Green Building Forum - Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

chrisduncan — Sat, 02 Jan 2021 15:58:03 +0000

Hello people. I recently finished converting a small barn into somewhere to live while our new house is being built. The barn is 60m2, single storey one main room with attached kitchenette and shower room, essentially one space. The walls are insulated on the outside with taped foiled PIR which is then clad in larch. Windows are thermally broken aluminium with 24mm IGUs.

I am feeling very depressed that after all my hard work, and with the recent cold snap, I have significant condensation on the inside of the frames. The place it really matters is in the living area where the bifold door frame butts up against engineered oak flooring. The condensation is enough to flow onto the floor and if I don't do something will start rotting or at least degrading the floor (see attached photo). Although a little condensation may be forming on the glass, most of it is forming on the frame.

After asking Mr. Google, there are plenty of websites which say it's not the window's fault, it's inadequate ventilation. I (perhaps wrongly) opted for windows which have a night vent setting rather than trickle vents as I couldn't bear spending lots of money on insulation only to throw money away on heating lots of cold air trickling in! I thought we could open windows a crack only when needed. Our problem is that we are not here all the time and I have to have an unattended solution to removing moisture from the air.

I suppose the humidity is coming from living in the barn without having the windows open, but a bit of me wonders if the two substantial stone walls are still drying out. The cladding was put on last spring so the barn has had 9 months in the dry and the walls had not been noticeably wet before anyway.

It seems there are two options unless someone can suggest something better. 1) I buy a dehumidifier and leave it running near the sink during the winter (eg Meacodry ABC 10L/day approx Â£140 and 160W) , and 2) I replace the shower extract fan with a single room MVHR unit (eg Blauberg Mini-air approx Â£370, 3.2W).

How can I tell if either of the above options will solve the problem? Has anyone had experience of dealing with condensation in this way? I would of course want to go for the most cost effective option, but I don't want to spend money on kit which doesn't in the end permanently remove the problem.

A couple more questions: both the dehumidifier and MVHR would be about 10m away from the bifold doors. I presume airflow in an empty (and fairly airtight, I think) building would be enough to ensure that the air next to the bifolds would be adequately dehumidified? Although the MVHR would have a far lower running cost, its efficiency would mean we would have to run the heating more to make up for this? Heating is underfloor from an LPG boiler. I have the temperature set to 20degrees when here and 15degrees when away.

What do I do? All comments gratefully received! Thanks

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

djh — Sat, 02 Jan 2021 16:16:42 +0000

If the moisture is coming from you living in the barn, then does the moisture/condensation stop when you're not there? If so, you don't have a problem?

If the condensation still keeps appearing when you're not there and the building has had a week or two to cool down to 15Â°C then I suppose it is coming from somewhere else and the stone walls would be my best guess (always assuming that your floor is properly dry and there are no leaks in the roof and walls).

I suspect that any ventilation will solve the problem. Maybe fit a cheap extractor fan? Computer fan? (and pay a bit more for LPG to make up for the lost heat) then fit an MVHR if it works.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

chrisduncan — Sat, 02 Jan 2021 16:47:27 +0000

Thanks djh. I'm not sure yet if the condensation stops if I'm not here. I only spotted it a few days ago when the weather got a lot colder. I will monitor over the next week as I will be away again for a few days.

The only old parts are the two stone walls and roof frames - everything else is new. Floor is suspended timber with membrane below, so no moisture from that. I injected a dpc in the walls about a year ago. Could they still be drying out?

Great idea about the extractor fan. I will leave the bathroom extractor going. It's 16W so it'll cost 40p to run it for a week! I'll report back ...

