Below are some pics: 1/hole for fan in my roof, 2/gravity flaps (to stop back flow) on top of fan on top of attenuator, 3/inside of attenuator.

The fan will be used to cool my house in the summer (28 degs for some weeks last summer with only slight diurnal changes - massive house) by extracting when the air outside is cooler than that inside - often 10 deg cooler outside overnight. Just under 500w, moving at least 2m3/s (yes I mean 2 cubic meters each second – giving approx 12 AC/H – looking for minimum of 2 deg drop in house temp). I have a mechanical 5 step speed controller and hopefully I will balance the draw with doors, windows, and vents as required. I have bought the kit, I will be installing it asap and whilst I will happily debate the rights and wrongs of my approach in the margins of this thread, what this thread is about is how to install the kit so as to avoid problems - especially of condensation.

See the pic; the roof is 70mm hollow clay blocks on beams, 100mm EPS, 40mm ventilation gap, 1.2mm OSB, breather membrane, concrete tiles. This hole is located in what will be the stair void (when I put in the stairs); the beam to the left (above wall) is the ridge (no conflict). The 3rd beam is 3m above the current floor and I will straddle from the top of this beam with 2 horizontally mounted steel box section supports (reusing an old hand rail) to the wall on the left and sit the attenuator/fan (70kgs ish) vertically on these. The attenuator is 900 tall and on the left it will just reach the OSB, obviously the right side will protrude through the roof. I will build a wall to just above the fan from light weight blocks probably directly on the OSB (weird I know but as long as I prevent slippage why not?). A lightweight removable bespoke copper cowl will sit on top protecting everything from rain and birds and allowing access for repair.

But will not a very large metal box open to the outside and inside at the top of a house drip continuously with condensation and let’s not forget the stack effect? I am not looking to use it during the winter so what about insulating the metal with 40mm XPS above the OSB and through to below the roof insulation, and leaving enough of a gap to fit a flat panel of XPS on top of the fan with alu-tape each winter? Will I also need to make an air-tight seal at the base of the attenuator?

Other thoughts: use up all my old pots of Hamerite on the galv steel and indeed on the powder coated fan frame; fit 40mm XPS onto the attenuator below the roof as further sound deadening; do I mount strongly the fan to the attenuator (by putting captured nuts on the underside of the attenuator) or mount the fan onto the block wall?

Sorry for long post but it is all a bit worrying - all thoughts welcome.

Thanks Frank, absolutely all comments are most welcome! I did a lot of research on this before opting for this seldom used solution. I'm afraid "convection" wouldn't "do"; there are 2 primary types of un-powered ventilation (though they are not mutually exclusive), 'stack' and/or wind driven', and this link explains them well I think.

http://www.angus-air.co.uk/natural_vent_pdfs/Mistrale_100_July_2009.pdf

The system in the link uses large numbers of sophisticated vents but there are other systems (eg Windcathcher) which specify one roof vent per room - the cost of these things (either system) defies imagination - one small windcatcher (I would need 6-8 ) costs £2000. I need a system which provides a constant stream of 20 deg outside air to my 28 deg house mass during the night and natural ventilation wouldn't come close. Plus, when it is hot (we live in Italy) we sleep naked on top of our beds with the ceiling fan on max plus another floor standing fan, that's 2 fans in each bedroom. So, I am hopeful that this fan will provide that much needed cool breeze AND maybe even save us money on running costs! Also, during the summer all the windows are open (with fly screens) and we have a 3 story (top 2 occupied) house with a balcony along 2 sides of each floor with a total of 6 very large double doors, also left open with fly screens, so there is no chance of an implosion! I am looking for at least a 2 deg drop in the avg temp of the house - so if we assume a delta T of 8 degrees inside as well as outside (instead of just outside) assume negligible diurnal change without the new ventilation and accept that the house needs approx 18kW to heat, can anyone tell me if that is enough info to calculate the likely result of ventilating over say 8 hours?

Ref the structure. Yes, as I said, they are gravity flaps of the type usually mounted horizontally; during the winter the hole will be air tight so no suction and maybe I'll lock the flaps down - during the summer there may be the odd time when they move but as they are mounted horizontally I reckon that will be a rare occurrence (the 150mm one vertically in our kitchen doesn't do it that often) - anyway if the wind is that bad I reckon we will hardly notice...As for the cap/cowl thing, I think I need a lot of free area to minimise back pressure/niose and also I need something beautiful - I am not sure that I get either with your suggestion. This first link does the free space thing well, though I am not so sure that a bigger overhang wouldn't be needed for weather protection and it is not so beautiful:

http://www.fireplacemall.com/Chimney_Caps/Chimney_Caps__Single_Square_or/chimney_caps__single_square_or.html

But this link (and they are less than 1 hour away) shows the sort of thing I will probably go for - I just need to spec the dimensions of the slats vs free space correctly and 45 deg or the normal 60 deg, or leave it to them, after all they are the experts. I also have to decide whether to make the thing go from square to round or not (look at the basamento per capelli in link below for example) it and I had already considered the upside down pyramid - some of this may come down to cost....

http://www.bolletta.com/cappello_lorenzo.htm

Now the last and biggest bone for me - the rubber mounts, what exactly (for my 25 kg fan mounted on 4 bolts) and where can I get them from? I have tried HARD and got nowhere. I was left wondering whether to use 4mm bitumen, or top grade mastic (25% flex) or some amazing bitumen substitute tape I have put on all the way around the meeting flanges. Help with this very gratefully received!

The resilience of rubber is given by its "Shore number" ( http://en.wikipedia.org/wiki/Shore_durometer). The stuff you intend to used is not rated by any number, so while it could be fantastic it looks rather like that ribbed stuff used as non slip matting.
The fixings must be left a bit "loose", i.e. use bolts that are a size smaller then the hole and use a nut and a lock nut so when they are tightened up there is a small clearance (~.010" thickness of a postcard)), a washer and a bit of soft rubber will stop any rattles.
If the fan assy. is designed to be floating so as minimise vibrations, this must also apply to any ducting connected to the assy. Leave it all loose with draft proofing foam as an air seal or a canvas section of ducting.
Frank