High humidity despite air infiltration: will MVHR really help?

[Sparrowhawk]

This forum is generally of the opinion that if 3-3.5ACH @ 50Pa is your building's airtightness target then MVHR is pointless, as the air entering through the fabric is bigger than the flow rate of the MVHR and the heated air will be lost faster that the MVHR unit can recover it.

 

My house is nowhere near 3.5ACH yet (a safe guess). So with plenty of air infiltration, why is the humidity averaging 70% in the lounge?

 

 

Lounge is south facing (spikes are when the sun hit the temperature monitor), no trickle vents, and contains a bunch of house plants.

 

I've been holding out that as we improve the airtightness then adding MVHR will fix this humidity, but if we're already well ventilated and this is the humidity, then MVHR is going to change nothing, right?

 

[For SteamyTea the absolute humidity is lower outside except for mid-summer. Natural ventilation between Sept-May should be bringing in less water vapour than is indoors. Battery ran out over the summer, hence the gap]

Edited October 6, 2024 by Sparrowhawk

[ProDave]

No trickle vents so how do you KNOW you it is "well ventilated"?

[Sparrowhawk]

17 minutes ago, ProDave said:

No trickle vents so how do you KNOW you it is "well ventilated"?

Up until September the cavity was open one side of the window. I stuffed it with stuff over winter but the curtains would still blow in the wind

Edit: our airbricks vent the wall as well as under the floor. They're not sheathed.

Edited October 6, 2024 by Sparrowhawk

[SteamyTea]

These houseplants, are they near a heat source and watered frequently?

Have you checked for other areas for water leaks?

 

My internal humidity is generally high, but then my outside humidity is very high.

 

Air infiltration only happens when there is a pressure difference, which means windy days.  A while back, I looked at the temperature drop in my kitchen when I left a window open, after the initial drop of about 1°C, there was little difference, but it was a calm day (calm down here is Force 6 or less).

[JohnMo]

You just need a robust ventilation strategy, my curtains blow about isn't a ventilation strategy.

 

Don't you need to measure the relative humidity instead of absolute?

[Sparrowhawk]

47 minutes ago, SteamyTea said:

These houseplants, are they near a heat source and watered frequently?

 

Watered once a week, but the biggest 2 take 2-3 litres each, and the rest probably another 2-3 litres combined. No greenhouse right now so 3 bougainvilleas are in here, plus 20 5" palm seedlings and a large pointsettia. I keep them away from the radiator but they are at 20C and in sunlight.

 

52 minutes ago, SteamyTea said:

Have you checked for other areas for water leaks?

 

Yes, everywhere I've had the floor up here is dry, no damp to be noiced on the (unfilled cavity) walls and the chimney has been taken down to loft level so no rain entering it either.

 

47 minutes ago, SteamyTea said:

Air infiltration only happens when there is a pressure difference, which means windy days.  A while back, I looked at the temperature drop in my kitchen when I left a window open, after the initial drop of about 1°C, there was little difference, but it was a calm day (calm down here is Force 6 or less).

And that's my disagreement with the "if you're above 3ACH don't do MVHR" position. Air infiltration aka natural ventilation isn't reliable ventilation. Even Passivhaus trust disagree with the 3ACH cutoff.

 

Force 6 - yep we're on the same calm day scale as you.

[Mike]

1. What are you measuring temperature & humidity with, and has it been calibrated / checked for accuracy?

 

2. You can download a small spreadsheet from this thread that will calculate the internal RH based on the external RH & internal + external temperatures. Variation from the calculated figures are the result of lag or other moisture sources / sinks. It may be of interest.

 

2 hours ago, Sparrowhawk said:

if 3-3.5ACH @ 50Pa is your building's airtightness target then MVHR is pointless

As I recall, the reasoning is that that's roughly the point where annual costs exceed savings (and when your EPC rating will suffer) - so uneconomic, which isn't the same as pointless. FWIW I've seen 5ACH quoted as the determining limit Europe. But it is universally agreed is that the more airtight the better, and with care and attention under 3 is readily achievable.

[Sparrowhawk]

1 hour ago, Mike said:

1. What are you measuring temperature & humidity with, and has it been calibrated / checked for accuracy?

A mix of  Zigbee temperature & humidity sensors Aqara, Tuya and a single Sonoff. I calibrated their RH reading last year using the NaCL test. The lounge has the sole Sonoff though, so I've added a Tuya to see if they match. After a night the Tuya is 0.6C / 6% RH lower than the Sonoff, which if it stays that different goes a way to explaining the higher recorded humidity in the lounge compared to the other rooms.

