Roof repair vs upgrade

[mjward]

Hello

 

I've spent a few months researching this but have now got to the point of exertion and feel I know less than when I started!

Short version: 100 year old house that had a loft conversion 25 years ago that added a non permeable membrane to just one side of the roof (the side where velux windows were also added). Fast forward to today, I've been renovating it over the past 2 years and the combination of modern plaster (the lime had failed) and adding UPVC windows (albeit with trickle vents) has caused a condensation issue that became noticeable in the loft (mould, 75% humidity) but not that bad elsewhere (1st floor 60-65% and ground floor 50-55%). A dehumidifier has a temporary control of that particularly issue but now with roof work required I'm thinking it's time for a better solution. I will add that there is no door between the 1st floor and the loft (not required at the time of conversion) and that there is no insulation other than the thinnest of sheets of polystyrene. Although I have now added 75mm of PIR to all the first floor ceilings.

 

From a basic roof work perspective, I have a few cracked/dislodged slates to repair and all the timber cappings are rotten and need to be replaced. Given one half of the roof as no membrane at all I also think at a minimum get all the slates lifted on that side and have one installed.

 

From an upgrade perspective, in terms of cost vs benefit, which of the following make sense to do at the same time (i.e. whilst the scaffolding cost is already factored in with the above)?

 

1. Remove both sides of the roof and replace the non-permeable membrane with a breathable one (although given the other side at a minimum will have one, do they both need to be etc?)

2. Fit insulation between the rafters (the rafters are very shallow, can't be more than 10cm in depth and given the need for the membrane to sag and have airflow I'm guessing absolute max would be 50mm of insulation. Realistically, nor could more be added below the rafters as it's already cramped in there. Given it's a conservation area I'm guessing can rule out adding it above the rafters as would change the profile/height)

3. Replace the Velux windows with modern versions (with trickle vents)

4. Anything else recommened to improve that loft space from both a heating & ventilation perspective?

 

Sorry I should also add there is no heating in the loft which obviously doesn't help the humidity issue combined with the fact the bathroom door is at the foot of the stairs to the loft.

 

 

Thank you for taking the time to read, these things are expensive so I guess I'm just wondering if its a fools errand doing more than the basic roof work given the setup.

[mjward]

Forgot one other bit with regards to insulation:

 

It seems if you're renovating a roof area then building regulations require you to do so with a minimum U-value for the insulation. This was the other bit I struggle with, I can't see how I can possibly get to those low values given the space restrictions i.e. if I don't add insulation it is a simple roof repair job that doesn't require planning permission or building control signoff but when I go down the insulation route both of those are involved.

[joe90]

Frankly how will they know? Do your best and keep stumm. 🥷

[mjward]

23 minutes ago, joe90 said:

Frankly how will they know? Do your best and keep stumm. 🥷

Well I would be lying if that thought hadn't crossed my mind but still need to figure out what elements I should be opting for given the situation. Guessing from a pure U-value perspective, any insulation better than none, but from a cost and general house warmth, would I notice the difference in just 50mm of insulation between the rafters etc

[joe90]

Others here are better at condensation mitigation so hopefully some else will give you guidance.

[Redbeard]

2 hours ago, mjward said:

It seems if you're renovating a roof area then building regulations require you to do so with a minimum U-value for the insulation. This was the other bit I struggle with, I can't see how I can possibly get to those low values given the space restrictions

Unless the 2022 revision changed it, and I don't think it did, insulating from the top, above an existing room-in-the-roof (R-in-R) is deemed to comply despite a higher (lesser) U value than the Part L target (this came in as part of the 2010 Regs). The thinking is that if you only have the rafter depth to deal with then rafter depth minus vent gap must equal insulation depth. Until 2022 ('21?) when the BEIS Best Practice Guidance docs came out this led to a typical set-up of 25mm vent gap plus 50 PIR (estimated U value approx 0.45ish). Now (as the BP doc requires 50mm vent gap) you'd only get 25mm PIR.

 

An interesting quirk is that if you subsequently take the ceilings down in the R-in-R you now have to install a total of 150 PIR and achieve a U value of 0.16!

 

And how do you tell if the 'from above' installation was done well? When you take the ceilings down, if all the insulation is on the floor, it wasn't! (Been there; seen that!)

