Green Building Forum - how to modify flow in roof ventilation?

how to modify flow in roof ventilation?

djh — Thu, 03 Jun 2021 22:22:56 +0100

On the east end of our house, there's a single storey extension, outside the thermal envelope. The larger southern half of the extension is a conservatory (well, actually a sun room with a big rooflight) and the northern part is the 'bin store'. The bin store was thus named because it suited the then relevant CSH assessment. It has since turned into a very useful pantry. There's a problem with it in that it overheats in summer; not something I thought about when designing a 'bin store'

Its construction is very crude. The external wall is built up with blocks from the hardcore base to the same level as the FFL of the house. The space between it and the main slab of the house on one side and the slab of the conservatory on another is filled with more hardcore, and topped by some paving slabs. The superstructure is just timber studding with OSB outside, followed by a breathable membrane and cedar cladding on battens. There's a flat roof, which is ply on timber joists with an EPDM membrane above.

The roof is shared with the conservatory, but the construction there is a bit more complicated. Starting from the outside this time, there's the EPDM membrane and ply below that with firring strips to provide a ventilation space underneath, followed by some mineral wool insulation between the joists and plasterboard on battens. There's a vapour barrier in there just under the joists too. The joists run east-west, being fixed to the wall of the house at their west ends.

The ventilation runs north-south through both the bin store and the conservatory. I think what is happening is that in summer the air under the roof over the conservatory heats up and flows out into the bin store, warming it up too much.

I'm wondering what, if anything, I can do to reduce or eliminate the overheating. The only thing I've thought of so far is to build an enclosure around the ventilation space where it emerges from the conservatory roof into the bin store, and fit an electric fan to blow air into that space, so hopefully reversing the flow above the conservatory and causing the warm air to emerge at the south end instead.

Does anybody have any better ideas? Or even think mine might work?

TIA, Dave

how to modify flow in roof ventilation?

owlman — Fri, 04 Jun 2021 11:44:42 +0100

Can't you cut through the outer roof covering and create an ventilated upstand to give the warm air another route out?
or:
Buy a small A_A heat-pump and use it to really cool the pantry, I guess you'll only be using it when the sun shines so your surplus PV will power it for nothing.

how to modify flow in roof ventilation?

djh — Fri, 04 Jun 2021 15:24:01 +0100

Posted By: owlmanCan't you cut through the outer roof covering and create an ventilated upstand to give the warm air another route out?

Sounds rather more invasive and error prone than I was hoping for.

Buy a small A_A heat-pump and use it to really cool the pantry, I guess you'll only be using it when the sun shines so your surplus PV will power it for nothing.

Increasing capital invested for active measures isn't very green even if the power is free.

how to modify flow in roof ventilation?

owlman — Sat, 05 Jun 2021 10:10:09 +0100

Posted By: djh<
" The only thing I've thought of so far is to build an enclosure around the ventilation space where it emerges from the conservatory roof into the bin store, and fit an electric fan to blow air into that space, so hopefully reversing the flow above the conservatory and causing the warm air to emerge at the south end instead.>

Can't you simply block between the roof joists , where they emerge. Why go to the trouble of building an enclosure and then introducing a fan. I've no idea as to how many joists there are but you may need a bigger fan than you imagine to "push/encourage " thermal currents to reverse.

how to modify flow in roof ventilation?

davidfreeborough — Sat, 05 Jun 2021 11:27:53 +0100

Hi Dave

If you want to avoid fans then best to use gravity. An upstand at the wall between the two spaces will tend to encourage air to flow in the same direction it does now. Reversing the direction requires some kind of downstand.

How about blocking the joist space and ventilating each cavity with a telescopic underfloor vent taking air from ~450mm below? High level and low level vents in the bin store should help ensure the temperature in there remains lower than that in the roof void, forcing air to flow north to south.