Filling chimneys

Hello again. When people talk of filling chimneys with LECA lightweight expanded clay aggregate or similar, I always assumed that the flue was removed first and the entire space was filled. Am I right? or do you have to keep the flue - maybe block it at the bottom with something breathable?

I have read this very helpful comment Chimney stack removal - #8 by don_helen from @don_helen - but it wasn’t 100% clear to me.

If filling a chimney, no voids should be left. This means that any flue liners have to be removed. If, on the other hand, the chimney is brought below the loft line, so that the remaining structure is within the thermal envelope, no filling is required and the space can be utilised for ducts, plumbing or wiring. You could even fit a dumb waiter if you wanted.

Hi Jonathan, to clarify our chimney wasn’t lined with a flue, just brickwork approx. 9” square.

The stack was removed and brickwork brought down into loft but above loft insulation depth.

We chose the foam glass bubbles (Geocell) over other mineral materials because of their inert, non wicking, low density, structural integrity & non clumping properties.

We didn’t airtight seal at fireplace end to allow some breathability.

I have noticed the original post needs editing it has the wrong size, we used the 4-8mm bubbles (not 2-4).

Thanks both. I think I have been under a misconception. Our chimney breast is 1.5 m wide at the ground floor. I always assumed it was a hollow space with a flue somehow supported up the centre. Now I’m thinking it’s 1.5 m of solid masonry with a narrow airway in the middle called the flue.

1920s brick terrace for what it’s worth

I can’t comment on your construction but can tell you about mine, 1936.

The chimney is as wide as yours and the void behind the fireplace is at least a metre wide. I discovered that when I bought a chimney sheep some years ago and it had nothing to hold it in place. As far as I can tell that void is at least as tall as me (180cm). I imagine that when the fire was lit the whole chimney breast worked as a radiator, and to some extent a storage heater. I dare say that at the time it was cutting edge technology.

That’s small compared with my childhood home, with a 3 metre wide inglenook with a concealed priests’ hole. I expect that any hidden priest would have got very hot but searching troops were unlikely to look too carefully behind the flames.

Our house is a 1910 semi. In rooms the chimney breast is a similar width to yours and runs from ground floor up through bedroom ceiling. It’s brick construction and accommodates two 9” holes/flues side by side one from ground floor & one from bedroom. Bedroom fireplace had long since been blocked off before we moved in.

We filled both from the loft as described previously.

Viewed from above in loft see two holes filled with geocell bubbles. Left was from the recently active fire place hence soot.

View from side see width of chimney breast brickwork which used to rise through roof. Picture taken before insulation

We swept the chimney from bottom to top & top to bottom before filling, pleased to say we didn’t find any priests or Victorian children.

Hope this helps. Don
Tried to include photos not sure if it’s worked