Disused chimneys – to block or to ventilate?

One of the more minor decisions that came up during my insulation retrofit project related to a disused chimney on the end wall. The chimney pot had an ‘elephant’s foot’ on top of it; a cover to prevent rain getting in, but with ventilation holes around the side. There was also a vent in the bedroom at the base of the chimney stack. Conventional wisdom is that there is a significant risk of damp stagnant air in disused chimneys that can condense and cause moisture problems in the wall. The idea of having an opening at the base and at the top of the chimney stack is to allow an airflow through the chimney, in order to prevent this occuring.

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  Vent in the disused fireplace

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  Plaster removed, revealing the blocked up fireplace and the brick gap over which the vent had been installed.

However, opinions vary on whether this is strictly necessary, and it is tempting from a heat loss and insulation perspective to block up the gap at the bottom in the newly insulated room. In practice it is probably fairly site-specific in terms of whether this represents a damp risk. It might not be a problem in a low rainfall area where the outside wall is fairly protected, and builders in these areas may have been blocking chimneys for years without causing problems. But in the current situation the chimney is in a north end terrace with significant wind driven rain. Regulations tend to go for broad applicability and fail-safe options, so ventilating disused chimneys is the standard practice. Being a bit reluctant do this, I was interested in alternative options. One option is to install a ventilation brick in the outside wall of the house, so that the chimney is ventilated but with less of an obvious draught/heat loss issue. This seemed like a good idea, albeit with an associated amount of ladders and work faff.

After a few feedback loops of indecision on my part, I decided that what I was lacking was evidence. So we have decided to block up the ventilation gap at the bottom, but cast a plastic pipe into it, with a plug of insulation in it. I can then put a temperature and humidity probe into the chimney space through the pipe and attach it to a data logger in order to get some actual data. At a later date, the pipe can either be used to provide ventilation if the data suggests that this is necessary, or I can leave it plugged.  

Pipe installed with miscan-lime cast around it. We will cap this off properly, and then simply remove the cap when it’s time to put the probe wire in.

In one sense this is a non-decision on my part, but it also feels like a no-regrets option. And it’s a great opportunity for a graph of some data in a years time! To be continued…