If you have a new account but are having problems posting or verifying your account, please email us on for help. Thanks :)
Hello all! Please ensure that you are posting a new thread or question in the appropriate forum. The Feedback forum is overwhelmed with questions that are having to be moved elsewhere. If you need help to verify your account contact
Hi all,
Vanilla are planning an update to the site on April 24th (next Wednesday). It is a major PHP8 update which is expected to boost performance across the site. The site will be down from 7pm and it is expected to take about an hour to complete. We appreciate your patience during the update.
Thanks all.

Blocking a chimney

  • 23-12-2021 10:30pm
    Registered Users Posts: 179 ✭✭

    So basically in the process of retrofit on my house and iv an unused chimney what are my options can I simply block it from the top or can I just fill it that it needs ventilation or might create damp


  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    Blues14. A lot depends on the construction of the chimney and the age of the house. In a house constructed since the 1960’s I would not fit a vent in the fire ope. It is not necessary if there is no dampness in the chimney from DPC level to the chimney cap at the top. This is very important. A vent in the room will cause heat loss and could also cause foul odours in the house. Have the chimney completely cleaned before blocking up the fire ope.There will be some salts remaining in the flue and it will be necessary to prevent any rainwater from entering the flue to ensure the salts do not get wet. It will also be necessary to ensure that birds cannot build nests in the flue. Fit a wide stainless steel cowl with bird blocker as seen in the photo.

    Also ensure that there is no possibility of ingress of rainwater down through the chimney cap.

    I would not fill the flue under and circumstances. It is not necessary.

  • Registered Users Posts: 6,622 ✭✭✭10-10-20

    I'd say cap at the top but first bung the opening at the chimney pot with a wad of rock wool to provide resistance to free air-flow but not act as a vapour barrier. The wad should be pushed in so that the rock wool isn't compressed, but fills the throat.

    My theory is that Rockwool will offer some resistance to air passage but will only minimally impeded vapour and hence prevent the over-ventilation of the void.

    As Mr Eastwood says, if the chimney is sound in the first place, the moisture intrusion should be minimal creating little concern.

    If you were to take the isolation of voids one step further then similar rockwool bungs should also be placed just slightly above the level of the joists on each floor so that the movement of heat is minimised between thermal zones - but frankly, that's pushing it for a void of the size of a standard chimney.

  • Registered Users Posts: 6,622 ✭✭✭10-10-20