Wood Fibre IWI: What about that fireplace?

[larry]

Hi all,

I've had some great advice on here before so I hope people don't mind helping me again.

 

Have started on a new room now for insulating with wood fibre board. I have used both Steicoflex (flexi wood fibre) between battens and Steicotherm (rigid wood fibre) in the first chunk of the house done so far, and very pleased with the results already. 

 

The new room has a fireplace and chimney on an external wall. It's a  solid brick wall. 1920s house, detached, East Anglia so quite dry, and we have fitted an MVHR.   It is a first floor room. Below is the living room which also has a fireplace and a single flue. We don't plan to use that fireplace either but might try and retain something when we eventually get to that room in about 2028...

 

Today's job: I have removed the fireplace and 100 years worth of soot, dust and sticks from bird nests. I think I must have removed something like 70kg of crud, it was pretty grim!  But all bone dry and no evidence (to my eyes) of any damp... 

 

Chimney is already capped - one of those half round caps so stops rain coming down at least but allows ventilation. 

 

We are not planning to reinstate the fireplace in this bedroom, so ideally would just cover over. . 

 

My questions (which are all linked as you will see!):

 

1. Can I cover the builder's opening on the fireplace over directly with rigid wood fibre board and lime plaster skim and just be done with it? (Or if not, can I cover it with anything else?)

2. How worried do I need to be about causing condesnation in the chimney/flue? What can I do to mitigate that?

3. Would a sensible plan to avoid condensation in the chimney/flue be to drill a few holes through the external wall at the base to then allow for through ventilation in the flue itself?  Or is this just stupid by making the chimney colder? 

4. Or do people fill their chimneys with insulation? If so, what and how?

 

I have temporarily shoved a pillow up there and boarded over the front in eager anticipation of the forum's collective wisdom...

 

 

 

 

[Redbeard]

If you are happy that it is dry you could insulate from the top with vermiculite or EPS beads. I did this for a client 5 years ago and no issues.

 

The flaw I find with ventilating unused flues as per DIY guides is that you introduce potentially warm moist air into the flue where it can potentially condense as the flue cools near the bedroom ceiling line.

[larry]

Just now, Redbeard said:

If you are happy that it is dry you could insulate from the top with vermiculite or EPS beads. I did this for a client 5 years ago and no issues.

Thanks. Sorry to be dense, but 'from the top', you mean literally pour a load down the chimney pot? 

 

Just now, Redbeard said:

The flaw I find with ventilating unused flues as per DIY guides is that you introduce potentially warm moist air into the flue where it can potentially condense as the flue cools near the bedroom ceiling line.

 

Yes, that was my worry, hence the suggestion about trying to introduce the ventilation externally and just keep it all cold. 

 

Thank you!

 

 

 

[Redbeard]

Yes, I poured it down and capped the flue. You could ventilate externally and apply IWI to the full chimney breast.

[larry]

Just now, Redbeard said:

Yes, I poured it down and capped the flue. You could ventilate externally and apply IWI to the full chimney breast.

 

That's super. Thanks. Glad that's not a completely off whack idea! Very appreciated. 

[Redbeard]

** Ventilation externally of course if you do *not* insulate the flue! We did both insulation to the flue and IWI.

[larry]

1 minute ago, Redbeard said:

** Ventilation externally of course if you do *not* insulate the flue! We did both insulation to the flue and IWI.

Thanks for clarity - but yes had guessed that. As much as anything I guess the polybeads wouldn't last long through the ventilation holes?! 

 

I suspect venting externally and insulating internally with IWI is probably what I will do - the chimney pot is a long way up, I hate ladders, and trying to find tradespeople around our neck of the woods is about as tricky as finding a dentist...