Wow! so many comments in such a short time. I'm working my way through them. I was able to "quote" but that isn't working now, so:
Victorianeco said "15c seems a little low in my opinion to leave a house, but then I don't know how long it is left empty for etc.

Is it just one window or all of them?"

The barn is only left for a few days. All the frames are showing condensation, but only the bifold is causing me concern as the other frames have tiled reveals.

Posted By: chrisduncanI was able to "quote" but that isn't working now

Yep, quoting only works directly from the last page of a thread.

You can get the effect of quoting from a previous page but it's a bit tricky: do a quote of a small bit of text from the last page of the thread then copy and paste the text and the user name to replace them in the blockquote element. Easiest if you have the page you want to quote from and the last page of the thread open in different tabs. Ideally you can quote from the same person to save having to do the name bit; if not (i.e., if they haven't posted on the last page) it's probably best to do the user name first so you don't forget and cause confusion by misattributing the text.

E.g., to do the following I quoted a word or two from your post above then copied and pasted the text (“So I think…main house”) from the previous page:

Posted By: chrisduncanSo I think these are up to date frames. If anyone disagrees I would be interested to know as we are planning to use the same company for the new main house.

Thanks everyone! I feel honoured to have all these thoughtful comments. More info tomorrow once I see what the RH and temp of frames is when I arrive.

Interesting that this came up - I just did an involuntary/accidental test on my house.

Our airtightness was 1.72 m3/hr. 200m2. new build with 3g fibreglass framed windows.

We've never had any noticeable condensation in 5 years on the inside (you do get some external condensation on the windows but that's a postive thing - it shows how well they're insulating from the house. It's only ever on the west facing windows early in the morning.

The worst insulated part of the house is our opening roof hatch to the roof terrace - it has an insulated (but metal) upstaged and is only double glazed. However, normally, even if we use the bathroom near the roof hatch and bath with the door open we dont get any condensation that's an issue (ie never enough to drip and only momentary). Likewise, only tiny amounts on the 3g roof window in the bathroom itself.

However, I turned our MVHR off on Saturday evening and didn't switch it back on until yesterday. With two people in the house and a few showers we did get enough condensation on the roof hatch upstand and window sufficient to drip off, but only on the roof hatch and not on any other windows.

I wouldn't build another house without MVHR - the absence of condensation, ability to dry washing in the house without worrying about damp and having dry bathrooms and towels (without any heated rails) are big pluses for me. (also lack of noise from extractor fans). A small unit for your barn would seem a sensible addition.

My air tightness at build was 0.72 m³/h/m² (from air tightness certificate) - design was "<10".

MVHR runs such that (over a year) mean CO2 is 540 and mean RH is 43%.

I do not see any condensation on my timber 3G windows.

Posted By: Simon StillOur airtightness was 1.72 m3/hr. 200m2.

Really? Mine was the best the guy had seen (granted 7 years ago) - that figure comes to 0.0086 m³/h/m².

BTW ACH and m³/h/m² are different measurements. ACH relates to the number of Air Changes per Hour the MVHR achieves, not the airtightness.

Posted By: borpinBTW ACH and m³/h/m² are different measurements. ACH relates to the number of Air Changes per Hour the MVHR achieves, not the airtightness.

ACH and m³/h/m² are both measures of airtightness (n50 and q50 respectively at the normal regime of 50 Pa differential). The two numbers n50 and q50 and variants with higher pressure differences are used by various testing regimes. Notably, the UK SAP uses q50 whilst the PHI uses n50. The PHI also insists on both pressurisation and depressurisation during tests and pressure excursions up to 100 Pa.

MVHR rates are usually expressed in units of m³/hr or l/s.

MVHR rates can be related to the building volume using either of the airtightness units if desired.

Here's the data to go with the original post. I got back this morning with a gun thermometer and a cheap RH meter.

Air temp outside 2 deg +/- 1
Air temp inside 14.5 deg +/- 0.5 (thermostats x 2 and gun agreed)
aluminium bifold frame inside top (2m high) 12.5 deg
aluminium bifold frame inside bottom(cill section) 8 deg
glass 12 deg (not sure if the IR gun thermometer would measure this accurately)
walls and ceiling all 14.5 deg
entrance door (44mm thick plywood with honeycomb paper core) 12 deg

RH was 60% on arrival and 59% after waving it around for 5 minutes. I'm not convinced of its accuracy.