 

 

[mjward]

On 26/02/2024 at 19:01, Redbeard said:

Unless the 2022 revision changed it, and I don't think it did, insulating from the top, above an existing room-in-the-roof (R-in-R) is deemed to comply despite a higher (lesser) U value than the Part L target (this came in as part of the 2010 Regs). The thinking is that if you only have the rafter depth to deal with then rafter depth minus vent gap must equal insulation depth. Until 2022 ('21?) when the BEIS Best Practice Guidance docs came out this led to a typical set-up of 25mm vent gap plus 50 PIR (estimated U value approx 0.45ish). Now (as the BP doc requires 50mm vent gap) you'd only get 25mm PIR.

 

An interesting quirk is that if you subsequently take the ceilings down in the R-in-R you now have to install a total of 150 PIR and achieve a U value of 0.16!

 

And how do you tell if the 'from above' installation was done well? When you take the ceilings down, if all the insulation is on the floor, it wasn't! (Been there; seen that!)

 

 

Thank you for that, ok so at least I can be technically compliant.

 

Given in practical terms it seems the best I would be able to achieve is 25mm PIR, in the real world now noticeable would it be? Any finger in the air guess at what improvement I might see with regards to energy bills? (it's a large house c400sqm and winter gas is running at £400 a month for 3 people). 

 

The loft is currently split into two bedrooms that are rarely used however the rooms are uncomfortably hot in summer and cold in winter (as expected). 5 south facing velux windows contributing to the above.

 

Aside from a bills/comfort perspective, would adding PIR solidly between the rafters worsen the humidity issue?

 

Thank you for your patience, there is so much info out there but I'm struggling to put it together in real terms 

[scottishjohn]

in answer to yuor question inthe title 

that will depend onthe roofs construction 

Iam guessing that there is no vapour barrier below the slates/tiles 

 and it was constructed as a cold roof with lots of ventialtion 

 

it sounds lke you now have a ppart sealed roof with no ventilation 

 would that be a good sumation of the situation ?

sounds like you need slate vents or eve ventilation above any insulation you have  to allow an air flow between your slates /tiles etc and your sealed off area below 

bottom line is old style cold roofs need to have a proper air flow at underside or you will get problems 

maybe a dry ridgge stsem will work as that will give you some air flow from eves outthrough the ridge ?

your modern vaulted ceiling type roofs 

have osb cladding and house wrap under the sltes .tiles then pir packed tight up to thenm so there is no cold air in there 

so its either totally sealed oor cold roof qwith air flow -anything else will not work  if the sealed sections with dormer have pir in but no gap above it with no venting from eves to ridge you will get stagnation and moisture .

 and no leakage of heat from ceilings below open areas 

so sum up you need airflow from eves to ridhe above any insulation you have 

 is the house constructed with cavity walls ?

if so there should be airflow up the gap from under floor  or brick vetns just above DPC to loft/roof area which rthen goes out at eves 

 

re reading your first post you are saying even downstairs is higher humdiity thaan you would like ,so maybe its a combination of ventilation problems  of whole structure  

If you feel I,m talking nonsense feel free to ignore , lack of heating or ventilation are the usual causes of your problems 

[mjward]

17 hours ago, scottishjohn said:

in answer to yuor question inthe title 

that will depend onthe roofs construction 

Iam guessing that there is no vapour barrier below the slates/tiles 

 and it was constructed as a cold roof with lots of ventialtion 

 

it sounds lke you now have a ppart sealed roof with no ventilation 

 would that be a good sumation of the situation ?

sounds like you need slate vents or eve ventilation above any insulation you have  to allow an air flow between your slates /tiles etc and your sealed off area below 

bottom line is old style cold roofs need to have a proper air flow at underside or you will get problems 

maybe a dry ridgge stsem will work as that will give you some air flow from eves outthrough the ridge ?

your modern vaulted ceiling type roofs 

have osb cladding and house wrap under the sltes .tiles then pir packed tight up to thenm so there is no cold air in there 

so its either totally sealed oor cold roof qwith air flow -anything else will not work  if the sealed sections with dormer have pir in but no gap above it with no venting from eves to ridge you will get stagnation and moisture .

 and no leakage of heat from ceilings below open areas 

so sum up you need airflow from eves to ridhe above any insulation you have 

 is the house constructed with cavity walls ?

if so there should be airflow up the gap from under floor  or brick vetns just above DPC to loft/roof area which rthen goes out at eves 

 

re reading your first post you are saying even downstairs is higher humdiity thaan you would like ,so maybe its a combination of ventilation problems  of whole structure  

If you feel I,m talking nonsense feel free to ignore , lack of heating or ventilation are the usual causes of your problems 

Hi John

 

