Insulated ducting for hob, suspended floor

[Super_Paulie]

hi gang.

Im starting the work on my kitchen island this week, more specifically the downdraft hob.

 

As my floor is suspended, (UFH in biscuit on 170mm PIR) i have left an opening where the hob is to go so i can bring the ducting up into the cabinet from the outside wall. Its a 2m straight run from the double airbrick so i can simply pass a 2m 220x90 duct through from outside, my joists go in the right direction, no biggy. But it just dawned on me that this duct should be insulated as it will be pulling hot air down into the cold void below the insulation.

Am i overthinking this or should i be looking to insulate the rectangular duct? if its at a slight dowanward angle away from the entry point under the cabinet then would it matter that there is condensation?

 

- should i insulate the ducting somehow? https://domusventilation.co.uk/catalog_products/megaduct-220x90-insulation-1m/

 

- should i be putting a shutter or butterfly inline somewhere to stop backdraft into my warm house?

 

Be interested in what other people have done here.

 

Edited Wednesday at 09:48 by Super_Paulie

[markc]

No need for insulation as the hot air will be moving and a flat duct will be plastic.

some condensation will form inside the duct but the moving air will carry this away to atmosphere.

a cold duct in a warm environment would form condensation on the outside which would be a problem.

[Super_Paulie]

Just looking to order the bits I need tonight. Should I be looking to put a shutter or back draft flap on this? The vent will exist low down at the same height as the existing air bricks which will in turn be under a suspended wooden decking eventually.

[JohnMo]

18 minutes ago, Super_Paulie said:

Just looking to order the bits I need tonight

Why bother pushing the air outside? Carbon filter and one with decent grease filter, and do it as recirculating?

 

No idea what your ventilation strategy is, if not MVHR, add a dMEV fan to kitchen and let it slowly tick away and auto boost on rising humidity.

[JohnMo]

21 minutes ago, Super_Paulie said:

existing air bricks which will in turn be under a suspended wooden decking eventually

Just seen this bit, sounds like an easy way to get a rotten deck - warm humid air pumped in

[Super_Paulie]

No MVHR. Need extraction for BC signoff and I've already got the venting hob. 

 

I'll extend the duct to the end of the deck when I've built it, but I'll only get 10 years from the deck at best anyways.

[JohnMo]

44 minutes ago, Super_Paulie said:

I'll only get 10 years from the deck at best anyways.

Why? Ours has been there 3 years and looks as good as the day it went in. Will be pretty pissed if it only lasts 10 years.

[Super_Paulie]

1 minute ago, JohnMo said:

Why? Ours has been there 3 years and looks as good as the day it went in. Will be pretty pissed if it only lasts 10 years.

My previous timber deck was looking tatty after 10 years on the coast and that's with oiling every year. It was made with new scaffold planks.

 

Anyways that's not my concern at the moment. I've ordered a back draft flap and I'll put it in, can't see it doing any harm.

[JohnMo]

46 minutes ago, Super_Paulie said:

Need extraction for BC signoff

Are you sure, if you have a dMEV fan you don't need any other ventilation, you move from intermittent to continuous section in building regs.

[Super_Paulie]

Yep, confirmed with the man himself. I need a vent or extraction to a certain level, either on the wall venting out or within the hob, which is what I've gone for rather than a dMev. Just because they looked fancy and I wanted to try it.