OSB?

http://www.norbord.co.uk/products/osb/sterling-osb3

Thanks - I knew someone would have a bright idea I will check out OSB3. An advantage of the Sarket was that you can use it uncovered for a fair while and without a membrane, so hopefully the obs3 is the same. : )

One minor point to mention is that SmartPly is manufactured in Clonmel Ireland rather than the UK.

Depends what the OP's motivation is, if it is consideration of carbon miles then depending on the delivery address in the UK the impact may be smaller than from a manufacturing plant that is in the UK perhaps for example in highland Scotland.

If the motivation is to support UK jobs then SmartPly do at least have a considerable workforce post manufacturing in the UK and the Irish will be grateful of the input to their economy.

Posted By: fostertomworry now is that it may also be much less airtight

I found in my informal test (single sample, side-by-side same building - we switched suppliers part way through due to short stock) that 18mm Smartply was as airtight as 18mm Norbord Sterlingboard, and that both had good airtightness (towards the top of the experimental range in the University of Leuven study).

The motivation is an interesting question.
If it is to support British Industry, you may do more good by buy cheap imports and spending the saving on something that has a direct impact at home, say a food bank or a homelessness charity.
Just like 'carbon miles', lots of research is needed.

Posted By: fostertomST as controversial lateral-thinking as usual!

How you get change

Posted By: TimSmallI found in my informal test (single sample, side-by-side same building - we switched suppliers part way through due to short stock) that 18mm Smartply was as airtight as 18mm Norbord Sterlingboard, and that both had good airtightness (towards the top of the experimental range in the University of Leuven study).

How did you test, and how did you calibrate to get actual figures i.e. 'top end of Leuven'?

If true, makes Smartply a remarkably useful product - high(ish) airtightness plus low(ish) vapour resistance (much lower than 'generic' OSB3) - and as untoxic, formaldehyde-wise, as natural timber - and almost home-grown. A real alternative to expensive woodfibre sheathing.

Viking House also confirms v gd measured whole-house airtightness using 18 T&G OSB3, joints mastic'd. My method has been 9 or 11 square edged OSB3 bubble glued and screwed to continuous support, all edges. In a way even more durably gap-filling, but maybe less airtight in that thickness. However I couple it with blown-in Warmcel between the studs, which has fair airtightness itself. So I think I get airtightness robustly in-depth, especially free from the slow-leak pinhole/hairline faults inevitable with tapes/membranes, which WUFI identifies as disastrous condensation-wise (strangely, bigger leaks are no prob, condensation-wise, because the air channel gets warmed above dew point).

All contrary to conventional wisdom about OSB airtightness (and about 'breathability' too). Of course it's got to be kept dry, otherwise Weetabix and destruction of all attributes!

> So, not massively brilliant methodology,

Not daft though. I'm pretty impressed.
I've not detected any pseudoscience yet. Awareness of the weaknesses looks pretty good too.
So far you could pass for yer actual scientist!
(well, to me anyway)

You sure you didn't wave any crystals about or blindly believe some PR tripe from products on that internet?

Blimey though... 250 Pa... lucky you didn't blow (suck?) the windows in. The standard 50Pa air tightness test is sometimes enough to pop taped joints in membranes.

> 250 Pa corresponds to 20 m/s (force 8, fresh gale).

Blimey. That makes me think...

Our rubble walls next layer in before internal insulation was a heavy Solitex membrane intended to stop wind coming through. It had a blue fabric finish and looked fine in principle but as we finished working with it we realised that the Tescon tape (as recommended) did not bond it anything like as well as with the internal membranes. Hearing that this layer could see something like 250 Pa (and after seeing effect of 50Pa on internal membranes before internal boards fitted) is quite alarming :-/

I suppose, thinking about it... 250Pa is 250 N/m2 and 250N is not much - only the downward force from 25Kg
If that could suck your windows in then those windows would have to be pretty poorly made :-)

Posted By: SprocketBlimey. That makes me think...

