Underfloor ventilation guidelines

[meow2222]

Required ventilation has to depend on vent area per floor area, wetness of = the ground, mean windspeed at vent, and how rot resistant the timberwork is= . Since most of those are unmeasured and uncontrolled, necessary vent area = per floor area must vary a fair amount. So its not possible to optimise ven= tilation with rule of thumb. Its also bound to vary from summer to winter.

I tested blocking vents on an uninsulated floor for a couple of weeks, and = the resulting change in heat loss was quite major. So better control of ven= tilation surely warrants attention.

What strikes me as the best option is not only to control ventilation accor= ding to RH & temp, but also to use a heat exchanger so the outgoing air doe= sn't take its heat out with it.

So let me propose contemplating the following setup: wallwart, humidistat, computer fan, and a tiny crossflow exchanger, perhaps= made from ali drink cans.

If that were put together for =A320-30, and 4 or 5 such vents saved =A3100 = a year, thats a cost of 80-150, and an ROI of 66%-125% pa. If I had more tu= its I might have done it.

NT

[robgraham]

f the ground, mean windspeed at vent, and how rot resistant the timberwork = is. Since most of those are unmeasured and uncontrolled, necessary vent are= a per floor area must vary a fair amount. So its not possible to optimise v= entilation with rule of thumb. Its also bound to vary from summer to winter= .

d the resulting change in heat loss was quite major. So better control of v= entilation surely warrants attention.

ording to RH & temp, but also to use a heat exchanger so the outgoing air d= oesn't take its heat out with it.

aps made from ali drink cans.

0 a year, thats a cost of 80-150, and an ROI of 66%-125% pa. If I had more = tuits I might have done it.

This topic came up for conversation with someone I recently met who has a 200+yr old stone cottage like mine (Scotland). He quoted a figure of 1 in^2 per foot run of wall for underfloor ventilation, so a nominal ventilation brick space of 9" x 3" should ventilate some 25 feet of wall length.

The 1920's work to put suspended floors into my house put in larger vents in greater numbers and everything is in favour of this being a dry site, so I reckon I can block the vents for a couple of weeks and see what effect there is on the floor temperature.

If I was to data log humidity in the floor timbers, what simple measuring sensor could I use ?

I'm intrigued by "tiny crossflow exchanger, perhaps made from ali drink cans." - how would this be made up and any gained heat extracted?

[meow2222]

and the resulting change in heat loss was quite major. So better control of= ventilation surely warrants attention.

ccording to RH & temp, but also to use a heat exchanger so the outgoing air= doesn't take its heat out with it.

rhaps made from ali drink cans.

100 a year, thats a cost of 80-150, and an ROI of 66%-125% pa. If I had mor= e tuits I might have done it.

The heat in the outgoing airstream is transferred into the incoming stream,= so you get fresh air with much less heat loss. It works well if the 2 stre= ams flow in opposite directions through the exchanger, and you've got enoug= h metal surface area.

One could be made from a stack of flat sheets of ali can metal, with maybe =

5mm wood strips used as spacers around the outer edges, with gaps for the 4= ports. I'd prefer a quicker easier design to make though.

NT

[robgraham]

, and the resulting change in heat loss was quite major. So better control = of ventilation surely warrants attention.

according to RH & temp, but also to use a heat exchanger so the outgoing a= ir doesn't take its heat out with it.

, perhaps made from ali drink cans.

=A3100 a year, thats a cost of 80-150, and an ROI of 66%-125% pa. If I had = more tuits I might have done it.

m, so you get fresh air with much less heat loss. It works well if the 2 st= reams flow in opposite directions through the exchanger, and you've got eno= ugh metal surface area.

e 5mm wood strips used as spacers around the outer edges, with gaps for the= 4 ports. I'd prefer a quicker easier design to make though.

Thanks NT - I see what you are getting at now, but surely the whole point of the underfloor ventilation is to get a cross flow such that air comes in on one side of the house and vents out the other side under the influence of the wind direction.

Rob

[meow2222]

so you get fresh air with much less heat loss. It works well if the 2 streams flow in opposite directions through the exchanger, and you've got enough metal surface area.

5mm wood strips used as spacers around the outer edges, with gaps for the 4 ports. I'd prefer a quicker easier design to make though.

The point is to dry the space to prevent rot. The above is the old way to do it, we can do better.

I'd want to add monitoring as an unnoticed failure could be costly.

NT

[Thomas Prufer]

wood strips used as spacers around the outer edges, with gaps for the 4 ports. I'd prefer a quicker easier design to make though.

Chap made a d-i-y heat exchanger for a basement where clothes were dried. As I recall, lint, condensation, and particularly icing were problems.

He tried two heat exchangers: one was thin plastic sheets separated by self-adhesive foam waetherstripping. The other was plastic straws.

Summary: Clothes dried more quickly, room remained warmer, wife happy. Talked about it in de.rec.heimwerken which is the German uk.d-i-y.

Prufer

[meow2222]

be 5mm wood strips used as spacers around the outer edges, with gaps for th= e 4 ports. I'd prefer a quicker easier design to make though.

Thats the type of construction I had in mind.

It would have to be tiny to fit in a 4x9 vent space. So the thinner and mor= e conductive the sheet the better. And the lower the air flow rate, the les= s surface area is needed for a given efficiency. The narrower the height of= each layer, the smaller it can all be.

There would be condensation, I had in mind to tilt the exchanger slightly, = and use a piece of plastic to run any condensate outside. Freezing shouldn'= t be a problem, even if outside is so cold that the warm air freezes it wou= ld only impede or block the exchanger short term.

There would also be build up of muck in the exchanger over time if not filt= ered, but filtering would cause blockage in time. So some sort of electroni= c monitoring really is necessary.

It might even be worthwhile to have 2 thermostats, one to run the 12v fan o= n 6v, and the other to run it at 12v when RH is further from the target fig= ure. This would reduce total heat loss further by improving exchanger effic= iency.

NT

[John Rumm]

The difficulty with rules of thumb like that is they take no account of the specific details. Like are the vents on opposing sides of the property to allow any cross flow of air, or are there sleeper walls etc that in effect void off areas of sub floor.

There were originally about 5 air bricks here (2 front, 2 one side, one the other, and none at the back) covering some 46 x 33 ft. However the sleeper walls sub divide that (they contain some gaps and missing bricks

- but obstruct the flow of air a little)

That was obviously not enough ventilation. I added 8 more, and that is obviously more than enough ;-)

I just got some ali "hit and miss" vents that I can mount over the outside of the air bricks. That should allow for some adjustment and experimentation.

I am going to experiment taking a damp reading from a wall in the corner of a room that was prone to dampness, since the added air seems to have dried it out, I will try throttling the air but checking it does not start getting damp again.