

Lots of crossflow is the answer

Due to the dividing wall you would benefit from (at least) two vents in each "room" at opposite ends. this will encourage air movement.

The moisture rises, which is why it is best to extract it from high up.

You are talking about vents at front and back of house only, do you live in a terrace then, with no side walls?

alprod · 27 Aug 2008

Yeah i live in a mid terrace. So i think all i could really do is put in an extractor in the bottim right corner?

Or would i get away with putting an extracor over one of the vents that are already in place?

I have also just thought, if i plasterbord ceiling of the basement there will be no ventilation to the joist and floorboards holding up the ground floor...im guessing this is a big no no?

JohnD · 27 Aug 2008

start by putting in more vents, so you have two or three at each end of the house. An airbrick every two metres is not too many.

you can make the holes by hiring a 105mm core drill which cuts a round hole with very little effort. you can use round vents similar o those used on extractor fans and tumble driers. then, if you want to add fans to one or more of them later, it will be an easy fit.

^woody^ · 27 Aug 2008

Intake and extract vents can be ducted, so the fan does not physically have to be where it extracts from the room - same for the vents.

Timber floors do not need to be ventilated if moist air can be prevented from getting into the floor void - ie insulate fully between the joists and fit a vapour check barrier or foil-backed plasterboards

alprod · 27 Aug 2008

Unfortunetly putting more air bricks in the front wall is going to be a problem!

My house is over 100 years old and the top of my basement wall is below the outside ground level. Any holes i drill through the front wall will be underground! Immediately outside my front door is a main road and pavement so i cant even excavate that.

The space for the air vent at the front has always been there, (think it may have been an old coal shoot or window) and causes me problems with surface water at the moment. See pic..

Untitled

alprod
8 Jul 2008
2

There is a small circular drain vent outside my front on the pavement which is which lets the air in. Under this part of the pavement the ground has been excavated so the air goes down and through the airbrick (which is below ground level). One problem is that all the surface water also goes down the drain vent and pools in the hole underground against the front wall and air brick and thats why the bricks are so damp under the airbrick in the pic above!

I don't know how on earth I could put some more airbricks into the front wall given that the highest point on my front basement wall is below the outside ground level??

JohnD · 27 Aug 2008

you can get a Z-shaped duct for this very job

it looks from your pic like you used to have cellar windows, there would have been a light well outside. if you can re-excavate this so it drains better, that will prevent water ponding against the wall.

alprod · 27 Aug 2008

My wall is only one brick deep so how would i fit this?

JohnD · 27 Aug 2008

air brick outside above ground

vent inside below ground

duct runs between them, built into the wall or enters between joists

it is even easier with 105mm pipe as you can use elbows

in some districts cellar vents are put beneath front doorstep

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Basement ventilation

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