Adapting ancient extractor fan duct

[Northman]

I have an old Xpelair 10" fan in the kitchen that is being replaced with a new 150mm fan. The exisiting ducting is pretty solidly in place, and well sleeved, so I'd like to avoid the mess and hassle of removing it and rebricking. My current thoughts are to use some 150mm ducting to run from inside to the existing outer grill which I'll leave in place; fitting an inline backdraught shutter, and using two pieces of plasterboard on the inner side. There is a solid circular shroud in place that will hold the plasterboard nicely in the centre, then another piece on the inner wall both holding the ducting in place for the new fan.

What would be the suggestions for an effective job particularly the outer side? Cement board,? just leave the duct to run up against the grill and let the shroud do the weatherproofing as it has done for decades?

 

[JohnD]

I wouldn't use plasterboard because there will be damp.

Rigid plastic or foam board might be better. An insulating product will prevent heat loss and condensation. You can cover the inside with plasterboard to decorate.

 

[Northman]

Yes, although the board would be a fair way inside the duct, its not going to last constant exposure, and insulation will be important. So then, I'm looking for a weather resistant insulating board that will fit both sides of the shroud, filling the space, and a piece of plasterboard on the inside wall for skimming. Would something like Kingspan Kooltherm K106 do the job? A 90mm depth would be more or less perfect to mount an internal leaf of plasterboard.

Kooltherm K106 Cavity Board

Kingspan Kooltherm K106 Cavity Board is a premium performance cavity wall insulation used to insulate cavity walls with just a 10 mm cavity

www.kingspan.com

 

[foxhole]

Cement board ?

 

[Murdochcat]

why a 150mm fan?

 

[Northman]

Cement board ?

As an outer leaf for weatherproofing? That would make sense, and I think I have an offcut somewhere. That would give me (from outside working in) cement board, 90mm insulation board, shroud, 90mm insulation board, plasterboard, skim. Does that sound like a plan?

 

[Northman]

why a 150mm fan?

To maintain the existing extraction capability? I think that's what is expected? I've no idea what the extraction rate of the original fan was, but it was big enough to be described (in the old Rolls-Royce description of their engine outputs) as 'sufficient', and the 150mm gives an extraction rate in line with the size of the room. I could possibly get away with a smaller one...

 

[endecotp]

A larger but slower-rotating fan will be quieter, for the same extraction rate. If you care about noise, fit a new 250mm fan with speed control.

 

[Northman]

Good point, and also a good reason not to go smaller than 150mm. I did wonder about keeping it large, but that does mean more heat loss when its not in use. I'll stay with the 150 and if I find it too noisy then with using the insulation boards inside the existing sleeving I always have the option of reverting to a large diameter in the future.