# Condensation in attic



## ALEXA (12 Nov 2011)

The floor of my attic is insulated between the joists with an added layer running perpendicular to this. The trap door is also insulated. The front of the house faces south so naturally the back of the roof space is colder. It is only a problem when we have frost at night followed by sunny days as we had recently. Heat is building up on the front slope of the roof and this is causing moisture on the underside of the bitumus membrane on the cold, north facing slope of the roof. When I checked the attic during the frosty period the membrane was quite wet and there were a few drips on top of the insulation. There are 2 ridge vents in the roof and they seem to have helped the membrane to dry out but I would appreciate suggestions as how this can be prevented in the first place as I am concerned about the moisture rotting timbers and affecting the performance of the insulation. Are extra tile vents or possibly insulating between the rafters the answer and how would be the easiest way to do this? The soffit is concrete so unfortunately air cannot get through there. As I say, this problem only seems to occur on frosty/sunny days but is something I would like sorted.


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## onq (12 Nov 2011)

ALEXA said:


> The floor of my attic is insulated between the joists with an added layer running perpendicular to this. The trap door is also insulated.


Can you confirm what they are insulated with and the thickness?


> The front of the house faces south so naturally the back of the roof space is colder.


That doesn't necessarily follow - the back roof surface may be colder.


> It is only a problem when we have frost at night followed by sunny days as we had recently.


Normally the problems with condensation seem  to arise with warm sunny days being followed by night frost.


> Heat is building up on the front slope of the roof and this is causing moisture on the underside of the bitumus membrane on the cold, north facing slope of the roof.


Heat on the outer surface of a south facing roof is unlikely to cause moisture to build up in and of itself - it may warm up the air in the attic so it absorbs more moisture.
Moisture build up in attics usually comes from within the house or the attic itself, particularly if there are down lighters penetrating the ceiling or vents from showers that aren't properly connected to a vent in the roof. I've inspected attics where the shower vents open directly to the attic.


> When I checked the attic during the frosty period the membrane was quite wet and there were a few drips on top of the insulation.


The points to moisture laden air not being properly vented from the attic and/or there not being proper vapour-checking from the habitable rooms below plus the decreased temperature in the attic due to the insulation.


> There are 2 ridge vents in the roof and they seem to have helped the membrane to dry out but I would appreciate suggestions as how this can be prevented in the first place as I am concerned about the moisture rotting timbers and affecting the performance of the insulation. Are extra tile vents or possibly insulating between the rafters the answer and how would be the easiest way to do this? The soffit is concrete so unfortunately air cannot get through there. As I say, this problem only seems to occur on frosty/sunny days but is something I would like sorted.


Are you suggesting the soffite of the eaves is concrete?
If there are no eaves vents then it may be possible that the attic is not ventilated in a compliant manner.
It may be possible to install low level roof vents close to the eaves level, but there are several other factors to consider besides venting as noted above.


ONQ

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 All advice on AAM is remote from the situation and cannot be relied      upon                                   as a defence or support - in and      of     itself  -         should       legal        action    be           taken.
 Competent legal and building professionals should be asked to advise in                                        Real Life with rights to inspect    and      issue         reports    on     the         matters    at          hand.


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## lowCO2design (13 Nov 2011)

ALEXA said:


> Are extra tile vents or possibly insulating between the rafters the answer and how would be the easiest way to do this? The soffit is concrete so unfortunately air cannot get through there. As I say, this problem only seems to occur on frosty/sunny days but is something I would like sorted.


your two vents are not adequate, install more vents, preferably close to the eaves to create air-flow in the attic space

insulating between the rafters is not the answer


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## ALEXA (13 Nov 2011)

The attic is insulated with 200mm Space Blanket between the joists and 170mm Knauf insulation rolled out over this. At first I thought the condensation was coming up flom below but I don't think this is the case. I only noticed it for the first time last winter when we had all the snow on the roof and recently when we had a few nights of frost followed by sunny days when the fromf of the roof heated up. I could feel the heat on the underside of the membrane so this is why I think this is the cause. We have a water tank in the attic too so could the water in it be heating due to the sun and causing vapour? I think perhaps the tank could have a lid fitted and insulated - it is already insulated around the outside but not underneath. When I say the soffits are concrete or something similar I mean they are solid and have no vents and it would be impossible to install vents in the soffit so I think extra tile vents will need to be fitted in the slope of the roof. Would it be sufficient to install 2 in the back or would they be needed in the front also? We only insulated the attic a few years ago and did not have this problem with the uninsulated attic. However the house is much warmer with the insulation  -just a pity it has given us a new problem.


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## lowCO2design (13 Nov 2011)

ALEXA said:


> The attic is insulated with 200mm Space Blanket between the joists and 170mm Knauf insulation rolled out over this. At first I thought the condensation was coming up flom below but I don't think this is the case.


do you have an airtightness/Vapour barrier on the ceiling? if not then you have moisture rising up from your house (from cooking, drying clothes, and heating our climates generally high RH, oh! and breathing)
so yes a certain amount of this issue is from miosture in your home froming condensation when it hits a cold surface ie your roof felt


> I only noticed it for the first time last winter when we had all the snow on the roof and recently when we had a few nights of frost followed by sunny days when the fromf of the roof heated up. I could feel the heat on the underside of the membrane so this is why I think this is the cause.


 yes this works both ways, warm in summer, cold in winter but regardless its ventilation you need


> We have a water tank in the attic too so could the water in it be heating due to the sun and causing vapour? I think perhaps the tank could have a lid fitted and insulated - it is already insulated around the outside but not underneath.


 yes cover tank and then insulate, with a rigid PIR type product (kingspan or similar approved) + pipework


> When I say the soffits are concrete or something similar I mean they are solid and have no vents and it would be impossible to install vents in the soffit so I think extra tile vents will need to be fitted in the slope of the roof. Would it be sufficient to install 2 in the back or would they be needed in the front also?


the B'regs call for 10mm continuous ventilation around the eaves, so IMHO (without a visual) you should be installing more than two vents..