Thanks for your response. Basically when previous owners did a loft conversion 25 years ago they put in velux windows on one side only, on that one side there is a non-breathable (thick plastic) membrane, nothing on the otherwise but yes, originally constructed as cold roof. Re current ventilation, looking at it from the outside and from the loft, I can't see where ventilation could occur, The loft conversion is full (i.e. plastered walls, velux without trickle vents, carpeted floors etc), can't see where air could escape/enter. I chucked in a few lap vents on the side that does have the membrane but not made much of a difference. The side without a membrane is boxed off at the eaves ie no access. Agree re vents/eve ventilation required although I'm with one roof not having any membrane I would have thought plenty of surface area for ventilation but it's not occuring and can only thing that's because of the loft being boarded up on the sides with plasterboard and no air vents etc. 

 

Glad you mention the ridge system, all this terminology I'm learning as I go but from what I can tell it already is a dry ridge system ie no mortar on the sides, just timber cappings so theoretically the roof is ventilated/breathing but once the humid air gets into the loft space I'm guessing there is no way for it to pass through the plasterboard and out through the roof if that makes sense. 

 

There is currently no PIR in between the rafters, in fact there is nothing between the rafters right now so could it be a case that the plasterboarded walls in the loft conversion need air vents added to allow the air to get to to the eaves and out?

[scottishjohn]

2 hours ago, mjward said:

Hi John

 

Thanks for your response. Basically when previous owners did a loft conversion 25 years ago they put in velux windows on one side only, on that one side there is a non-breathable (thick plastic) membrane, nothing on the otherwise but yes, originally constructed as cold roof. Re current ventilation, looking at it from the outside and from the loft, I can't see where ventilation could occur, The loft conversion is full (i.e. plastered walls, velux without trickle vents, carpeted floors etc), can't see where air could escape/enter. I chucked in a few lap vents on the side that does have the membrane but not made much of a difference. The side without a membrane is boxed off at the eaves ie no access. Agree re vents/eve ventilation required although I'm with one roof not having any membrane I would have thought plenty of surface area for ventilation but it's not occuring and can only thing that's because of the loft being boarded up on the sides with plasterboard and no air vents etc. 

 

Glad you mention the ridge system, all this terminology I'm learning as I go but from what I can tell it already is a dry ridge system ie no mortar on the sides, just timber cappings so theoretically the roof is ventilated/breathing but once the humid air gets into the loft space I'm guessing there is no way for it to pass through the plasterboard and out through the roof if that makes sense. 

 

There is currently no PIR in between the rafters, in fact there is nothing between the rafters right now so could it be a case that the plasterboarded walls in the loft conversion need air vents added to allow the air to get to to the eaves and out?

you do not state if walls are cavity type =2bricks with a gap  bewtween

presbt spec suggests at least 25mm -- some like 50 mm

 also the side with the non breathable plastic sheet --is that on top of OSB or sarking --or is it jusst a sheet and tile battons --as they used to use in english type houses?

If you look at modern brickwork they now use vertical plastic vents in between the bricks every  couple of metre or so  right up the walls and top of walls have a fire stop --so air flow will in ina bottom vents and out at top 

that is to make a draft between the 2 brick walls and the top of the walls should not be covered,as that will stop airflow --eg your loft flooring going over the gap in the walls 

certainly a vented dry ridge system is good but still needs vents at eves to make  an airflow out of the top,as warm air rises.or lots of slate vents on both sides of roof  

 other wise the air will not move very quickly and maybe become stagnant

 a " cold roof" is meant to have an airflow  to stop condensation and  humidity building up 

 all your insulation should be on outside of living area and well sealed as any leakageoof warm air  will cost you in heating and make condensation worse 

 there should have been a vapour barrier behind any plaster board that is ajoing any ouside walls or roof space.

replacing your plaster section on one side with foam backed plasterboard after a vapour barrier should address all your problems without taking up too much space

 if there is no insulation behind the plaster board then heat will escape and possibly cause condensation on the vapour barrier at inside -so hence it should be vapour barrier ,insulation then plaster board --which is why i suggest using foam bacled plaster board to do both jobs 

 are your tilles /slates fitted to counter battons or just one row of battons

one row does not give an air gap between tiles and breathable membrane -- and dirt will gather up on cross battons  and over time can cause problems,usally at 3 corners of roof will be first place to cause rot of trusses -double battons first row is vertical ,so no water or crap gathers on them 

scottish roofs  due to extra wind up here  use either osb sheeting or sarking boards then breathable membrane  and slates are then nailed dicrtly to the boards ,over ime the sarking boards shrink as they dry out and you end up with little gaps whicgh allow some ventilation through the breathable membrane 

 this is still a cold roof so needs ventilation at eves into roof space and out the top ideallyand totally sealed and insulated from living space 

 have search for diagrams on cold roofs and warm roofs

[mjward]

No cavity to the walls (c1906 building)

The side with the plastic sheet has this sheet on top of the rafters (fairly tight ie not much of a sag) then the roof batons and slates on top of this. No OSB.

