Green Building Forum - Ideas for sealing chimney stacks Sat, 02 May 2026 10:02:53 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Ideas for sealing chimney stacks https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304744#Comment_304744 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304744#Comment_304744 Sat, 30 Dec 2023 11:53:08 +0000 Warm Jake
Now, on to the subject. Idea's to run past you lot for blocking the airflow up my chimneys safely - I know it's a big no no, but I think there must be some smart ideas that could work.

Some notes:

1. I'm in a top floor 1890s Glasgow tenement with access to the attic space. All rooms have a fire place, mostly covered with a vent. I plane to open out all the fireplaces and insulate the ones which are on external walls.
2. I've got daft plans to make the flat as low-energy as possible while respecting the building and not putting it at risk of decay. The flat has a lot of amazingly crafted victorian features which I'm trying to keep.. extensive cornicing.
3. As part of my "air-tight" plan, I want to use the chimney flues to run fresh air ducting from the MVHR. Core access holes into the flue in the attic space and send a duct down to the sealing plate in the throat of the fireplace. The fireplace is normally the furthest point from the room door, so fresh air coming from here seems like a good idea.
4. I won't have the permission or desire to remove the chimney stack down to the roof level as seems to be often suggested.

1st Idea.

Simple cap on the chimney top to stop rain getting down but still vented. Airtight seal the fireplaces off at the throat with a latched gate on the seal plate. During winter months, seal off the latched gate, then in warmer months, open the gate to allow through flow. Question is, would sealing the flue from the bottom for 4-6months a year cause a serious issue?

2nd idea.

This one is a lot more involved...

Seal off the bottom end at the throat. In the attic, open a hole into the flue as near the roof height as possible. Install a sealed shelf into the flue to create an open section of flue between chimney pot and just below roof height. You then have a open flue section, about 2m deep with a sealed bottom. The issue I see with this is that air can't easily flow down to the bottom of this section and any damp ingres may not dry out. So I'm thinking of making a special chimney cap which covers the pot from rain ingres but remains vented. The top of the cap would have a rotating venturi duct with a hose that drops down to the bottom of the chimney flue void. Wind blowing across the venturi duct would create a vacuum, sucking air up the hose creating an air circulation, drawing air in from the vented chimney cap, down into the void then back up venturi hose and out.

The lower section of the flue would be sealed from elements and essentially be part of the building envelope. I would then run the room fresh air ducting down as mentioned before. The flue could then be filled with EPS balls up to the opening in the attic, especially useful for the flues on external walls.

A lot to get through there and would be better explained with some drawings. Maybe I'll draw something up.

Would be great to hear your inputs and other ideas/methods if you know of any..

Thanks,

Jake]]> Ideas for sealing chimney stacks https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304747#Comment_304747 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304747#Comment_304747 Sat, 30 Dec 2023 16:41:02 +0000 philedge Ideas for sealing chimney stacks https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304748#Comment_304748 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304748#Comment_304748 Sat, 30 Dec 2023 17:05:08 +0000 Peter_in_Hungary Except the sides of chimney stacks are usually structural, I would make the opening in the front.

I would make the opening as close to ceiling level as possible, put in the ducting and then seal the flue above the opening, then seal up the opening and cover the whole lot with the loft insulation (MVHR input ducts are warm air so should be insulated or better within the heated envelope). Above the insulation (and therefore above the flue division) remove a brick for stack ventilation.

Re the 1st idea, I am generally against any idea that requires seasonal changes to something to ensure proper working. Changes get done in the first flush of enthusiasm - then it is done late and then missed and forgotten.]]> Ideas for sealing chimney stacks https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304750#Comment_304750 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304750#Comment_304750 Sat, 30 Dec 2023 17:52:05 +0000 Warm Jake
I've got eyes on using the roof space in future, so I wouldn't vent directly into it, however, I could easily set up a duct hose from the eaves up the wall to feed into the external part of the flue.

