I have a twin chimney stack but one side is no longer used and I want to cap it off. I am getting conflicting advice from builders/roofers about whether the cap can be a complete seal, or whether it should be vented to allow a small amount of air circulation. What is the approved best practice?
If you plan on capping your chimney you should always vent the flue. The best practice for this is to fit a vent at the wall where the original fire would have been and to fit a vented cap on the chimney stack at the chimney can that way it allows for free air circulation around the inside walls or as we call it the flue. The reason been is that if you don't allow the free passage of air you can end up with condensation and all other sort of nasties that can occur in time. When your builder does the work get him to carry out a smoke test to make sure that all is fine. This is done by placing a smoke pellet up the opening and making sure everything is in place.
Good luck with your project,
I fix the problems caused by not ventilating capped chimneys , condensation will build up in the flue or even in the remnants of a chimney through the absorption of moisture to a chemical within the brick work called ammonium sulphate.
If you don't ventilate top to bottom, it will cost you plenty to re render the interior later.