It's now 8 hours later and the RH is still showing 60%. Either it's not reading accurately or it's a coincidence, which it could be as it is 4 degrees warmer and I've been in the building for about 5 hours and boiled the kettle 3 times.

Sorry to make you wait for it but the really important bit (for me) was there was absolutely no sign of condensation anywhere on arrival, and there still isn't now. So to recap, this was after leaving the building to cool down from 19 to 15 degrees while I was away for 4 days with the bathroom extractor fan running permanently (thank you djh for that idea), and outside air temperature similar to before. So the previously damp air must have been gradually replaced with cold air from outside via leakage pathways, which when heated would then have been drier?

What I take from the above is that the huge amount of condensation must have come from 2 people being in the barn for a couple of days with inadequate ventilation. I won't know for sure as I didn't have the RH meter then, but I will leave the fan off overnight and see what the condensation/RH levels are in the morning.

So having just finished the build and wanting a long term cheap solution to potential future condensation risk (without major disruption), I'm thinking that perhaps I just replace the extractor fan with a humidistat extractor fan or possibly keep the existing fan and ask my electrician to wire in an override to turn it on when the outside air temp drops below a certain temperature. Are humidistat fans any good these days (I fitted one about 30 years ago and it was useless)?

We had no perceived problem with air quality before it got cold, so I don't think I need a solution which operates all year round. However someone did mention CO2 levels. Is that something to be concerned about?

I wait with bated breath for comments and suggestions about the fan solution (MEV?), or other options or calculations ... Thanks again - it's been fascinating reading everything so far.

Posted By: tonyMy rule of thumb known as Tony’s rule is that any surface at or below 12 C in a home is at risk of condensation.

This should not be possible in a new well designed and detailed new or converted building.

So how does that square with Smart Systems' (self declared on their website) comment "Smart has grown to become the UK’s leading supplier of aluminium glazing systems and bespoke aluminium extrusions"? Smart extrusions are used by large numbers of fabricators - 10 fabricators with 10 miles of centre of Bristol). Sections have a polyamide thermal break. Are they just not up to the job?

What is spec on 24 mm double glazed panels ? Long shot - Is the glass in correct way around with soft coating on cavity side of inside pain of glass. Best glass is about half heat loss of old/worst double glazed units with no coatings and warm edge bits.

When house is warm and you are by the doors is there a cold down flow of air going on near the windows ?

I tend to see condensation on my double glazing on day when it first turns cold. House then dries as it pulls in colder drier air from outside - condensation is then reduced on windows on future cold days if it stays cold.

I have just fitted a humidstat fan on bathroom refurb (Vent Axia Solo Pro SELV ) it seems OK ish. A bit slow to come on for my liking and a bit variable when it comes on versus RH meter. It has a switched live feed you could run off a temperature probe outside too. Like your idea there. Unit has manual override on little cord so you can keep it on when it and I find I use that on occasion.(eg when it drops cold)

We have dehumidifier in bathroom for drying washing so this stops it running sometimes too.


60% RH seems low for new build instinct rather than deep knowledge, thought it would be drying out ?
60% when 4 degrees warmer is a fair increase in the % of water in the room than the 14 C at the start.

Stick a piece of paper to the glass then fire the gun at that

Posted By: chrisduncanAir temp outside 2 deg +/- 1
Air temp inside 14.5 deg +/- 0.5 (thermostats x 2 and gun agreed)

OK, so the dew point was 6.5°C

aluminium bifold frame inside top (2m high) 12.5 deg
aluminium bifold frame inside bottom(cill section) 8 deg

The top sounds quite reasonable, the bottom not so much. As Tony asked, is there some other possible heat path besides the frame itself? Otherwise maybe it's time to look at cross-sections and ask manufacturers etc.

glass 12 deg (not sure if the IR gun thermometer would measure this accurately)

As Ed already said, the trick is to put a bit of sticky tape on the glass, wait until the temperature equalizes and then measure that.

walls and ceiling all 14.5 deg
entrance door (44mm thick plywood with honeycomb paper core) 12 deg

RH was 60% on arrival and 59% after waving it around for 5 minutes. I'm not convinced of its accuracy.

Yeah, I've got several cheap RH meters and I stand them all together every now and then to try to convince myself of their relative accuracy. I also hired a calibrated meter once to check they were reasonable. Turns out they are generally fairly reliable, but not exact.

It's now 8 hours later and the RH is still showing 60%. Either it's not reading accurately or it's a coincidence, which it could be as it is 4 degrees warmer and I've been in the building for about 5 hours and boiled the kettle 3 times.