Definitely think adding a few slate vent tiles will show a decent improvement

Re vapour barrier, the main walls are uninsulated, just plaster onto stonework. In the loft there is plasterboard to box in the eves area but standard plasterboard, no foil backed/vapour barrier on them in the roof. I think you're on the right track, get some PIR in between the rafters (allowing enough of air flow gap) but also take down the old plasterboard and fit foil backed insulated plasterboard to lose a bit of room space but should make big difference re comfort and humidity

From the access I have, hard to sell if slates on counter battons as the ridge board covers the view from the side and the plastic sheet stops me seeing whats happening above it.

I'm in a deep valley in Yorkshire so probably not far off the winds you've got up there

Will do some more digging on cold/warm roofs but you're giving me a few good bits to think about and research

 

Cheers John

[joe90]

This sound typical of a bad loft conversion with no understanding of building practice. If it were me (and it’s what you want to do) I would put PIR between rafters as you suggest with an air gap above then additional PIR under the rafters for the whole loft, this will give you better insulation for the whole space. 

[mjward]

23 hours ago, joe90 said:

This sound typical of a bad loft conversion with no understanding of building practice. If it were me (and it’s what you want to do) I would put PIR between rafters as you suggest with an air gap above then additional PIR under the rafters for the whole loft, this will give you better insulation for the whole space. 

Oh it is but was done at least 20 years ago when the standards were much low but nevertheless even at that time it was done very poorly.

 

My primary concern is ventilation/humidity i.e. given its a converted loft space and plasterboard etc isn't great for breathability, what should I be changing to allow the humid air from inside the house to ventilate in a way that doesn't cause mould etc?

[Mr Punter]

32 minutes ago, mjward said:

what should I be changing to allow the humid air from inside the house to ventilate in a way that doesn't cause mould etc?

 

Extractor fans in bathrooms and shower rooms to expel the moist stale air plus trickle vents / wall vents in bedrooms and living rooms to allow fresh outside air in.

[Iceverge]

The warm moist air from the house is coming up and condensing in the colder roof area. 

 

Put a continuous extracting fan up there pulling air out of the loft space. £50 would buy a reasonably one if you can tolerate eBay. 

 

I put a continuous extract fan in the attic of my uninsulated parents house drawing air from the landing space 24/7. 

 

Although it isn't directly connected to any of the wet rooms it has notable dried out all of the house by ensuring a steady flow of fresh air gets pulled in all the time. 

 

 

 

 

 

 

 

 

Edited March 1 by Iceverge

[joe90]

1 hour ago, mjward said:

what should I be changing to allow the humid air from inside the house to ventilate in a way that doesn't cause mould etc?

 

5 minutes ago, Iceverge said:

The warm moist air from the house is coming up and condensing in the colder roof area. 

So if you insulate the whole loft (warm roof) you have no cold surfaces for the moisture to condense on but still provide a little extraction as it’s the top of the house. In my build along passive lines I had a warm roof and no ventilation and no mould at all up there.

[Iceverge]

19 minutes ago, joe90 said:

In my build along passive lines I had a warm roof and no ventilation and no mould at all up there.

 

 

Correct but you did build your house to a high level of airtighess and install MVHR. 

 

The absolute humidity of the air in the house would have been very well controlled by the MVHR so even if air did ever get to a cold spot it would have been unlikely to have been carrying enough vapour to condense. 

 

The airtighess you did to your attic ,(OSB and foamed joints I think from memory) would have played a very important role.  Think of it like blocking a pipe at just one end. It still stops all flow. Your attic would have had very little air from the house coming up there because it had nowhere to go. 

 

This isn't the case in @mjwards house as a non airtight loft conversion. Best to work with the problem in my opinion and extract the damp air from where it's causing the issue.

 

Think of it like putting a bilge pump in the area where all the water is gathering in a boat with lots of small leaks. 

 

 

 

 

 

 

 

 

[joe90]

51 minutes ago, joe90 said:

no cold surfaces for the moisture to condense on but still provide a little extraction as it’s the top of the house.

