How to Block a Chimney That is Not in Use

If you have an old chimney that is no longer used, you can block or close it off to prevent birds from nesting, moisture or rain getting in, heat loss up the chimney, noise from outside entering your dwelling.

The most important thing to remember when blocking a chimney is 1 + 0 or 0 + 1 but never 1 + 1. What we actually mean is if you want to block the chimney at the top, you must leave an air gap at the bottom i.e. at the fireplace level.

If you block it completely at the fireplace level, you must leave an air gap at the top where the flue terminates. But you you should never block at both the top and the bottom.

Why? You need air to be able to get inside the chimney and breathe as the temperature will still rise and fall inside as the house warms up and cools down. This is very important otherwise you may get dampness coming through your walls over time.

Flues can be blocked in many ways, each method having its own benefits and drawbacks. Below is how we do it.

2 Methods To Block A Chimney When Not In Use

Here are two ways to block a chimney that is no longer working: 

Method 1: Close off your chimney on top.

This can be done by installing a chimney cap or cowl at the top of the chimney. This method of blocking off a chimney is considered one of the most secure. Chimney caps or cowls are designed to fit over the top of a chimney pot and can be made from different materials, the most popular and durable being stainless steel.

Professional chimney service providers typically install them, but you may purchase an stainless steel cap that you can install yourself if you are comfortable climbing a ladder to reach the top of your flue You can buy a chimney cap or cowl from any fireplace / stove shop or general builders merchant or hardware store. A cap comes in many different sizes, so get one that fits your chimney correctly. You should measure the outside diameter of your flue before you purchase the cap – it should be wider than this outside diameter measurement so that rain runs off outside the flue.

We often use one of our anti bird / anti rain chimney cowls to do this. The height of the stainless steel mesh can be reduced so that cap of the cowl can be pulled down lower over the pot, this may be necessary if your chimney suffers from extreme driving rain where rain can fall sideways in windy, exposed locations. Remember if you close the chimney completely on top, you must leave it open it the bottom.

There are other ways you can close off your chimney on top. You could glue or silicone an angle grinding disk flat onto the chimney pot. The hole at the center of the angle grinding disk allows air to enter the chimney with minimal rain water entering too.

You could cut a piece of lead and shape and press it so that it covers the chimney pot – this method does not allow air to enter the chimney from the top, therefore, you must allow air to enter the chimney from below.

Which ever way you do it, it is worth remembering that chimney caps / cowls are the most effective way of keeping rain and birds out of chimneys.

Method 2: Close off your fireplace at the bottom.

This can be done in a number of ways. But remember, if you block it completely and make it airtight at the bottom, there needs to be an air gap on top of your chimney.

You could stuff rock wool up inside the fireplace. The rock wool will have to be inserted up and inside the flue opening above the fireplace. This is a semi permanent way of doing it and is a bit messy

You could remove the fireplace and block up the opening with bricks and plaster. This could be made airtight or instead it could be done but with an air vent installed where the chimney opening is situated. This is a good option if you want to create more space in the room by removing the fireplace completely. This method creates a uniform solid barrier but it can be messy and difficult to do.

This method is perfect for aesthetic reasons, as it allows you to keep the look of your chimney while blocking it off. The downside is that it can be difficult and expensive to install.

Depending on the shape of the flue opening at the bottom or if there is a flue gather in place, it may be possible to install a steel or stainless steel plate to cover the flue. This is relatively simple way of closing off at the bottom with out the need to remove the fireplace and if you ever want to use the fireplace again, just remove the plate.

Conclusion: Best Way To Close Off a Chimney

When sealing an old chimney, using the right resources from a reliable chimney store is essential. Improperly sealing the chimney will cause it to leak, which can lead to unwanted dampness or moisture ingress.

A Chimney store may also suggest using devices like the anti bird/rain chimney cowls instead of permanently blocking your chimney. This is better when you can’t decide if you’ll need your old chimney later. Regardless of your chosen method, make sure air can enter you chimney from either the top or bottom.