Condensation in loft

Geegee

I went into the loft today and noticed alot of condensation droplets on the roof felt. The loft was noticably cold and is well insulated (two layers criss-cross) - is this normal? I was in the loft for about an hour yesterday with the loft access removed - would the warm air entering it during that time have been sufficient to cause such condensation?

I went up today to ensure that the area underneath the water tank was not insulated to allow some heat up to prevent it from freezing...it was insulated! I have pulled back all of the insulation under the tank - was this the correct thing to do?

Victor

This is the new house?

You will get a lot of moisture as the house dries out. Talk to rooferpete, but I think that large amounts of condensation would be unusual. Can you check to make sure there are vents to allow the attic to ventilate. Is there a vapour barrier in the ceiling (foil backed plasterboard, plastic sheeting, etc.)?

Is the tank fully insulated otherwise?

rooferPete

Hi,

When you refer to "well insulated" I take you mean the amount of insulation say 2 > 300 mm in thickness ?

Victor has covered many of the items to look for such as vapour barrier or foil backed plaster slabs.

Where I would almost agree fully with him is a new house drying out because there must a lot of moisture within the structure.

What should prevent the condensation forming on the under felt is proper ventilation at the eaves and in the roof space allowing a through flow of air.

If the attic is properly ventilated and has no mistakes like vents from ensuites venting into it the temperature at both sides of the felt should be the same, that would avoid condensation from forming on the inside.

Also the hatch door should be ventilated and if needed a foam seal around the sides as that would prevent the heat from inside the house entering the roof space.

This is a good time to check these items because of the cold weather, condensation left uncorrected could cause your roof timbers to start weakening due to the water soaking into the timbers.

The sign of a well ventilated attic is it should be as cold inside as it is outside.

The sign of well insulated is you can feel the heat difference when you go back into the living area.

.

JohnBoy

rooferPete wrote:

Hi,

Also the hatch door should be ventilated and if needed a foam seal around the sides as that would prevent the heat from inside the house entering the roof space.


.

is that not a contradiction? it should be vented, but sealed? I'm confused

Victor

rooferPete wrote:

Also the hatch door should be ventilated and if needed a foam seal around the sides as that would prevent the heat from inside the house entering the roof space.

I think this word is meant to be "insulated". We'll forgive Pete,seeing as he was up "working" until 3am.

The temperature inside and outside the roof won't be exactly the same, especially given wind and solar factors, but they should be very close and the attic should be closer to the temperatur outside than the temperature of the rest of the house.

The best time to check would be early on a dull, calm day with the heating on in the rest of the house.

JamesM

Just on the slight chance that your heating system is not a sealed system - do you have a header tank in the attic - a small tank for the heating system ?
If you have and you were recently trying to get that radiator to heat by turning up the heating temperature - you could get hot water in the header tank and this would cause condensation. Just a maybe
Jim.

Eurorunner

JamesM wrote:

Just on the slight chance that your heating system is not a sealed system - do you have a header tank in the attic - a small tank for the heating system ?

I don't mean to interupt the flow of this thread but one quick question;
I have what you describe above ie. large tank/small tank. Neither of them are covered/sealed (Nor are they wrapped with lagging/insulation). Should they be?

Victor

Eurorunner wrote:

I don't mean to interupt the flow of this thread but one quick question;
I have what you describe above ie. large tank/small tank. Neither of them are covered/sealed (Nor are they wrapped with lagging/insulation). Should they be?

Yes they should be insulated. However, some attics are insulated between the rafters, making it less necessary.

Unless you fancy the whole tankful of water coming through your ceiling some cold night.

You should insulate the tanks and pipes on all sides, except the bottom of the tank.

Geegee

Is there a vapour barrier in the ceiling (foil backed plasterboard, plastic sheeting, etc.)?

No, it just seems to be ordinary plasterboard.

Is the tank fully insulated otherwise?

Around the sides, yes but not on the lid. Should I have pulled back the loft florr insulation underneath it?

Just on the slight chance that your heating system is not a sealed system - do you have a header tank in the attic - a small tank for the heating system ?

There is a small cylinder type tank in the loft directly above where the HW cylinder is.