My caveat.

 

it also depends on budget, if it’s affordable to make the loft fully insulated and draught proof the whole house will benefit (but still with ventilation especially as….

 

Edited March 1 by joe90

[joe90]

On 26/02/2024 at 16:05, mjward said:

the fact the bathroom door is at the foot of the stairs to the loft.

 

[mjward]

Thanks all, to address the points raised:

 

1) bathroom has a high output & efficient extractor, I recently upgraded it and has certainly helped as well as changing behaviourial issues (making the family leave the bathroom door closed until the fan has done it's job

2) all windows in the house has (open) trickle vents. This has definetely helped with overall humidity in the house, although not a fair test with a dehumidifier running although latest readings are 34% Ground Floor 48% First Floor, 58% Loft. So all absolutely fine but...that's largely due to the powerful Meaco dehumidifier doing a wonderful job. 

3) With regards to the continuous extractor fan, is that between the loft and external (i.e. fitted in the gable walls) or between loft and house below? The reason I ask is that earlier research had brough up a product called a Positive Input Ventilator (I believe Nuaire are the leading brand). From what I could tell this really helps with humidity issues although by default I would ideally like a non-mechanical/electrical fix due to future mainteance and running costs.

4) In terms of cost I look at it from a value perspective as opposed to affordability. My thinking is, I will have the slates lifted to fit/replace the underlay with something breathable i.e. with the sunk cost of that work, scaffolding etc, I'm thinking that the incremental cost of adding insulation will be as low as I'll get it. That said, I fitted the ceiling insulation myself so know the cost/time it takes to do and what I've learnt is that rarely is useful when it comes to judging what I will be quoted! If I can pay X to add insulation and it (1) makes the room much more comfortable throughout the year and (2) reduces my heating costs then on paper it's the way forward but obviously all depends on X and if it makes sense over a 10 year+ horizon.

5) To add to the insulation element, where it becomes complex is where it is applied. At 6'2 I can only stand up in the centre of the loft, move a foot either direction and it's a crouch. That's with 12.5mm plasterboard. Given the 75mm rafters give me limited "between" space for insulation, if I went below rafters (say 150mm PIR each side) it would pretty much render the space useless for me. But if I go above, I enter the unknown from a planning permission perspective given I'm in a conservation area and from what I read they really don't like roof heights being raised (not to mention the additional work to the ridge boards)

 

Not that it's a complicated setup but I put this quick sketch together to easily visualise:

 

[JohnMo]

I will jump in on the ventilation issues. You currently have from the sounds of it, an intermittent high flow fan in the bathroom, you shower or bathe and ensure the door remains closed until the fan stops. You have trickle vents in the windows. Plus you run a dehumidifier. 

 

First have I missed anything?

What other wet rooms (kitchen, utility, toilet, shower rooms etc) do you have and how are they ventilated?

 

What temperature do you keep you house at?

 

Trickle vent as useless on their own at ventilation.

 

Intermittent fans are great at short term moisture clearance issue, but do not really provide a sound ventilation strategy for the whole house. For this you need a continuous supply of fresh air, ideally controlled so you ventilate as much or little as required.

 

19 minutes ago, mjward said:

Positive Input Ventilator

It works but continually blows cold air from the loft or outside or heats it via an electric heater element. Not an ideal solution.

 

Cross ventilation has been shown to work well. For this you need continuous running extract either a central unit (MEV) or room dMEV fans in wet rooms and trickle vent in dry room (bedrooms, living etc). Plus door undercuts so air is free to move when doors are closed. Add to this humidity sensing trickle vents and humidity boosting on the fan. Greenwood CV2GIP dMEV fans are excellent, silent in operation automatically boost on humidity require 2-4W electricity.

 

In addition to the above the house needs to warmer than the air it pulls in from the outside.

[mjward]

26 minutes ago, JohnMo said:

I will jump in on the ventilation issues. You currently have from the sounds of it, an intermittent high flow fan in the bathroom, you shower or bathe and ensure the door remains closed until the fan stops. You have trickle vents in the windows. Plus you run a dehumidifier. 

 

First have I missed anything?

What other wet rooms (kitchen, utility, toilet, shower rooms etc) do you have and how are they ventilated? kitchen on ground floor with AGA ie flue extraction no electrical extractor fan. Will have another 1st floor bathroom but not at that stage of the renovation yet

 

What temperature do you keep you house at? mid-renovation so not all radiators in so currently, the ground floor is between 21-22c, the first floor is on average around 16c (albeit the renovated bathroom has a working rad and is 21c). No radiators in loft, currently 14c

 

Trickle vent as useless on their own at ventilation.

