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  1.  
    Hi,

    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
    • CommentAuthortony
    • CommentTimeOct 6th 2017
     
    Even with MVHR you will still get steaming up on tiles and mirrors as steam is already condensing into droplets in the air it will steam up mirrors.

    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.
    •  
      CommentAuthordjh
    • CommentTimeOct 6th 2017
     
    I have as much lime-plastered wall in my showers as possible - just the immediate area around the shower is panelled. Given that plus a passivhaus environment I don't suffer from condensation on mirrors to any serious extent. If the mirror begins to cloud over at the end of a shower, I just open the door and it clears straight away. I certainly wouldn't bother with electrical heating.
  2.  
    We did not achieve extreme air tightness, but our mirrors do fog up while we we take a shower. We have an open shower cubicle.

    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.
    • CommentAuthorCerisy
    • CommentTimeOct 6th 2017
     
    Our house was designed to PPH standards and in our en-suite shower room (enclosed shower) our mirrors clear pretty quickly. I considered / researched heated mirrors, but the budget didn't stretch to it. No problem for us - we do put the MVHR onto high speed to clear the moisture.
    • CommentAuthorGreenfish
    • CommentTimeOct 10th 2017
     
    Our mirrors do steam up after a long shower i.e. does for wife but never for me, but clears very quickly with MVHR on boost. A handy squeegie solves those few times it is not quick enough to clear. No mirror heaters, and don't miss them. But a power supply to the back of the mirror has been useful for some creative low-level room lighting (LED back lights around edge of mirror), great for those middle of the night trips to the loo etc.

    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.
    •  
      CommentAuthordjh
    • CommentTimeOct 10th 2017
     
    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.
    • CommentAuthorGreenfish
    • CommentTimeOct 10th 2017 edited
     
    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.
    •  
      CommentAuthordjh
    • CommentTimeOct 10th 2017
     
    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?
    • CommentAuthorcjard
    • CommentTimeOct 11th 2017 edited
     
    Cheapest way to verify on site:

    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
    • CommentAuthorGreenfish
    • CommentTimeOct 11th 2017
     
    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.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 11th 2017 edited
     
    Was a miserable summer this year.
    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.
      Culdrose.jpg
      Westhall.jpg
    • CommentAuthordelprado
    • CommentTimeOct 11th 2017
     
    Very useful thread, thanks for starting.

    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
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 11th 2017
     
    A little bit of analysis and it seems that during this period, Suffolk had higher RH, with it skewed to the top end. It was warmer though.
    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.
    • CommentAuthorborpin
    • CommentTimeOct 11th 2017
     
    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.
    •  
      CommentAuthordjh
    • CommentTimeOct 11th 2017
     
    Westhall is another place near the coast; Wattisham would be a better sample. Still higher than I thought though.
    •  
      CommentAuthorSteamyTea
    • CommentTimeOct 11th 2017
     
    Posted By: djhWesthall is another place near the coast
    Apart 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.
  3.  
    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.
    • CommentAuthorCerisy
    • CommentTimeOct 11th 2017
     
    delprado - we have electric towel rads in both bathrooms and use them occasionally in the winter when we can't put the towels outside to dry. They have a 2hr button and are only 500w so it isn't too extravagant!

    Now the electric underfloor heating mats are a little extravagant, but we never said we were going hair-shirt!!
  4.  
    Agreed, the electric underfloor matts are very nice.
    • CommentAuthorGreenfish
    • CommentTimeOct 12th 2017
     
    MVHR 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.

    I do have towel rails on the UFH, mostly used in summer as a heat soak for the solar thermal to avoid stagnation.
    •  
      CommentAuthordjh
    • CommentTimeOct 12th 2017
     
    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.
    • CommentAuthorGreenfish
    • CommentTimeOct 12th 2017
     
    Our Sheila Maid is in the "plant room" (walk in cupboard with thermal store, UFH manifold + pumps, MVHR unit, and the expansion vessels for borehole, solar thermal and ASHP - pipes everywhere!) the warmest space in the house. A Sheila Maid is an excellent drying device for all those raining days when things have to have washed.
    • CommentAuthorrevor
    • CommentTimeOct 12th 2017
     
    Our en-suite mirror will mist up if don't switch it on and if the en-suite door is closed so we have the door to the bedroom open. When on boost the extract needs some extra incoming air so where can it come from?. Gap under the door does not allow enough air in. Think if I was doing again would have an inlet into the bathrooms.
    • CommentAuthorcjard
    • CommentTimeOct 15th 2017
     
    Take the door off, plane a few millimetres off the base, and put it back on
    • CommentAuthorrevor
    • CommentTimeOct 15th 2017
     
    Need more than a few mm. Leave the door ajar and see how far the extract pulls the door naturally.
  5.  
    VG airtightness, UFH, MVHR

    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.
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