Yeah, in the case of warm air ducts, they would be insulated running down the flue... even more so if I do the EPS balls fill idea.

Do either of you know if this is a known idea? I've not found anything like it. It could be a transferable idea to other people in my situation if it works out not too complicated.]]> Ideas for sealing chimney stacks https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304751#Comment_304751 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304751#Comment_304751 Sat, 30 Dec 2023 18:21:24 +0000 djh Posted By: Peter_in_Hungarythen seal up the opening and cover the whole lot with the loft insulation (MVHR input ducts are warm air so should be insulated or better within the heated envelope).I'm confused. MVHR ducts fall into two groups. The external group: intake and exhaust. And the internal group: supply and extract. The internal group contain air at nearly internal temperature, so they don't need insulation if they're within the heated volume, and do need it if they're outside. The external group contain air at nearly external temperature so they do need insulation if they're within the heated volume, and don't need it if outside. The MVHR unit itself is like the internal group of ducts.

From Jake's description I'm not clear where he's planning to put the MVHR unit - in the unheated attic, or somewhere in the heated space. But the answer will decide which ducts need insulation and where. If he is thinking about putting the unit in the attic, don't forget you need easy regular access to change the filters.]]> Ideas for sealing chimney stacks https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304754#Comment_304754 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304754#Comment_304754 Sat, 30 Dec 2023 21:07:49 +0000 Peter_in_Hungary Posted By: djhI'm confused. MVHR ducts fall into two groups. The external group: intake and exhaust. And the internal group: supply and extract.

I assumed by saying
Posted By: Warm Jake3. As part of my "air-tight" plan, I want to use the chimney flues to run fresh air ducting from the MVHR. Core access holes into the flue in the attic space and send a duct down to the sealing plate in the throat of the fireplace.

Warm Jake was talking about the supply to the rooms which would need to be insulated. Terminology check - I assumed that fresh air ducting to rooms from MVHR is warmed supply.]]> Ideas for sealing chimney stacks https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304755#Comment_304755 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304755#Comment_304755 Sat, 30 Dec 2023 21:19:28 +0000 Peter_in_Hungary Posted By: Warm JakeI've got eyes on using the roof space in future, so I wouldn't vent directly into it, however, I could easily set up a duct hose from the eaves up the wall to feed into the external part of the flue.
You must not vent into the roof space even if you don't plan to use the space.
Vent and external input should be from the outside, preferably on the same plane of the building (to ensure balanced wind effects) and be should spaced at least 2m apart.

Posted By: Warm JakeYeah, in the case of warm air ducts, they would be insulated running down the flue... even more so if I do the EPS balls fill idea.

The warm air ducts need insulating from the moment they leave the MVHR unit which itself should be insulated either by being within the heated envelope or separately if not as per djh's post]]> Ideas for sealing chimney stacks https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304756#Comment_304756 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=18055&Focus=304756#Comment_304756 Sat, 30 Dec 2023 23:24:42 +0000 Warm Jake
The aim is to have the MVHR in the kitchen, or perhaps in the roof space. I have a fairly good understanding of the necessities of insulating/not insulating ducts whether in or out the heated space.

Posted By: Warm Jake
I've got eyes on using the roof space in future, so I wouldn't vent directly into it, however, I could easily set up a duct hose from the eaves up the wall to feed into the external part of the flue.


This mention of ducts and roof space is in relation to the upper chimney section. What's been mentioned above sounds like a good idea. Section off the very top of the chimney flue - cap the chimney pot with a venting cap - seal the bottom of the section as close to the roof as possible (within the roof space) - run a duct/hose (Nothing to do with MVHR system) from the base of the newly formed upper chimney section to a vent at the roof eaves.

This would allow the sectioned off upper chimney flue chamber to remain well vented in case of damp ingress while allowing me to use the lower section of the chimney flue for whatever I want. Run ducts down to the rooms for the MVHR as well as probably insulate the lower chimney void.

I think it's a good plan! Thanks for pointers]]>