Breathable roof underlay?

Phil4432

So in our 1930's semi, the dirt in the attic is unbelievable.  We'd like to be able to store things up there, without them getting bombarded by a tornado of dirt.
I understand that the only complete solution would be to have the insulation moved to the actual roof, as opposed to where it is now, which is the floor of the loft.  That however, would be very expensive.

I know its not a complete solution, but perhaps a breathable membrane would help?  Question is, how breathable is it?  Being a 30's semi with modern double glazing, its prone to the usual mold issues and we have to diligently vent and run a dehumidifier to manage this.  The roof is prone to condensation as can be seen on the inside of the roof, but nothing excessive.

A membrane such as this;
https://www.roofinglines.co.uk/product/klober-permo-ecovent-breathable-roofing-underlay-50m

Any thoughts, comments much appreciated. 

grumbler

Do you mean re-laying the tiles and adding the membrane under them?

Don't know about Klober membrane, but Dupont Tyvek one is very breathable (and waterproof at the same time).

Ganga

Board out the loft and cover the stuff you want to store ,much cheaper option.

Phil4432

Ganga said:

Board out the loft and cover the stuff you want to store ,much cheaper option.

I've considered that, the problem would that would be ventilation. 

Phil4432

grumbler said:

Do you mean re-laying the tiles and adding the membrane under them?

Don't know about Klober membrane, but Dupont Tyvek one is very breathable (and waterproof at the same time).

Would you know how breathable in hot weather?  Reason I ask; if you take Gore Tex its only breathable when its hot on the inside of a garment, and cold on the outside.  In warm/hot weather, it stops breathing.

grumbler

It's sort of fleecy and you can see though it.

I don't see why the breatheability can depend on the temperature.

Don't know about Gore Tex, but what you say makes no physical sense to me. I think all these materials are porous and water-repellent.

Phil4432

grumbler said:

It's sort of fleecy and you can see though it.

I don't see why the breatheability can depend on the temperature.

Don't know about Gore Tex, but what you say makes no physical sense to me. I think all these materials are porous and water-repellent.

I'm a keen hiker, Gore Tex doesn't breathe that well in warmer conditions but a similar material called e-vent is a bit better .  So some breathe better than others, in certain conditions.  Was wondering if underlay for roofs is that same.  May give the manufacturer a call and ask them.

theonlywayisup

As Grumbler, I would go with Tyvek. Tyvek Supro.  Dupont do a range of membranes.  I'm not entirely sure where you are trying to apply it, membranes usually go on the top of the truss/rafter under the batten (airflow) and then the slate as the pic in this link.

https://www.dupont.co.uk/products/tyvek-supro.html

TELLIT01

I think the OP is looking for a solution which can be attached to the underside of the rafters rather than doing a total reroof.

Chickereeeee

I asked a similar question a few months ago (with zero replies) but for my garage, which has the same problem. (The 1920s house has non-breathable  underfelt on its - original - roof).  
I would suggest addressing the damp problem as a separate issue, with e.g. soffit or other roof ventilators. The new membrane (which is supposed to be breathable, but i wonder how much it is after a few years of dirt) should go under  the tiles/slates, but would give some protection from dirt if installed under the rafters, although it will build up over time. I also considered the spray-on foam insulation you can have installed on the underside of the roof. I imagine it would work, but look into it carefully first.
(Nb, Gortex works by allowing water vapour through, but not water in its liquid state. Generally this means water migrates from the warmer to the cooler side)

FreeBear

Chickereeeee said: I also considered the spray-on foam insulation you can have installed on the underside of the roof.

Spraying foam over the underside of a roof is possibly the worst thing you can do. Aside from the fire risk, the stuff traps moisture in the roof timbers which eventually leads to rot. When you come to replacing the roof, most of the tiles will get broken during removal so can not be reused. The foam gives of toxic fumes as it cures, and again if/when it burns.

There are also a growing number of mortgage companies refusing to lend on properties that have had spray foam put in. Surveyors are also wising up to the problem and are starting to down value their valuations.