Condensation on roof felt

[fattony]

hi

Putting the Christmas decorations away, I was surprised to see so much condensation in the loft, to a point where the roof trusses were visibly wet where they are against the felt.

The loft is insulated and I'm guessing the response will be to ventilate the loft so warm air goes up into the loft, but then what would be the point in insulating it.

Appreciate any advise as I'm sure it won't do the roof trusses any good over time

Thanks

 

[Burnerman]

There should be plenty of insulation above the ceilings up there, but the eaves should also be free to ventilate fresh air......any comment Tony?
John

 

[fattony]

I would need to check if there are vents in the soffits, I'm guessing that's what's required?

 

[Burnerman]

Sure - I live in one of the coldest places in UK.....or so it seems!
In my loft I have 400mm of glass wool above the ceilings but I keep the roof under felt completely clear and the same goes for the soffits which are ventilated. The cold water tanks are well lagged too, and I have no condensation issues. Bathroom and kitchen ventilators are ducted to the outside, of course.
John

 

[KenGMac]

fattony, good evening.

Suggest you have a look at the "Pinned posts" on this forum in Stairs and lofts.

In general terms.

1/. The Insulation in the loft will keep the heat in the habitable parts of the property.
2/. General living in the property will produce massive volumes of so called " Free Water"
3/. Free Water is produced by boiling the Potatoes, Pasta, Showers, Baths, Etc, Etc, and as an aside you will be "Expiring" water vapour into the air in the Property.

So where are we?

We have a nice warm insulated property, BUT?

There is generally a BUT somewhere?

In this scenario the BUT is that in the loft, the HEAT! cannot get into the loft, because of the Insulation, [here it comes] BUT moisture vapour [as above] can and will percolate into the loft space, there being NO Vapour Barrier such as a layer of Plastic or some such between the top side of the Plasterboard and the cold loft space.

What then occurs is that all the Moisture vapour from simply living and cooking Etc. gets into the loft area, BUT/ because there is no heat up there the Vapor WILL CONDENSE ! the insulation and Plaster board or lahth and plaster cannot stop the Water Vapour getting into the loft space.

With the lack of heat in the loft Space, --- Condensation --- MUST occur!

Remedy? ventilate the loft space, if you have sufficient Insulation in the Loft, the more ventilation in that space will stop dead your Condensation problem.

Hope this gives you some assistance??

Ken.

 

[fattony]

Very informative post Ken, thanks

I havent checked but I suspect I dont have vents in the soffits, should I cut some grills in, something like below?

http://www.naplesuk.com/vent-soffit-grille-with-fly-screen-round-100mm.html

Thanks

 

[jackthemac]

hi



The loft is insulated and I'm guessing the response will be to ventilate the loft so warm air goes up into the loft, but then what would be the point in insulating it.



Thanks

Your loft isn't insulated only the ceiling is, the loft supposed to have air circulation. Look up cold and hot roof construction and you will see the differences.

You need better vents in the roof space, this wont affect your heat loss as its above the level of the insulation.

 

[fattony]

Valid point, just trying to find a vent which I can cut in relatively easily, don't fancy having a jigsaw up there, any suggestions?

Edit, Ive seen these

https://www.amazon.co.uk/SOFFIT-PUSH-AIRFLOW-VENTILATOR-GRILLE/dp/B006ZLKEEK

Any good?

 

[jackthemac]

yes that will be fine, make sure not drill where a rafter is, make sure your glass wool isn't blocking the eaves it should be pulled back a bit from the sides. You can also get the plastic tile vents that match your existing tiles. hth

 

[noseall]

Poorly fitted loft hatches can have some bearing on moisture migration, as well as.....

Non-breathable roof membrane.

Lack of additional venting i.e. overfascia vents, soffit vents, gable air bricks etc.

Excessive moisture in habitable zone i.e. drying of clothes on radiators, zero extraction when cooking or bathing, high occupancy numbers.

Zero VCL at loft ceiling level i.e. zero duplex plaster boards, zero polythene membrane etc.

 

[fattony]

I had a quick look on my way out this morning, seems we have some vents in the soffits, I need to check they are clear and go from there.

If they are clear what can I do, still add more?

We have 2 loft hatches, 1 timber one and 1 insulated one, presume it's fibreglass with a plastic coating.

Thanks

 

[fattony]

Sorry I should have said my boiler is in the loft would that contribute to the issue?

Thanks

 

[noseall]

Sorry I should have said my boiler is in the loft would that contribute to the issue?

Thanks

No.