Opening windows or closing them might be good advice against humidity.

Disclaimer: Humidity is an important topic at Portugal; and at a lesser level at Italy; and at an even lesser level at the North of Europe. Moisture is not a focus of my profession. I offer this webpage for gratis as hints to protect you and your properties from fungi, but I do not perform truly conclusive experiments or analyses of mildew. I do not know! Clyde Windows and Construction Limited and Community Energy Plus advise that condensation is complicated. A good source contradicts a different good source. A single recommendation might be good but its opposite might contrarily be good at a different time (e.g. night versus day; or Summer versus Winter); and/or at a different location (e.g. west Portugal versus east Portugal versus Italy); and/or with a different climate; and/or with different conditions which I do not enumerate! Misinterpretations and invalid generalisations are too easy! Do not rely on this webpage and do not prosecute me over it! Buy a hygrometer and a dehumidifier and open windows and doors and close windows and doors to find out what technique or techniques works or work for you. If you offer evidence or counterevidence or data or counter-data, then I might consider it or them. Thanks!

Table of Contents

A good real-estate agency (Nest Property Management at Louth) gave me a pamphlet about mustiness. I failed to find an Internet version. Similar webpages are WWW.FNREM.com.Au/pages/real-estate/blog/4526/the-menace-of-mould-and-how-to-manage-it and WWW.PropertyTribes.com/down-in-the-damps-t-2084.html and HTTPS://CEP.org.UK/landlords/damp-condensation-and-mould-advice-for-landlords and WWW.ClydeWindows.com/blog/windows/internal-window-condensation/?srsltid=AfmBOoqcZ_pLawhwSvMLowDgwFgEhKB3CHnemvmvGaeNgW-XnmkBepGX and WWW.DIYdata.com/problem/condensation/condensation-overcoming.php and WWW.Netmums.com/coffeehouse/home-garden-194/cleaning-14/110656-condensation.html etc.

A different good real-estate agency (Anthony-Byrne Property Services at the same town) gave me a booklet from a Viking town. That booklet about mustiness is like the document which is hyperlinked to by this sentence, which is available in many languages.

Coimbra and Oporto and Montemor-o-Velho and Matosinhos and Condeixa at Portugal are too humid. The map of humidity which this sentence hyperlinks to does not show Azores, which is even worse more humid than Coimbra according to a lady who used to reside at both Azores and Coimbra.

I bought a dehumidifier (during 2008) and 2 hygrometers at Portugal. A dehumidifier significantly reduces a house’s humidity. A hygrometer also showed me that open doors and open windows during non-night times much more significantly reduce a house’s humidity than a dehumidifier accomplishes. WWW.Netmums.com/coffeehouse/home-garden-194/cleaning-14/110656-condensation.html alleges that a "a positive pressure unit" is better than a dehumidifier.

The 2^nd Republic of Portugal robbed a hygrometer etc. of mine. “I keep my expectations low.” said an advocate about regaining my properties that Portugal robbed on 5^th July 2023.

The metrological forecasters here predicted that humidities here can be e.g. worse than 80% throughout 23^rd August 2025 and between 80% and 100% between 7p.m. and 2p.m. earlier during the same week or similar humidities between 8p.m. and 10a.m. even earlier during the same week. (E.g. 100% at 3a.m.; 80% at 7p.m.; and 86.3% at 9a.m.)

“indoor humidity of 80% or higher
[. . .]
•In winter, you still need to open windows on a daily basis (wait till the warmest time of day)
[. . .]
•[. . .] opening blinds during the day”
says First National Real Estate Maitland at Australia. Cf. WWW.PropertyTribes.com/down-in-the-damps-t-2084.html and HTTPS://CEP.org.UK/landlords/damp-condensation-and-mould-advice-for-landlords and WWW.DIYdata.com/problem/condensation/condensation.php etc.

However Clyde Windows and Construction Limited says: "During the beginning of the winter when you start to heat your home, condensation occurs. After a few weeks, your home will begin to dry out and you’ll see less condensation. Opening a window briefly is a quick temporary solution. The dryer cold air will enter the room while the moist air is allowed to escape."