Yep, that's one reason why it would be fun to do some airtightness testing on 5 year old houses, rather than just a week after construction and before the first windy winter.

Beaufort force 11 (strong gale, just short of hurricane) could be around 30 m/s. ½ρv² = 0.5 * 1.2 * 900 = 540 Pa. That's the pressure increase on the upwind side and the decrease on the downwind side - open a door or window or something on the downwind side so the inside of the house equalizes with that and the pressure difference across the upwind side becomes 1080 Pa. And that's assuming nice smooth air, not the buffeting pressure caused by gusting.

Posted By: Ed DaviesThat's the pressure increase on the upwind side and the decrease on the downwind side - open a door or window or something on the downwind side so the inside of the house equalizes with that and the pressure difference across the upwind side becomes 1080 Pa. And that's assuming nice smooth air, not the buffeting pressure caused by gusting.

On a breezy (& gusty) day in Broadford a few years ago I opened the driver's side car door, heard a pop & felt something fly past. What I'd felt was some of the granules that were all that was left of the rear passenger window which had shattered in response to the sudden change in pressure distribution.
I try to be very careful about opening doors (both car & house) gently on windy days now...

Posted By: fostertomSurely Ed, that buoyancy effect is too great by a factor of 10?

Take a 1 m² horizontal-area column up through the house. 2.4 metres per storey so 4.8 metres tall. The density of air is about 1.2 kg/m³ so the column has a mass of 5.76 kg. Force of gravity is 9.81 N/kg so the weight of the column is 56.5056 N so the pressure at the bottom is 56.5056 Pa greater than that at the top.

Note, this isn't buoyancy (due to different temperature or humidity) but simple pressure difference.

Posted By: fostertomAmazingly, Fraunhoffer's (WUFI) tests have shown that constant, weak buoyancy completely dominates actual airchange effects, long-term; breezes/winds/gales they recommend to simply ignore, because intermittent - and also reversible.

Yeah, but is unsticking tape reversible?

Hi all - thanks for the ideas and comments.

If all is equal then I would rather support UK businesses and help to develop the UK timber industry at the same time. For example the struc. eng had specified C24 for floor joists when C16 would have been, and are, perfectly adequate. So I have changed to C16 and we are now able to source these from the UK. The rest of the wood is from the UK, the floor boards are from just down the road - well half an hour, etc.

It looks like osb is on the cards - just got to double check a few details as there seem to be quite a few different figures for vapour permeability knocking about for osb3.
Sterlingboards figures seem a little confusing.

They say:

The water vapour resistance factor (μ) of OSB, as given in BS EN 13986 : 2004, should be either taken as
the design value given in BS EN 12524 : 2000 [30 (wet cup), 50 (dry cup)] ***so this agrees with what we found out so far

....or determined in accordance with
BS EN ISO 12572 : 2001. Such values may be used in any interstitial condensation calculations to BS 5250 :
2002. Experimental values determined in accordance with BS EN ISO 12572 : 2001 (wet cup) for Sterling OSB/3
are given in Table 1.

Table 1 Water resistance factor (μ)
Panel thickness (mm) Water resistance factor (μ)
9mm 219
15mm 147
23mm 107

So the experimental factors would appear to be very different.

Make any sense to anyone?

: )

Sune

Posted By: fostertomMeans that exposed windy sites have negligible effect on actual air change, long-term!

I don't think it does. If the wind blows from the west and changes the air once, then blows from the east and changes the air again, the result is two changes of air not none! Lovely and fresh and freezing cold.

Granted you may be able to net out moisture flows to some degree, but not air changes.

Posted By: fostertomwhereas any airflow direction causes heat loss and isn't netted. However, the bit about the 24/7 nature of stack effect outweighing the intermittency of wind, still holds.

Wind is more intermittent in some places that others.
It seems reasonable to guess that the relevance of wind will also vary with the exposure of the property? A terrace in London & a detached property on an exposed hillside or coastline are very different things surely?