Do I need to cap chimney?

[F]

Boiler removed so that branch is redundant, living room fireplace now bricked up so the remaining branch is also redundant.

Do I need to cap the chimney or can I simply leave it?

TIA

[visionset]

You really need to have an airflow through the whole stack, so long as you have an airbrick at the bottom it is not necessary to cap it. Whatever rain ends up in there will quickly dry out. The ideal though is to cap it with a rain shielding device that lets air out, usually this is a mushroom pot., though other methods are equally valid such as a ridge tile leaving the ends open. You really should have the vent at the bottom whatever though.

[al]

Hi Slightly different question, why do you need an airflow? Surely it becomes just like a cavity wall -which in my last house (50years old) did not appear to be vented? Al

[Andrew Gabriel]

Chimneys are constructed to be airtight. Any moisture which gets in (typically through parts which are single skin walls) gets trapped, and moisture gradually builds up inside. I opened one out which had been sealed for 20 years (had a vent at bottom but not at the top), and it was soaking wet with condensation running down the flue. It had soaked the whole chminey breast, but this wasn't very visible inside as it had caused the plaster to all come away from the wall and hang in mid-air, creating a moisture break. (Once I realised, one thump with a hammer and all the plaster came crashing down.) It took 3 months to dry out the chimney breast once the bottom and top were vented, before I could replaster it and finish off the decorating.

BTW, if the chimney is on an outside wall and you are going to be bricking up the fireplace, provide the bottom ventilation from the outside rather than the room, as this will lose you less room heat up the chimney. When I did mine, I actually blocked the flue off at the top of the fireplace (piece of plasterboard with loft insulation on top), and vented above that to the outside. Then I am left with the fireplace alcove open to the room, which can be used decoratively or for storage. The rear, being a half brick wall, I lined with cellotex to prevent it forming a cold spot and condensation. I dropped a lighting wire down the flue from the loft to allow a hidden light to be fitted in the alcove, which gives a very nice effect.

I don't think cavity walls normally have anything blocking off the top, and they aren't built to be airtight -- often the odd perp (vertical pointing) allows air through.

[al]

Thanks Andrew. Al

[F]

Pity, that's what I really wanted to avoid as it would spoil a fairly large expanse of flat wall...

[F]

I really want to avoid a vent on the wall in the room (the chimney is on an inside wall). Could I get away with simply not capping it?

[Owain]

Can you vent to or via an underfloor void?

Owain

[Andrew Gabriel]

No you can't. Have you got a sub floor space you could vent it to? What does the fireplace back onto?

I suppose in theory, if you capped the top against rain but vented it, and there's not much exposed stack above the roof, you might just get away with it, but then again, you might not.

[F]

It's sitting on a concrete slab.

Another room.

Looks like I'm stuck with a vent. Is there a minimum size?

[visionset]

You mean available sizes or regs?

Anything will drastically improve things. To a certain extent it won't matter much, small vents will have a faster flow and make up for their smaller size - to a certain extent! So long as there is flow top to bottom I doubt you will have a problem - you would probably get away with as little as 25cm² You can get proprietory brass grilles at that size - 5x5cm.

[Andrew Gabriel]

The top vent on mine is half a brick taken out (with wire mesh to prevent animal entry). It's in the loft just below the roof line as I had no external access at the time, although it would have been better nearer the top of the exposed chimney. The bottom is a whole air brick to the outside as it was easier.