Clyde Windows and Construction Limited says: "During the winter months, condensation will be seen on the inside of the window. Condensation will present itself on the outside of the window during the summer months." Moisture on opened and closed windows here was on their outsides, not insides, on 18^th August 2025 (i.e. a warm day during a warm month) shortly after 6a.m. o'clock.

"I bought a wardrobe and chest of drawers for my lo and the backs of them have gone mouldy I keep wiping down but it returns. As I said before the windows are so wet in the morning and I never leave windows closed fully plus we have a few wall air vents in every room." says a person who reports worse dampness than I ever lived with.

Mother owns a printout of a document. That document seems to be a webpage of a newspaper. It recommends to avoid fustiness by never opening curtains. That document does not convince me. Cars’ windows do not have curtains. Cars’ windows gain moisture before 6a.m. o’clock. I keep a subset of this house’s curtains opened and different subsets closed and opened for different amounts of times. I do not detect an overwhelming difference of moisture between any of these windows.

Italy is too humid. Many persons attempt to dry clothes outside at Italy during nights. An educated gentleman at Italy said that that is normal and bad because nighttimes humidities are against drying. He said that many persons at Italy do not know that that is a bad technique.

My hygrometers at Portugal have digital displays. My hygrometer here is analogue with its display graduated only by multiples of 5%, so my estimated readings thereof lack precision.

I quote numbers by HTTPS://WWW.Met.Ie/ga - that website declares that it is copyrighted. (More about copyright.)

These two graphs show measured (not predicted) humidity at a weather station which is located near where I reside. Their minimum is 54% relative humidity. Their maximum is 99% relative humidity. It was least humid at circa 2p.m. or 4p.m. and circa 80% or worse before circa 10a.m. and after circa 6p.m.
These two graphs about September 2025 show (the same) 29,429 datapoints each: san_aice_umam_tableView_September2025.csv and san_aice_umam_tableView_September2025_moments.csv

Humidity measurements near where I reside

The metrological forecasters here predicted that humidities here can be e.g. worse than 80% throughout 23^rd August 2025 and between 80% and 100% between 7p.m. and 2p.m. earlier during the same week or similar humidities between 8p.m. and 10a.m. even earlier during the same week. (E.g. 100% at 3a.m.; 80% at 7p.m.; and 86.3% at 9a.m.)

My analogue non-electric hygrometer here professed circa 75% relative humidity on 23/08/2025 at 10:22a.m.

September

These graphs show measured (not predicted) humidity at a weather station which is located near where I reside. Their minimum is 54% relative humidity. Their maximum is 99% relative humidity. It was least humid at circa 2p.m. or 4p.m. and circa 80% or worse before circa 10a.m. and after circa 6p.m.
These two graphs about September 2025 show (the same) 29,429 datapoints each: san_aice_umam_tableView_September2025.csv and san_aice_umam_tableView_September2025_moments.csv

Many days during September 2025 here were humid. E.g. a prediction for a less (sic!) humid day here (i.e. for 29^th September 2025 predicted also on 29^th September 2025) by HTTPS://WWW.Met.Ie/ga was 79.3% and 74.6% and 75.9% for 2p.m. and 3p.m. and 4p.m. and that day's all other hours' values being at least 80.6% (5p.m.) and at most 94.4% (10p.m.).

My analogue non-electric hygrometer here professed circa 61% relative humidity on 23/09/2025 at 1:07p.m. when HTTPS://WWW.Met.Ie/ga predicted 66.0% humidity for here for 2p.m. on the same day.

It professed circa 65% relative humidity on 23/09/2025 at 9:17p.m. when HTTPS://WWW.Met.Ie/ga predicted 82.3% humidity for here for 10p.m. on the same day.