Could well be, the dew point will have shifted to 10.5°C or so in that case. Depending what exposed surfaces you have inside there'll be a greater or lesser extent of humidity buffering going on which will complicate things further but may well tend to keep the RH more constant than basic theory might predict.

Longer term you can just open windows more when you are there and turn the heating up a bit so it doesn't get cold as a result. Or fit some combination of humidistat and thermostat to the extractor. I don't know what's available off the shelf.

Posted By: bhommels

Posted By: Simon StillInteresting that this came up - I just did an involuntary/accidental test on my house.

Our airtightness was 1.72 m3/hr. 200m2. new build with 3g fibreglass framed windows.

So assuming an average ceiling height of 2.40m, your leak rate would be 1.72m3 for 200m2x2.4m=480m3, or 0.0036 ACH. Even if you made a typo and it is 1.72m3 on 200m3 it would be 0.0086 ACH which is frankly unbelievable. I think the ISS would be only slightly better. I suspect there is something wrong with your accidental measurement method.

How did you arrive at these numbers? Were they taken at 50Pa pressure difference?

Sorry - obviously wasn't sufficiently clear in what I'd written.

The 'accidental test' was whether and where I would get condensation in my house if I didn't have an MVHR. As expected, on the worst insulated window though I was surprised that was the only place.

The test result was from a properly done blow test at the end of construction and would was written as
1.72 m3/(h.m2) @ 50 pascal

Posted By: GreenPaddy
- that said, I'm not convinced that the frames are doing a great job, but with the room at only 15oC, 12.5oC for the frames is not terrible. Measure the frame temps again once you've warmed the barn up internally.

This afternoon, internal air temp 18 deg; outside -3 deg; top of frame 13.2 deg; bottom of frame 8.1 deg

The temperature of the frame is consistent from top down to 400mm from floor, then drops 1 deg per 150mm and drops steeply for the last few mm. Do you think this is likely to be because cold air falls, or because there's worse thermal insulation in the cill? I will look at the Smart section drawings, but I think most of the sections use the same polyamide breaks.

I should also have said that I turned the extractor fan on again this morning and by this afternoon the condensation is starting to diminish.

So:
I can just run the extractor fan permanently when using the barn in cold weather. The fan at 16W won't cost much to run, but I will be paying to heat up an indeterminate amount of cold air seeping in to the barn from outside.

I can change to a humidistat extractor that will only run while the RH is above a certain level, but I haven't found one yet. The Xpelair one will only kick in when humidity changes by more than 5% in 5 minutes (to avoid "unwanted operation" !!) and another will only operate above 60%RH. Can any of you recommend one?

Or I can leave a window on the lockable night vent setting while we are here and put up with wasting heat and having cold draughts (you can tell this is my least favourite!).

I am thinking that single room MVHR is probably overkill as the cost of one will probably never be recouped through lower heating costs vs just leaving the extractor fan running. But I will have to monitor gas use to check this.

Chris,

I use a psychrometric graph, to determine dew point which gives the relationship between moisture, temp, hum, enthalpy, entropy, and all sorts. It takes a little getting used to, but there are little tutorials as to how to use the basic elements of the graph. That's quite different from dew point calculators looking at interstitial condensation.

I tend to agree that changing the fan will have its own carbon footprint, which might not be outweighed by running the existing fan when needed. When is "when needed"? You could add a remote humidistat to the existing fan, maybe something like this...
https://crwltd.co.uk/extractor-fan-inline-bathroom-run-on-timer--humidity-sensor-switch-socket-2335-p.asp?

What is it you're trying to control? RH in the bathroom, at the window...You might want to be able to de-activated it so it can't come on at night, as say room temp falls, and RH rises. I think I might just put a timer on it, and be done with it. Increase the time period if you see a bit more cond. Off between 10pm and 7am, or what ever.

That doesn't really help with the main thrust of your question - how to not repeat this in the house. As others have said, you have a cold bridge at the foot of the window. It may be resting on the metal cill, and bridging the cavity. So metal cill from inside to out, and possibly cold air directly underneath from cavity. Is the cill thermally broken? Is the unit sitting on insulation? Door thresholds and floor based window cill details are often a problem. Been discussed on here before - basically need to buy some expensive high compressive strength insulation. But you don't need a lot of it, so not so bad. Just getting the builders to be bothered following the detail in the underbuild is tricky. I sometimes don't bother telling them, and just cut out the few blocks and insert at the appropriate time (or note on the drawing to leave the block course out at that area).