 

Intermittent fans are great at short term moisture clearance issue, but do not really provide a sound ventilation strategy for the whole house. For this you need a continuous supply of fresh air, ideally controlled so you ventilate as much or little as required. I have the overun timer set to 1 hour so with the three of us the bathroom fan is on for a good portion of the day but that's the only mechanical way air is currently transferring internally/externally

 

It works but continually blows cold air from the loft or outside or heats it via an electric heater element. Not an ideal solution. agree, I had heard this was the downside (either cold air or expensive to heat and not very good at the heating part)

 

Cross ventilation has been shown to work well. For this you need continuous running extract either a central unit (MEV) or room dMEV fans in wet rooms and trickle vent in dry room (bedrooms, living etc). Plus door undercuts so air is free to move when doors are closed. Add to this humidity sensing trickle vents and humidity boosting on the fan. Greenwood CV2GIP dMEV fans are excellent, silent in operation automatically boost on humidity require 2-4W electricity. how would this differ from changing the setting on the current bathroom extractor to continuosly run? This is the one I have https://www.vent-axia.com/range/lo-carbon-svara-black

 

In addition to the above the house needs to warmer than the air it pulls in from the outside.

 

Thank you so much for that, I've tried to address the points above.

 

In the interim I'm doing more research on the insulation element, certainly feels like between and over rafters is the best solution when it comes to comfort/heating (although I don't believe it addresses the ventilation issue). I will play around with calculators but I'm guessing a flexible insulation between the 75mm rafters followed by 150mm PIR on top of the rafters will get me in the right U-value ballpark for building control whilst not being shot down by planning permission for excessive roof height increase.

[JohnMo]

1 hour ago, mjward said:

how would this differ from changing the setting on the current bathroom extractor to continuosly run? This is the one I have https://www.vent-axia.com/range/lo-carbon-svara-black

That fan looks ideal also. Set to run continuously, with automatic humidity sensing and you could minimise the overrun timer as the unit would look after itself. However 10l/s isn't enough to ventilate the house.

 

I had a look and the AGA requires a 30cm2 air inlet grill permanently open, is this in the kitchen?

 

Your ventilation strategy will have to take account of the air requirements of the Aga so you don't start to depressurise the house.

[mjward]

16 minutes ago, JohnMo said:

I had a look and the AGA requires a 30cm2 air inlet grill permanently open, is this in the kitchen?

 

 

 

I don't know too much about it as was fitted by previous owners but I know there is a pipe on the Aga that has its own dedicated external inlet as per photo

 

[Iceverge]

5 hours ago, mjward said:

bathroom has a high output & efficient extractor, I recently upgraded it and has certainly helped as well as changing behaviourial issues (making the family leave the bathroom door closed until the fan has done it's job

Say it runs 5 mins on average for 15 times a day at 30l/min. 

 

That's 2250l/day

 

A continuously running dMev fan would at 8l/min would be more like 11520l/day. 

 

That's 5 times the fresh air.

 

Intermittent extractor don't have a good track record of good indoor air quality. 

 

 

5 hours ago, mjward said:

So all absolutely fine but...that's largely due to the powerful Meaco dehumidifier doing a wonderful job. 

 A dehumidifier might be 200w. An order of magnitude greater than a mechanical ventilation fan. EG at least 10 times the running cost. 

 

Also remember whilst it does remove moisture from the air it does nothing for the CO2 and VOCs in the house. 

5 hours ago, mjward said:

With regards to the continuous extractor fan, is that between the loft and external (i.e. fitted in the gable walls) or between loft and house below? The reason I ask is that earlier research had brough up a product called a Positive Input Ventilator (I believe Nuaire are the leading brand). From what I could tell this really helps with humidity issues although by default I would ideally like a non-mechanical/electrical fix due to future mainteance and running costs.

 

All mechanical ventilation system are the same ultimately . They just suck air out or it push in. MVHR is fancy in that it does both and recovers some heat. 

 

PIV works but it has doubled in price the last few years and it creates cold spots under the vent. That Greenwood fan is about 1/3 of the cost and does the same thing. 

 

The gasses in a house tend to diffuse pretty quickly given half a chance. EG an open door for a few minutes. 

 

You've seen that with the loft. It works in reverse too. You can put your ventilation fan in any room that suits and it'll dry the whole house if run continuously.