My analogue hygrometer reported circa 67% at 10:05a.m. and 65% at 11:35 on 24/09/2025 when HTTPS://WWW.Met.Ie/ga predicted 81.5% and 78% for 11a.m. and noon respectively when all windows and external doors were closed since circa 8:10p.m. the previous night (except 1 door for a few seconds at 11:07a.m. to collect mail). My analogue hygrometer reported circa 69% at 6:05p.m. when HTTPS://WWW.Met.Ie/ga predicted 88.9% for 7p.m. when all our windows and external doors remained closed since 11:07a.m. except for a few seconds to exit once and to enter once. Shortly before 3p.m. on the same day HTTPS://WWW.Met.Ie/ga predicted 82.2% and 88.2% (instead: i.e. not 88.9%) for 6p.m. and 7p.m. respectively.

October

Predictions for 1^st October 2025 here predicted also on 1^st October 2025 by HTTPS://WWW.Met.Ie/ga were at least 87.0% or 89.1% (for 4p.m.) and e.g. 93.1% or 89.5% (for noon) and 92.3% or 91.6% (for 10p.m.).

Opened doors and windows at our house at the North of Europe took many tens of minutes on 5^th October 2025 to lower my analogue hygrometer's display by circa 9% from circa 75% to circa 66%. HTTPS://WWW.Met.Ie/ga predicted on 5^th October 2025 for 5^th October 2025 here values like 79% when these doors and windows were opened.

Various days during October 2025 here were humid. E.g. a prediction for here (i.e. for 13^th October 2025 predicted shortly before 9a.m. also on 13^th October 2025) by HTTPS://WWW.Met.Ie/ga was for all that day's values to be at least 83.6% (for noon) and at most 97.2% (for 11p.m.). I bathed at circa 1a.m. that morning. My analogue hygrometer reported almost the same relative humidity at circa 2a.m. (i.e. shortly after that bath) as it reported almost sixteen hours later at 5:54p.m. that evening. My analogue hygrometer reported 74% at 5:54p.m. HTTPS://WWW.Met.Ie/ga predicted one minute later (at 5:55p.m.) 90.1% for here for six minutes later (6p.m.).
My analogue hygrometer still reported 74% at 6:08p.m. Instead of 90.1%, 90.2% was predicted for 6p.m. by HTTPS://WWW.Met.Ie/ga shortly before 9a.m. Such a 5:55p.m. prediction predicted 96.8% (instead of 97.2%) for 11p.m., with all values being at most 96.8% and at least 90.1%.
I maintained the windows here closed since the night of 12^th October 2025 to after 13^th October 2025. I maintained the external doors here closed since circa zero o'clock on 13^th October 2025 (to 1:09p.m. on 13^th October 2025 when I opened only one door for a few seconds to speak to a canvasser (I closed this door before speaking to this canvasser)). The only other times that I opened an external door on this day (until after the measurements by me for this day) are at circa 2:55p.m. for a few seconds to check a mailbox and for a few seconds to depart to a post office to post registered mail and for a few seconds when returning at 4:38p.m.

Predictions for 17^th October 2025 here predicted also on 17^th October 2025 (predicted shortly before 10a.m.) by HTTPS://WWW.Met.Ie/ga were: 88.5% for 10a.m.; 82.7% for 11a.m.; 78.0% for noon; 75.7% for 1p.m.; 77.6% for 2p.m.; 83.4% for 3p.m.; 89.0% for 4p.m.; 90.3% for 5p.m.; 90.5% for 6p.m.; 93.2% for 7p.m.; 92.6% for 8p.m.; 95.5% for 9p.m.; 91.5% for 10p.m.; and 91.1% for 11p.m. Our house had many windows and one external door open from circa 9a.m. to circa 11:25a.m. on 17^th October 2025. Therefore humidity became decreased by circa 4 percent during circa 2 hours. I.e. from circa 74% at 9:27a.m. to slightly worse than 70% at 11:27a.m.

November

My analogue hygrometer reported slightly less than 65% relative humidity in our house with all its windows and external doors closed at circa 10a.m. on 30^th November 2025. I then opened many windows and 1 external door. After many minutes, e.g. at circa 10:12a.m., it reported circa 60%. It did not rain during this morning.
A neighbour lives scarcely a minute's walk away in a similar house. He bought a digital hygrometer in Japan. It reported 41% during rain at circa 5:23p.m. on 30^th November 2025.
HTTPS://WWW.Met.Ie/ga predicted 92.8% humidity in these houses at 6:29p.m. on 30^th November 2025 for 7p.m. during the same evening.