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

tony — Sat, 02 Jan 2021 16:56:14 +0000

Whole house MVHR will solve it

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

revor — Sat, 02 Jan 2021 19:12:47 +0000

Is the area quoted at 60 m2 correct? Part of reason may be the ratio of volume of air to sq meterage of glazing. The larger the volume of air the more moisture it can hold and if the condensation area is small then the moisture more likely to stay in the air. My house at 360 sq m will hold a lot of air and we have stone walls with no DPC and EWI on them and a large glazed area on a gable end with aluminium framed glazing on a timber frame. We have MVHR and we sometimes turn it off at night so that the cold air outside does not cool the house down and using heat to keep up with it regardless of how efficient the unit might be. That is the only time we see a bit of condensation and that is on bedroom windows but not enough to pool. As others suggest ventilation is key along with some heating. I suspect your windows are not as thermally broken as you may think. My SIL had aluminium framed windows to replace old timber windows and has had issues with condensation ever since. ( Being more airtight could be a contribution though.)
A thought about your EWI on stone it won't be able to breathe out and any moisture in the wall will come inwards to the house. Our stone walls where not forming part of the interior are EWI with cedar cladding but interior is lime plastered so can breathe into the house. A major contribution may be your stone walls so a temporary fix would be a dehumidifier that you can run a hose from it to drain so you don't need to empty the container. In my previous house I had one permanently running in the basement to keep my workshop dry. Worked a treat. Longer term think of a MVHR unit.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

djh — Sat, 02 Jan 2021 20:02:16 +0000

Posted By: tonyWhole house MVHR will solve it

Yes, but that's vast overkill, AIUI.

edit: depends on what Chris' long-term plans are for the barn, of course.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

Victorianeco — Sat, 02 Jan 2021 21:18:50 +0000

Basic dewpoint calculation required:

What are the humidity levels of the room?
What is the air temperature of the room?

What is the temperature of the window/frame?

Without these, just throwing guesses into 'wet thin air'

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

tony — Sat, 02 Jan 2021 21:23:00 +0000

We know the answer to that, the windows are below dewpoint, condensation on them is running down them and is likely to wreck the wooden floor.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

Victorianeco — Sat, 02 Jan 2021 21:31:18 +0000

We know the frames are below dewpoint but the property must be extremely wet (humidity wise) or they must be extremely cold

Even at 60% RH (the higher end of 'normal') and an air temperature of 15c (provided) the dewpoint would be ~7c (unlikely)

That's a very low temperature for the frames.... Or the RH is much higher than 60% RH?

A condensing dehumidifier would provide 2 benefits, background heat along with dehumidifying. Simple problems call for simple solutions....

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?

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

djh — Sun, 03 Jan 2021 00:01:24 +0000

Posted By: VictorianecoEven at 60% RH (the higher end of 'normal') and an air temperature of 15c (provided)

Wrong. The air temperature provided at which 60% might be 'the higher end of 'normal'' was 20Â°C.

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.

Your opinion seems to have been formed without the basis of any facts.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

Peter_in_Hungary — Sun, 03 Jan 2021 08:14:16 +0000

chrisduncan
You could perhaps hire a dehumidifier and see what results you get.
The long term solution would depend upon the long term use or the building. The best solution would IMO be MVHR and as from the description of the building it is small and mostly open plan a single unit with 1 input and extract fitted to give cross flow should do the job. Small MVHR units can be got for about 300 quid.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

Victorianeco — Sun, 03 Jan 2021 10:09:44 +0000

Posted By: djh
Posted By: VictorianecoEven at 60% RH (the higher end of 'normal') and an air temperature of 15c (provided)

Wrong. The air temperature provided at which 60% might be 'the higher end of 'normal'' was 20Â°C.

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.

Your opinion seems to have been formed without the basis of any facts.

Not sure where you are basing any of this information from, if you re-read my original reply:

"Basic dewpoint calculation required:

What are the humidity levels of the room?
What is the air temperature of the room?

What is the temperature of the window/frame?

Without these, just throwing guesses into 'wet thin air'"

I never mentioned at what temperature the humidity reading was taken as a humidity reading simply hasn't been taken nor provided? I based my calculation at 15c 60% RH just as a working 'example' not as a conclusive answer

The 'fact' that the original poster has clearly mentioned the property (when empty) has an air temperature of 15c is where I based my calculation.

