Green Building Forum - Mirror demisters needed with MVHR? Tue, 19 Dec 2023 08:26:22 +0000 http://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=261883#Comment_261883 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=261883#Comment_261883 Fri, 06 Oct 2017 10:06:52 +0100 jamesheath
I'm in the process of building a house with MVHR, extreme airtightness etc. (Passivhaus principles & levels, PHPP designed etc)

My understanding is that mechanically ventilated houses have
a) a higher rate of ventilation then normal houses
b) dryer air generally
c) more evenly warm rooms

With that in mind, do I need mirror demisters?
in a bathroom with an enclosed shower
in a bathroom with an open shower

(OK - no-one NEEDS mirror demisters - we've never had them & never been too fussed...)

Our builder is suggesting installing them & then not using them if we don't need to.

I'd rather gather some experience from someone who lives in a similar house - do the mirrors steam up?

Thanks,

James]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=261889#Comment_261889 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=261889#Comment_261889 Fri, 06 Oct 2017 12:32:00 +0100 tony
it will go fairly soon after showers and cant be stopped.

if you need mirrors right after shower, then electrically headed mirrors will help you.]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=261898#Comment_261898 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=261898#Comment_261898 Fri, 06 Oct 2017 15:18:34 +0100 djh Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=261901#Comment_261901 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=261901#Comment_261901 Fri, 06 Oct 2017 16:22:04 +0100 richardelliot
We have some electric heat pads underneath the mirrors that are controlled by the lights and work a treat. I haven't measured exactly how much energy they use but it isn't a big drain. Yes I could wipe the mirror when I want to have a shave after my shower, but it is very nice not to have to, and I wouldn't want to be without them now.]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=261908#Comment_261908 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=261908#Comment_261908 Fri, 06 Oct 2017 20:39:39 +0100 Cerisy Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262020#Comment_262020 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262020#Comment_262020 Tue, 10 Oct 2017 09:36:24 +0100 Greenfish
One thing I do wonder if we will miss in old age is an electric bathroom heater - somethimg to get just the bathroom extra hot at short notice. UFH is lovely but slow to respond, and the uniform heat in a draft-free house does mean we comfortably run rooms at a lower temperature than the old house. However Mrs G grumbles now and then that stepping out the shower is a little too cool for her. Standing/sitting beneath a fan heater is also a great way to dry off a slower moving disabled elderly person in comfort.

So no mirror heaters, but do think about power in the bathroom.

Posted By: jamesheathHi,
My understanding is that mechanically ventilated houses have
a) a higher rate of ventilation then normal houses
b) dryer air generally
c) more evenly warm rooms

I think you mis-understand.
a) I don't think you can make that statement. It will depend on how drafty the old house, and how fast up you run the MVHR. Also local is different from total. Old house extractor fan had greater short term evacuation in the bathroom than MVHR now provides, but MVHR is on 24/7, and old extractor ran for 15 mins.

b) MVHR means that the Relative Humidity inside the house will track the RH that is outside. If you live in the warm and damp western side of the UK (> 80% outside, and only 10deg cooler) then inside it will never feel dry or even rarely be below 60% inside unless you run an air conditioner or dehumidifier. However if you have lived with lots human generated moisture, that is lots of people in a small space, with clothes drying on the rads, boiling pans on the stove and no extraction or open windows, then yes MVHR will remove that artificial wetness (and smell).

c) Air is a poor transporter of heat, so I don't think the MVHR contributes in that way. It is both the lack of drafts that air tightness brings, lack of convective heat currents previously produced by radiators that comes with UFH, and lack of cold spots from having 3G and being surrounded by insulation witout thermal bridging, that make for more evenly warm rooms.]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262028#Comment_262028 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262028#Comment_262028 Tue, 10 Oct 2017 16:20:59 +0100 djh Posted By: GreenfishSo no mirror heaters, but do think about power in the bathroom.
Indeed. I have some currently unconnected power radials in the shower rooms for exactly that purpose.

b) MVHR means that the Relative Humidity inside the house will track the RH that is outside. If you live in the warm and damp western side of the UK (> 80% outside, and only 10deg cooler) then inside it will never feel dry or even rarely be below 60% inside unless you run an air conditioner or dehumidifier.