December

More about copyright

Furthermore Met.Ie warns that it shows data which are not checked (and data which are checked), which I republish for this webpage.

Weather data for near other places where I lived

I do not deem the humidity data from Aveiro PTGAFNAZ from HTTPS://WOW.Met.Ie/stations/report?id=1f60e669-3082-e911-80e7-0003ff597316 to be correct. For examples, my digital hygrometers reported that the duration for a change of a few percent of relative humidity is many minutes at the place near Aveiro where I resided, but Aveiro_PTGAFNAZ_January_2025_tableView.csv - i.e. an excerpt from HTTPS://WOW.Met.Ie/stations/report?id=1f60e669-3082-e911-80e7-0003ff597316 - has: "2025-01-01T16:35:00+00:00,59
2025-01-01T16:40:00+00:00,61
2025-01-01T16:45:00+00:00,62
2025-01-01T16:50:00+00:00,45.1
2025-01-01T16:55:00+00:00,7.22222222222222" which I interpret to be a false report of a change from 45.1% relative humidity to 7.22222222222222% relative humidity during 5 minutes. HTTPS://WOW.Met.Ie/stations/report?id=1f60e669-3082-e911-80e7-0003ff597316 has: "Offical Station No".

For example, opened doors and windows at our house at the North of Europe took many tens of minutes on 5^th October 2025 to lower my analogue hygrometer's display by circa 9% from circa 75% to circa 66%. HTTPS://WWW.Met.Ie/ga predicted on 5^th October 2025 for 5^th October 2025 here values like 79% when these doors and windows were opened. This country is less humid than Portugal.

I imagine that many of these data from Aveiro below 40%-60% more or less before 8a.m. more or less or after 7p.m. more or less or below 30% more or less at any hour could be incorrect: too good to be true.

January

February

March

April

May

June

July

August

September

October

November

December

Example commands

gnuplot
set datafile separator comma
set terminal png
set ylabel "Relative humidity"
set title "humidity here"
set xlabel "when"
set xdata time
set timefmt "%Y-%m-%d"
set format x "%A\n%d\n%B\n%Y"
set yrange [ 0 : ]
set output 'some_name.csv.png' #Based on HTTPS://StackOverflow.com/questions/14047061/how-to-call-gnuplot-from-cli-and-save-the-output-graph-to-the-image-file
plot "some_name.csv" using 1:2, [ : ] 80 title 'humidity too big' lt rgb "red" #"If time/date information is to be plotted from a file, the using option must be used on the plot or splot command." says HTTPS://GNUPlot.SourceForge.net/docs_5.4/gnuplot.pdf

cat san_aice_umam_tableView_September2025.csv | sed -e 's/.*T//' | sed -e 's/+.*,/,/' > san_aice_umam_tableView_September2025_moments.csv

gnuplot
set datafile separator comma
set terminal png
set ylabel "Relative humidity"
set title "humidity here"
set xlabel "when"
set xdata time
set timefmt "%H:%M:%S"
set format x "%l%p" #This incorrectly shows noon as "12pm" and and midnight as "12am". Cf. HTTPS://WWW.EnglishClub.com/efl/articles/vocabulary/12am-12pm-whats-the-difference and HTTPS://WWW.RMG.co.UK/stories/time/noon-12-am-or-12-pm
set yrange [ 0 : ]
set output 'san_aice_umam_tableView_September2025_moments.csv.png' #Based on HTTPS://StackOverflow.com/questions/14047061/how-to-call-gnuplot-from-cli-and-save-the-output-graph-to-the-image-file
plot "san_aice_umam_tableView_September2025_moments.csv" using 1:2, [ : ] 80 title 'humidity too big' lt rgb "red" #"If time/date information is to be plotted from a file, the using option must be used on the plot or splot command." says HTTPS://GNUPlot.SourceForge.net/docs_5.4/gnuplot.pdf