So back to my original post without a humidity level it is meaningless. Be it with my calculation or indeed yours.

Dewpoint simple diagram for reference:
https://en.wikipedia.org/wiki/Dew_point#/media/File:Dewpoint-RH.svg

So unless a RH% is given you're arguing over nothing....

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

GreenPaddy — Sun, 03 Jan 2021 10:27:54 +0000

Your windows would appear to be one of the coldest part of your new build, which you could say is depressing, or you could say is great, cause you've done a great job with the rest of the envelope, and all the moisture is getting forced to the window areas. Would you prefer not to be able to see it at rafter tails etc?

I would also say it's not surprising that you have so much moisture as with (at least) 2 people in quite a small volume, the RH will rise rapidly, and you will be liberating litres of water each day, which have to go somewhere.

Ideally that moisture would be extracted by some means, but whilst not having the whole picture, it would seem you've opted for a keep all the warmth inside by not ventilating. That I think is maybe your mistake to resolve.

I would say you've answered your question well, by the first step of leaving your shower room extract on 24hrs a day. That should certainly reduce the levels of moisture, and so reduce the cond on the windows. The extent to which it succeeds, will help determine the longer term solution.

For me, you have a couple of questions...

- how much air do I have to remove/replace to get rid of the excessive cond problem.
- is the moisture solely from the occupants, or is it also from short term drying out, or even continuous infiltration of water from outside by some mechanism.

These two are of course linked. Running the extract fan will help you get a handle on the magnitude of the problem. You're always going to need to remove the occupant moisture part of the equation, so I'd say some form of ventilation/extraction is required permanently. Let's see what you find empirically with the fan, and then we can all shout for our favoured method of effecting a permanent solution.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

philedge — Sun, 03 Jan 2021 10:38:09 +0000

As its only appears to be temporary accomodation and only a problem during short term cold weather, Id just get a window vac and dry the windows as and when the weather gets cold enough for condensation to form.

If the barn is to be habitable after your house is built then look again at the problem- make sure its designed out of the new build!!

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

WillInAberdeen — Sun, 03 Jan 2021 11:33:51 +0000

A helpful exercise is to visualise how much moisture vapour two adults create in a day. There are lots of numbers out there but the ones I used were very roughly 1kg per person per day from breathing, plus 1kg per day for each of : showering, cooking, drying washing, houseplants, pets.

Work out how much your household scores, and run that amount of water into a bucket and look at it. Chances are that it is wayyy more than you could scrape off the windows, so congratulations you are already removing most of the vapour in other ways, probably ventilation. You just need to top up the ventilation a little more to get on top of it, or accept that you are doing well and live with it.

Paradoxically, you need least ventilation in the winter months when the fresh air you draw in is cold and doesn't carry much moisture. You need to increase ventilation in the spring and autumn on milder days, despite that the window frames are warmer and don't alert you to the problem!

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

tony — Sun, 03 Jan 2021 12:06:56 +0000

Extractor fan left on in an airtight house will depressurise the house very slightly then keep it slightly depressurised but not shift any air ðŸ˜¢

Re dew point, it is the frame/glass temperatures that are relevant to us here and we can see that they are below dew point. What then happens is that moisture is literally pumped from the air in the house with only water molecules moving condensing them onto the glass until the air in the WHOLE HOUSE is so dry that the dew point at the frame/glass is in the safe zone. Effectively the windows are trying to dehumidify the house.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

Victorianeco — Sun, 03 Jan 2021 12:18:12 +0000

Yep, so a simple RH% monitor and surface reading of the windows along with the temperature of the air will give all you need to start with...

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

Ed Davies — Sun, 03 Jan 2021 17:34:37 +0000

Posted By: VictorianecoYep, so a simple RH% monitor and surface reading of the windows along with the temperature of the air will give all you need to start with...

