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Thanks, yes this is first time I have tackled this. Pretty sure there is a problem with the sub-floor ventilation, just based on the stale smell and signs of previous dry rot in the centre of the house. The extension is new, but there was no better ventilation out of that side of the house...

Thanks. I'm now thinking maybe a very quiet regular bathroom fan at 2, accessible through a hatch in the cupboard floor, and ducted out to air brick 1. Something like this: https://www.superlecdirect.com/ventaxia-498095-fan Should it run continuously, like all the time, or better on a timer...

I'll go for ducting from near 2 in the deadest part to the air brick at 1. I'll try for 110mm rigid circular but might have to go for the rectangular stuff if there's too many sleeper walls to get through. And direction pushing air from the dead area out of the air brick. What about the fan...

No ventilation out through the extension. Building Control did question it, but there were no air bricks in the old outrigger so no obligation to provide more insulation than was already there (probably why the dry rot was advanced even before the extension). With all the floor up I should...

Yes thinking about it over the day I was thinking that would be the best way to mimic natural circulation of air through air bricks. I am also thinking of sealing in the insulation with aluminium tape to try to stop any air movement between the subfloor and the house - for heat efficiency and...

I think getting a full concrete slab poured would be too much hassle but I did wonder about foam. Even if we leave the main void open, is it better to fill in tiny gaps like this between the external wall and the sleeper wall that the beams rest on? I could just use a can of foam for that, but...

I've got an old semi-detached house with a suspended timber floor across most of ground floor (area in green on drawing), mainly 4x2" timbers across a maze of sleeper walls with about 250-500mm underneath them. I have all the timber floor up so I can replace a few rotted timbers. I will also...

Been up and looked and getting a vent vertically on to that branch would be a pain to get 900mm above nearest window, it would have to stick up about 1500mm unsupported above gable roof. But I could run the vent at 45degrees up towards the other stack. In that case is there any point keeping...

Thanks. And the vertical section does make a run that long with two bends ok?

Like so? What length of vertical A-B do you think?

Trying to put a new bathroom right in middle of semi-detached house, on the second floor. It will have to be a 90degree pan connector and then a 4.5m horizontal run just to get to the external wall (can get a 10cm fall over the 4.5m), then the main soil stack is about another 2.2m and...

I was going to put 2" bed of mortar up to top of liner in anyway as clamp is wider than the bottom of the pot. And then a normal flaunching round the whole stack after.

It was from flexiflue, all new stainless steel. But yes I wasn't blown away by quality as all three bits were about 40 quid. How might it fail?

Lining the chimney while I have good scaffolding up on roof. I had to rebuild top of chimney and pots were off anyway. So I have gone for a top clamp on a flexible 5" /125mm stainless steel liner. Flue went in chimney pretty easy. And this top clamp seems to hold it ok. The bolts are tightened...

I've laid about 30sqm of normal 16mm underfloor heating pipe under a new concrete floor. One reel of 100m and another of about 70m. I have a new 30kW combi boiler. I read you should have a direct connection to the boiler for the UFH manifold. I have 22mm flow and return pipes quite close to...

Yes I will leave it ventilated! Perhaps with one of those C-caps on top of the stack to reduce the heat loss. Thanks everyone for the advice. I guess bricks are less waterproof than I thought.

Within the flue, but what about the void around the flue inside the chimney?

Thanks yes looks like that's it. But... so why do lined chimneys blocked with register plates at bottom not need air bricks?

Here is the stack up close from the scaffolding. George answered my main question - that moisture would get through chimney brickwork eventually so better to ventilate. But why doesn't that apply to any brick cavity wall? If we do have to keep ventilation then putting an air brick in the...

Is that true even if you cap it thoroughly? I haven't looked into it but I'm thinking Celotex at top inside flue, then some ply or slate, then dpc or lead, then a few inches of cement tapered to run water off. I've even got a spare coping stone with drip grooves I could put over it if that would...