 

2 hours ago, Mike said:

2. You can download a small spreadsheet from this thread that will calculate the internal RH based on the external RH & internal + external temperatures. Variation from the calculated figures are the result of lag or other moisture sources / sinks. It may be of interest.

Thanks I will play once I have found how to get HomeAssistant to give me the average figure.

 

13 hours ago, Mike said:

As I recall, the reasoning is that that's roughly the point where annual costs exceed savings (and when your EPC rating will suffer) - so uneconomic, which isn't the same as pointless. FWIW I've seen 5ACH quoted as the determining limit Europe. But it is universally agreed is that the more airtight the better, and with care and attention under 3 is readily achievable.

👍

[marshian]

On 06/10/2024 at 15:23, Sparrowhawk said:

This forum is generally of the opinion that if 3-3.5ACH @ 50Pa is your building's airtightness target then MVHR is pointless, as the air entering through the fabric is bigger than the flow rate of the MVHR and the heated air will be lost faster that the MVHR unit can recover it.

 

My house is nowhere near 3.5ACH yet (a safe guess). So with plenty of air infiltration, why is the humidity averaging 70% in the lounge?

 

 

Lounge is south facing (spikes are when the sun hit the temperature monitor), no trickle vents, and contains a bunch of house plants.

 

I've been holding out that as we improve the airtightness then adding MVHR will fix this humidity, but if we're already well ventilated and this is the humidity, then MVHR is going to change nothing, right?

 

[For SteamyTea the absolute humidity is lower outside except for mid-summer. Natural ventilation between Sept-May should be bringing in less water vapour than is indoors. Battery ran out over the summer, hence the gap]

 

Great data

 

I was worried about mine (lounge) being in the high 50's to low 60's.........

 

No trickle vents and very little air ingress (due to insulating under the floor) and Patio doors are rarely opened (due to spiders apparently)

 

PIV unit runs 12.5/24 hrs and certainly lowered the overall humidity

[marshian]

9 hours ago, Sparrowhawk said:

A mix of  Zigbee temperature & humidity sensors Aqara, Tuya and a single Sonoff. I calibrated their RH reading last year using the NaCL test. The lounge has the sole Sonoff though, so I've added a Tuya to see if they match. After a night the Tuya is 0.6C / 6% RH lower than the Sonoff, which if it stays that different goes a way to explaining the higher recorded humidity in the lounge compared to the other rooms.

Ahh be interesting to know how to do that for the Sonoff units I have - 6 % difference is significant

[Sparrowhawk]

21 minutes ago, marshian said:

Ahh be interesting to know how to do that for the Sonoff units I have - 6 % difference is significant

https://www.engineeringtoolbox.com/salt-humidity-d_1887.html and https://imatrixsys.com/multi-point-relative-humidity-calibration/

 

I did mine in a marge tub with a small pot of salt and water and multiple sensors next to it. The important thing with NaCl is that for room temperatures, the relative humidity over the salt solution stays pretty constant in the 75-75.75% range. Leave the sensors to reach equilibrium (2-3 days) and record the difference for each sensor and then in HomeAssistant change the offset value.

[marshian]

5 hours ago, Sparrowhawk said:

https://www.engineeringtoolbox.com/salt-humidity-d_1887.html and https://imatrixsys.com/multi-point-relative-humidity-calibration/

 

I did mine in a marge tub with a small pot of salt and water and multiple sensors next to it. The important thing with NaCl is that for room temperatures, the relative humidity over the salt solution stays pretty constant in the 75-75.75% range. Leave the sensors to reach equilibrium (2-3 days) and record the difference for each sensor and then in HomeAssistant change the offset value.

 

Very useful thank you - I know for my Sonoff Temp sensors I can change the offset value in the app - I've had a look in the app and I can change humidity offset too so looks like a calibration is on the cards for this weekend

[marshian]

Just to round the calibration element of the thread - I’ve had both sensors over a NaCl slushie for a couple of days at room temp 

 

one required an adjustment of 2.7 and the other 4.5 (both cases they were over reading) so I’m a smidge less concerned about my humidity levels now 

 

thank you @Sparrowhawk