Exactly, the first question is are the windows too cold or is the house air too wet? Obviously they are relative to each other, as tony says, but we still need to establish which is the real source of the problem. Might be both, of course.

If it turns out the air is too wet then the next question is what's causing that. Insufficient ventilation for the occupants or additional sources of vapour?

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

borpin — Mon, 04 Jan 2021 11:46:32 +0000

The first step is to measure the humidity.

You might find that, as this is recently completed, there is still excess moisture inside (ant solid concrete floors?). I though I had a leak when I first was watertight, but discovered exactly that - humidity was too high.

Posted By: chrisduncanbut a bit of me wonders if the two substantial stone walls are still drying out.

Highly likely unless you did something to actually dry them out

So before you do anything get something to measure the humidity, and if high, I'd start with a dehumidifier.

I'd also measure the CO2 levels. If you have limited ventilation, they might be really high too.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

dickster — Tue, 05 Jan 2021 11:10:35 +0000

Hang on a minute!

We've got lovely triple glazed windows and 2g French Windows with insulated (kind of) blinds as well. Log burning stove (horror of horrors) keeps us at around 20-21 degrees to bedtime, drops to maybe 17-18 on a cold night, stove lit again in morning.

The air inside is too dry most of the time, even with cooking in the room. Don't use the externally vented cooker hood or MHRV and leave wet room door open to allow drying wet floor to add to humidity.

We get up to 30 mm of condensation on the glass at the bottom pane on some of the windows/doors if it has been below freezing outside, as shown in Chris Duncan's pic. We also get external condensation on the outer pane and ice forming on the pane when the right circumstances prevail, (warm front coming in after cold night).

I personally do not view our condensation as a problem and assume that the warm air at the top of the pane starts to cool and flows downward over cold glass and has had time to cool enough to dump moisture at the bottom on the coldest part of the pane.

Rivers of condensation top to bottom would be concerning. Condensation on Chris's beams might be that they take a lot longer to warm up when the barn starts to be used, so human activity initially creates condensation. Perhaps after a week of use the beam condensation might disappear?

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

bhommels — Tue, 05 Jan 2021 12:56:57 +0000

Posted By: dicksterHang on a minute!

We've got lovely triple glazed windows and 2g French Windows with insulated (kind of) blinds as well. Log burning stove (horror of horrors) keeps us at around 20-21 degrees to bedtime, drops to maybe 17-18 on a cold night, stove lit again in morning.

The air inside is too dry most of the time, even with cooking in the room. Don't use the externally vented cooker hood or MHRV and leave wet room door open to allow drying wet floor to add to humidity.

We get up to 30 mm of condensation on the glass at the bottom pane on some of the windows/doors if it has been below freezing outside, as shown in Chris Duncan's pic. We also get external condensation on the outer pane and ice forming on the pane when the right circumstances prevail, (warm front coming in after cold night).

Have you measured the RH indoors? I am really surprised you get condensation on the inside of your 3G windows while saying the indoor air is too dry. I never got any condensation at all on the inside of our new-ish 3G, and quite a lot on the DG (old PVC framed, cold spacer stuff). Now, in the first winter with MVHR, even the DG is almost condensation free (hurray!), with RH hovering between 40 and 50 % depending on what is going on in the house.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

Pile-o-Stone — Tue, 05 Jan 2021 13:25:27 +0000

I would have thought that a MHRV system would be mandatory in a highly insulated and airtight building?

I'm currently fitting one in our old mill because we are now getting condensation after insulating parts of the house. We didn't get condensation until we started our (long) program of insulating and airtightening the house. We tried a dehumidifier and although it does fill up with water after a couple of days, the house is still full of condensation. I think the house is just too big for it, plus we have three floors.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

revor — Tue, 05 Jan 2021 13:40:18 +0000

For interest I have measured my indoor RH and temperature this AM and it was 45 % at 18 deg c 3 deg outside no heat on and if we had carpets would expect to be getting static. Don't get condensation and only 2G but does not get really cold here.
We have noticed over the last 11 years warmer winters as evidenced by pipes not freezing (famous last words). We have an outbuilding and a cold water feed to a tap inside comes over at height and is insulated. 11 years ago it would freeze up for several days and over the last decade has gradually not frozen for so long then got a period of slushy water then last couple of years or so not frozen at all. We do not need convincing of global warming.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

