new roof and membrane tomorrow, just looked, purlin supoort botched and signs of mould.

[^woody^]

It seems like you are thinking of doing a loft conversion without the necessary building regulation approval? And also you probablly need to read up on how condensation forms in lofts and cold spaces, as you will most likely make things much worse by your proposals.

As a guide, once any air hits a cold felt or membrane surface, it will condense, and will need to be removed by ventilation. Not alll breathable felt is breathable enough.

As to the existing problem, I would suggest that the condensation you are experiencing now is not because the insulation is preventing the house from "breathing", but that the insufficient insualtion is allowing more warm air into the loft to condense in the first place. A loft needs to be cold and it needs to be ventilated. And for that reason, the plan to insulate a loft at rafter level is a very bad idea.

On the plus side, the purlin supports are OK

 

[mart321]

It seems like you are thinking of doing a loft conversion without the necessary building regulation approval? And also you probablly need to read up on how condensation forms in lofts and cold spaces, as you will most likely make things much worse by your proposals.

As a guide, once any air hits a cold felt or membrane surface, it will condense, and will need to be removed by ventilation. Not alll breathable felt is breathable enough.

As to the existing problem, I would suggest that the condensation you are experiencing now is not because the insulation is preventing the house from "breathing", but that the insufficient insualtion is allowing more warm air into the loft to condense in the first place. A loft needs to be cold and it needs to be ventilated. And for that reason, the plan to insulate a loft at rafter level is a very bad idea.

On the plus side, the purlin supports are OK

Thanks for the reply, I wouldnt do something Iike this without planning permission, I will probably be getting a tradesman In to do the work aswell due to time constraints.
You say insulting a loft at rafter level is a bad idea? Isn't this a very common thing to have done? .
The roofer says ventilation should be fine with the new membrane, says he could put some vent tiles in but there isn't any need. . He said using Kingspan and insulating fron the rafters is a good idea?

 

[^woody^]

Isn't this a very common thing to have done?

The insulation should be around a heated room, not a cold space. If the loft is being converted, and therefore heated, then insualtion at rafter level is fine. If not and it is to remain a cold space, then the insualtion should be a ceiling level to keep the roof void cold.

If you are converting the loft, then you dont want vent tiles. Rather you need ventilation from eaves to ridge. Or you have the roof designed to not need ventilation at all. In fact roof vents are rarely installed as they should be, in the places they should be and in suitable numbers. Many times, a complete waste of time and money.

It's been found that breathable membranes are not performing as they should. So there are specific types for high or low vapour permeability, and in most cases, the roofer will just get whatever is cheapest at the merchants - which wont be the higher breathable type.

Above all though, if you are converting the loft then it should be designed first as you may need to upgrade the rafters or other supports before installing insulation, and selecting the membrane. And the amount (if any) of ventilation required, needs to be thought about too before just installing any old insualtion.

Basically, plan ahead now, dont just get any old thing done without considering what might be needed later

 

[datarebal]

Over fascia vent and ridge vent will be fine. There is not an issue insulating between the rafters properly, if you stick to the rules.
Your loft space simply becomes a heated room , your heating bill might rise, boiler working harder... enjoy your train set.

 

[^woody^]

Your loft space simply becomes a heated room , your heating bill might rise

It doesn't work like that. It's not just a case of increased heating bills, but one of increased condensation risk. The loft will remain cold and doors not perform anything like a heated room.

 

[datarebal]

of course it works like that. it acts as a bigger room. the idea is keeping the warm moist air away from the cold underside of roof or what ever. more heat will find its way into the loft at the cost of rooms below.
if you have no insulation at ceiling level then the heating will in fact have to work harder.
condensation risk is then higher.
if the op insulates as i said and ventilates as i said then he will be fine.

 

[^woody^]

of course it works like that. it acts as a bigger room. the idea is keeping the warm moist air away from the cold underside of roof or what ever. more heat will find its way into the loft at the cost of rooms below.
if you have no insulation at ceiling level then the heating will in fact have to work harder.
condensation risk is then higher.
if the op insulates as i said and ventilates as i said then he will be fine.

The condensation risk profile is much higher as warmer air, containing more moisture is being allowed into the cold loft area, where it cools and then releases it's moisture. That can't be stopped unless the loft is kept at higher temperatures - which the OP can't do unless he provides heating.

He wont be fine by insulating and ventilating as you say. He will be worse off financially, and at a much greater risk of causing problems related to dampness and timber rot.