I think it's less confusing to say that the specific humidity inside tracks that outside, since the relationship between the relative humidity inside and outside isn't simple tracking but also depends on the temperature difference.

My house doesn't follow that rule though and it markedly differs from the PHPP predictions (it's more even and mostly stays between 45-50% all the time) so humidity buffering can clearly be important in some circumstances.]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262042#Comment_262042 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262042#Comment_262042 Tue, 10 Oct 2017 22:45:50 +0100 Greenfish Posted By: djh
b) MVHR means that the Relative Humidity inside the house will track the RH that is outside. If you live in the warm and damp western side of the UK (> 80% outside, and only 10deg cooler) then inside it will never feel dry or even rarely be below 60% inside unless you run an air conditioner or dehumidifier.

I think it's less confusing to say that the specific humidity inside tracks that outside, since the relationship between the relative humidity inside and outside isn't simple tracking but also depends on the temperature difference.
You are absolutely right, that is a better way to express it.

My house doesn't follow that rule though and it markedly differs from the PHPP predictions (it's more even and mostly stays between 45-50% all the time) so humidity buffering can clearly be important in some circumstances.

Where are you located a dry part of the country? And curious any build specifics that could provide the buffering?

Just want to emphasise that location/local climate makes a big difference to internal RH. I was quite worried when I first got a hygrometer and measured the RH to find we were above the golden 60% value. Yes the new plaster was still drying, but even after that RH below 60% is rare unless I heat the house up more than we need. Then I did the sums. If it is a persistent 20C and 80% outside like we had this summer then the MVHR will pull in that moisture, and at 20C indoors the RH will also be 80%.

Of course even in the dry East if you have lots of washing drying, a damp dog, and brewing a stew then the RH will be higher inside and MVHR will reduce it.]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262044#Comment_262044 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262044#Comment_262044 Tue, 10 Oct 2017 23:47:02 +0100 djh Posted By: GreenfishWhere are you located a dry part of the country?
Yes, Suffolk.

And curious any build specifics that could provide the buffering?

Well, straw bales covered by lime plaster might have something to do with it :bigsmile:

Just want to emphasise that location/local climate makes a big difference to internal RH. I was quite worried when I first got a hygrometer and measured the RH to find we were above the golden 60% value. Yes the new plaster was still drying, but even after that RH below 60% is rare unless I heat the house up more than we need. Then I did the sums. If it is a persistent 20C and 80% outside like we had this summer then the MVHR will pull in that moisture, and at 20C indoors the RH will also be 80%.

Our house is always warmer than outdoors so we never have RH as high as outdoors, but our outside RH is never as high as 80% except when it's raining, plus or minus a bit. Are you close to some permanent water?]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262048#Comment_262048 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262048#Comment_262048 Wed, 11 Oct 2017 06:08:23 +0100 cjard
Before plasterboarding, run a wire from the lighting radial in that room, to where the mirror will go
Fit a normal mirror
See if it steams up beyond your capacity to tolerate
If it does, mount that mirror somewhere else
Smash hole in wall and retrieve wire
Obtain huge designer led backlit, clock containing, radio playing, touch sensitive, auto self demisting mirror off eBay for about 100 quid (from Poland) and fit

The sooner you stop on this list, the less it costs :)

I went all the way to the end of the list, because she liked the mirror. It's still not steamed up, and it's not wired in to power yet, but then the bathroom doesn't have a door on either]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262050#Comment_262050 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262050#Comment_262050 Wed, 11 Oct 2017 06:55:44 +0100 Greenfish Our house is always warmer than outdoors so we never have RH as high as outdoors, but our outside RH is never as high as 80% except when it's raining, plus or minus a bit. Are you close to some permanent water?
Cornwall - everywhere is close to the sea! We also get days of "missle" when the house is in a damp cloud. Ask Steamytea, I'm sure he has posted graphs of our weather for you Easterners to laugh at.

cjard - very funny, if it were not only so true. When none of the designer ones were big enough for SWMBO I skipped straight to the final step - bought a big mirror from Ikea, mounted it infront a wood frame and fitted an LED ribbon from Ebay.]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262052#Comment_262052 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262052#Comment_262052 Wed, 11 Oct 2017 08:21:25 +0100 SteamyTea And here is the data:
(ignore my labelling error, it is mean temp)
And just for a laugh, here is some Suffolk data, looks similar.]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262054#Comment_262054 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262054#Comment_262054 Wed, 11 Oct 2017 09:47:38 +0100 delprado
What is the view on towel radiators? I guess electric so it can be on at any time of year when heating is off?