JamesM

Geegee wrote:

There is a small cylinder type tank in the loft directly above where the HW cylinder is.

If it is a red sealed cylinder, it is an expansion vessel - so you have a sealed system. If it is an open tank, a smaller version of your main tank, with a ballcock, it is a header tank - and you have an open system. It is only an open system that can cause condensation. If you have a sealed system, then the condensation has nothing to do with the heating system.
Jim.

rooferPete

Hi John Boy,

Of course you are correct, spelling errors are almost forgiveable but a complete word ?

Thanks to Victor for correcting my error

With regards to insulating the storage tanks in the attic I recommend you make sure the expansion pipe that usually overhangs the tank can overflow into the tank when necessary.

If it is blocked or a lid put on the tank preventing the water flowing into the tank it can lead to serious flooding should the thermostat on the immersion fail.

.

Geegee

Should all first floor ceiling plasterboard have a vapour barrier within it to comply with building regs? Is there a way of identifying it?

Victor

Geegee wrote:

Should all first floor ceiling plasterboard have a vapour barrier within it to comply with building regs? Is there a way of identifying it?

You should normally have a vapour barrier where you have insulation.

This is because warm air in a house contains a lot of moisture from kitchens, bathrooms and people. Because of the sharp drop in temperature from one side of the insulation to the other, there is a strong risk of condensation on the cold side of the insulation. With (especially) fibre type insulations, the risk is that the condensation is within the insulation itself, thereby creating a thermal bridge out of the insulation. By placing the vapour barrier on the warm side of the insulation, you prevent the moisture laden air getting to the insualtion. Vapour barriers need not be absolute, but should be substabtial.

Geegee

By placing the vapour barrier on the warm side of the insulation, you prevent the moisture laden air getting to the insualtion. Vapour barriers need not be absolute, but should be substabtial.

So this would be directly above the plasterboard and underneath the insulation? Is a vapour barrier at the builders discretion or a building regulation?

Victor

Geegee wrote:

So this would be directly above the plasterboard and underneath the insulation?

It is usually part of the plasterboard, but could be separate. Alternatively you could have foil backed insulation. On insulation it looks like this http://www.moyisover.ie/duct_wrapsh96.gif The foil is not unlike the foil used in packing chocolate (more liek the plastic/metal ones than the pater / metal ones).

Geegee wrote:

Is a vapour barrier at the builders discretion or a building regulation?

It is not discretionary, although I would be hard pressed to find it in the Regulations.

patrido

Victor wrote:

It is usually part of the plasterboard, but could be separate. Alternatively you could have foil backed insulation. On insulation it looks like this http://www.moyisover.ie/duct_wrapsh96.gif The foil is not unlike the foil used in packing chocolate (more liek the plastic/metal ones than the pater / metal ones).

It is not discretionary, although I would be hard pressed to find it in the Regulations.

it's dealt with in the ventilation section of the regs, not the insulation section.

on my interpretation, it is not specifically required, but it is recommended where adequate ventilation cannot be guaranteed on the cold side. this might be fine with products like rock/glass wool where the vapour can permeate easily, but i'm not sure about closed cell products like xtratherm or kingspan, and it's possible that the regs were not written with these in mind.

interestingly, xtratherm recommend that a vapour layer is used even though their products are foil backed. even when using xt/tl to insulate underneath rafters (in addition to insulation between rafters), which has foil between the plasterboard and the attached insulation, you are supposed to use a vapour layer underneath the rafters.

kingspan on the otherhand claim that their product does not need an additional vapour layer.

i have contacted both companies to clarify, and will post back their replies.

YAPP

This prob had 5 of us baffled one winter,
drops on condensation on one side of the roof and none on the other......

turned out that it was due to the attic hatch being open while the house was being skimmed and the excess moisture was rising and meeting the underside of the felt/membrane.

The airflow was not constant in the roof space due to the trap door not being fitted/closed....! Roofing Membrane Manfacturer looked into this for us but had heard of this before from other customers.....

We found that the house next door (being skimmed too) had the hatch fitted and there was not condensation in the roof at all, and both insulated to same spec...!