RobinB — Tue, 05 Jan 2021 14:42:58 +0000

The Blauberg unit rather annoyingly is an Enthalpy heat exchanger which puts recovers both heat AND moisture and reintroduces both into the building.
Is there some attractive way to reduce the cold bridge at the bottom of the window by cladding it in some way?

FWIW we built an airtight home (under 1.8 ach), effective MVHR and yet we can get condensation overnight at the bottom of timber 3G windows behind thermal blinds in the kitchen. On other windows we get condensation on the outside of the glass. RH about 50%, typically 17/18 degrees by morning.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

revor — Tue, 05 Jan 2021 15:03:57 +0000

@robinb. Bet if you open the curtains a little you will stop the condensation as the window will become warmer.
Condensation on the outside is understandable with 3G as the outer pane is colder than would be the case with 2G where heat loss from room is greater. It is one of the reasons we did not go with 3 G only used 3G in velux roof windows.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

Pile-o-Stone — Tue, 05 Jan 2021 15:16:26 +0000

Posted By: chrisduncan
It seems there are two options unless someone can suggest something better. 1) I buy a dehumidifier and leave it running near the sink during the winter (eg Meacodry ABC 10L/day approx Â£140 and 160W) , and 2) I replace the shower extract fan with a single room MVHR unit (eg Blauberg Mini-air approx Â£370, 3.2W).

Apart from the shower room, do you have extracts elsewhere such as for the laundry room (either in the room itself or an externally vented dryer) and above the cooker?

Our dryer is supposed to vent externally but it's getting old and so maybe the seals have gone because a lot of vapour seems to congregate in the utility room. Our cooker vent doesn't have an external vent (it has a charcoal filter to get rid of smells) so it's useless at getting rid of condensation from cooking.

Our house is like a Turkish bath if our clothes washing days coincide with a meal of boiled veg!

I'll be fitting MHRV extract vents in both the kitchen and utility room and I'm hoping this will solve both those problems.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

djh — Tue, 05 Jan 2021 16:07:43 +0000

Posted By: Pile-o-StoneOur house is like a Turkish bath if our clothes washing days coincide with a meal of boiled veg!

I'll be fitting MHRV extract vents in both the kitchen and utility room and I'm hoping this will solve both those problems.

Should do - I presume you're putting extracts in shower rooms etc? We're drying clothes indoors today since it's raining and we always steam veg, but humidity doesn't seem to notice.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

djh — Tue, 05 Jan 2021 16:10:27 +0000

Posted By: RobinBwe can get condensation overnight at the bottom of timber 3G windows behind thermal blinds in the kitchen

This thread is interesting to me partly because it seems paying extra for PH windows with thermally-insulated frames did actually buy me something! We don't see any sign of condensation on the inside of windows, even behind thermal blinds.

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

chrisduncan — Tue, 05 Jan 2021 17:43:18 +0000

Posted By: revorIs the area quoted at 60 m2 correct? .

Thanks revor. Yes - main room 40m2 with kitchenette and shower room 15m2 and 5m2

Help please! Condensation in newly converted barn. MVHR vs dehumidifier?

chrisduncan — Tue, 05 Jan 2021 17:53:19 +0000

Posted By: revor I suspect your windows are not as thermally broken as you may think. .

Windows are made from Smart Systems sections "Over the past forty years, Smart has grown to become the UK's leading supplier of aluminium glazing systems", fabricated by a local well respected company. Alitherm 600: https://www.smartsystems.co.uk/product/122/alitherm-600.

So 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. I do however, need to be sure of the cause of the condensation so that I can make sure it doesn't happen in the new house. New house will have MVHR so it should be better.