Thanks]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262057#Comment_262057 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262057#Comment_262057 Wed, 11 Oct 2017 10:02:08 +0100 SteamyTea Culdrose mean temp 15°C
Westhall mean temp 16°C
Culdrose mean RH 84%
Westhall mean RH 89%

Looks like we were not the only ones to have a bad summer.]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262063#Comment_262063 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262063#Comment_262063 Wed, 11 Oct 2017 13:13:50 +0100 borpin Posted By: jamesheathWith that in mind, do I need mirror demisters?Yes. I love the fact that (a portion of) the mirror never steams up. I just bought the really small Demista, £20, 5W does the job perfectly.]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262067#Comment_262067 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262067#Comment_262067 Wed, 11 Oct 2017 13:32:56 +0100 djh Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262070#Comment_262070 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262070#Comment_262070 Wed, 11 Oct 2017 14:01:09 +0100 SteamyTea Posted By: djhWesthall is another place near the coastApart from the altitude difference they seems closely matched.
Things is, and supporting your point about AH, and without going looking for real extremes, there is probably not much difference when you get a wet summer.]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262072#Comment_262072 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262072#Comment_262072 Wed, 11 Oct 2017 16:09:30 +0100 richardelliot Posted By: delpradoWhat is the view on towel radiators? I guess electric so it can be on at any time of year when heating is off?


We have a radiator that is connected to the gas fired central heating system in the usual way and also has an electric heating element. The electric element is useful for Spring and Autumn when the central heating is off but you'd like a little burst to dry the towels.

I have found that towels do dry with the MHVR. The issue for us comes when you have a lot of towels on the radiator that are all folded up.

If you do go fully / partially electric make sure you have a timer on there. We just have a fused spur and it is annoying to manually turn on and off and we sometimes leave it on.]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262075#Comment_262075 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262075#Comment_262075 Wed, 11 Oct 2017 16:34:52 +0100 Cerisy
Now the electric underfloor heating mats are a little extravagant, but we never said we were going hair-shirt!!]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262081#Comment_262081 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262081#Comment_262081 Wed, 11 Oct 2017 19:03:04 +0100 richardelliot Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262095#Comment_262095 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262095#Comment_262095 Thu, 12 Oct 2017 14:21:18 +0100 Greenfish
I do have towel rails on the UFH, mostly used in summer as a heat soak for the solar thermal to avoid stagnation.]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262100#Comment_262100 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262100#Comment_262100 Thu, 12 Oct 2017 16:52:05 +0100 djh Posted By: GreenfishMVHR does a good job at drying towels, but it helps if they are opened out not tightly folded. More open hanging space for them would be good.
Indeed that's a very good tip. We hang ours over the landing banister and over one of the support struts in the shower, FWIW. They all dry well when opened up like that. We have a Sheila's Maid in the bathroom for more serious indoor drying when necessary.]]> Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262104#Comment_262104 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262104#Comment_262104 Thu, 12 Oct 2017 17:22:58 +0100 Greenfish Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262106#Comment_262106 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262106#Comment_262106 Thu, 12 Oct 2017 19:17:23 +0100 revor Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262166#Comment_262166 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262166#Comment_262166 Sun, 15 Oct 2017 07:50:02 +0100 cjard Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262178#Comment_262178 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262178#Comment_262178 Sun, 15 Oct 2017 22:51:08 +0100 revor Mirror demisters needed with MVHR? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262291#Comment_262291 http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15331&Focus=262291#Comment_262291 Thu, 19 Oct 2017 09:37:27 +0100 Simon Still
Our bathroom mirrors do mist up. I fitted some of the keenly priced powered mirrors that are available but rewired them so that the LED lighting is connected to the room lighting and the demister pad only operates when the lights are on and humidity level is “high” (which also boosts the MVHR).

2 Winters in and haven’t felt any need for additional heating in the bathrooms (though a cable is there if needed in future.

Towels dry very well with MVHR but I fitted a pair of 90cm towel rails (one above the other) to the bathrooms so that they could just be folded in half when hung.]]>