> We only insulated the attic a few years ago and did not have this problem with the uninsulated attic. However the house is much warmer with the insulation  -just a pity it has given us a new problem.


while the insulation reduced the heat escaping into the attic. this is not your problem. your problem is a lack of ventilation 

Alex, when was your home built?


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## bstop (13 Nov 2011)

when the roof inner surfaces are colder than the external air, condensation will be formed from this warmer external air entering your attic. This will occur and ventilation will not prevent this but will in fact assist it. There is no way to prevent this type of condensation and it occurs in all ventilated attics. It only occurs when low temperatures are followed by warmer weather as you describe. This condensation will dry when the temperature of the roof rises and will not cause any major problems. My advice is not to worry about it.


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## onq (14 Nov 2011)

+1 what lowCO2design has posted and I comment as follows -



ALEXA said:


> I could feel the heat on the underside of the membrane so this is why I think this is the cause.


Warm moist sir generated during the day lodging in an attic space condenses on coldest roof surface as the structure cools.


> We have a water tank in the attic too so could the water in it be heating due to the sun and causing vapour?


No, not unless you have the tank situated directly under a rooflight where the sun could get at it.
Temperature increases due to the attic generally warming up could certainly contribute to the rate of evaporation from the tank.


> I think perhaps the tank could have a lid fitted and insulated - it is already insulated around the outside but not underneath.


An insulated cover and sides will assist.
The cover will help prevent evaporation into the attic
No insulation below the tank will keep the water from freezing.
The side insulation should run into the "floor" insulation - no gap.


> When I say the soffits are concrete or something similar I mean they are solid and have no vents and it would be impossible to install vents in the soffit so I think extra tile vents will need to be fitted in the slope of the roof.


You could possibly install vents in the roof slope just above the level of the quilted insulation (so they are not blocked).
However you need to get a calculation done to ensure the venting is adequate.
You can drill through most normal construction materials.
Someone should inspect before you do any work.


> Would it be sufficient to install 2 in the back or would they be needed in the front also? We only insulated the attic a few years ago and did not have this problem with the uninsulated attic. However the house is much warmer with the insulation  -just a pity it has given us a new problem.


You seem to have super insulated the attic without installing a vapour check to reduce moisture migrating through the insulation into the space.
The effect of this has been exacerbated by the relative lack of ventilation overall and in particular the lack of low-and-high level vent arrangement that promotes airflow to carry away moisture from surfaces.


ONQ

 [broken link removed]

 All advice on AAM is remote from the situation and cannot be relied       upon                                   as a defence or support - in  and      of     itself  -         should       legal        action    be            taken.
 Competent legal and building professionals should be asked to advise in                                         Real Life with rights to inspect     and      issue         reports    on     the         matters    at           hand.


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## onq (14 Nov 2011)

To summarize - 

The three principal ways of reducing condensation are -

- Heat the surface itself (like the rear windscreen of car - south facing roof effect)
- Prevent moisture from reaching it (vapour check the surface of spaces below)
- Ventilate to help moisture evaporate (vent the space itself)

Here is some guidance from the Department of the Environment

Technical Guidance Document for Part F 2009 

See Page 28, Diagram 11


ONQ

 [broken link removed]

 All advice on AAM is remote from the situation and cannot be relied        upon                                   as a defence or support - in   and      of     itself  -         should       legal        action    be             taken.
 Competent legal and building professionals should be asked to advise in                                          Real Life with rights to  inspect     and      issue         reports    on     the         matters     at           hand.


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## ALEXA (14 Nov 2011)

Thanks guys for all the advice. The general consensus seems that I need more vents installed which I will get done asap. I will also cover and insulaie the water tank and hopefully these measures will reduce the problem.

The space blanket has a moisture barrier on the underside but maybe it is not sufficient. The house was built around 1940s and is an ex-Dublin corporation house that we have extended. The walls are made of poured concrete and the soffits are solid with no vents. When the houses were originally built insulation in the attic was probably noy of great importance!

 I would not worry about the condensation if I thought it wouldn't do any damage to the roof timbers but would feel batter to get some improvement done.


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## onq (14 Nov 2011)

Do get someone competent to look at the situation before you act, Alexa.
Consider letting us know how you get on.


ONQ

 [broken link removed]

 All advice on AAM is remote from the situation and cannot be relied         upon                                   as a defence or support - in    and      of     itself  -         should       legal        action     be             taken.
 Competent legal and building professionals should be asked to advise in                                           Real Life with rights to   inspect     and      issue         reports    on     the         matters      at           hand.


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## roger diesel (12 Dec 2011)

Hi all, first time poster here with a similar issue. i live in an end of terrace 1940s ex-corpo house also and am considering insulating my attic as the bedrooms get cold very quickly i've noticed. the attic is quite small, only 1.75m to underside of ridge beam, and is hipped, but i'd like to use the space for storage nevertheless, rather than leaving it cold as present. There is no roofing felt, only the roof tiles (concrete?) on battens laid directly onto the rafters (175mm deep). some very old insulation has been laid flat between the joists and is probably ineffective. My intention was to insulate between the rafters with rigid board leaving a gap of 50mm between the insulation and tiles, so as not to encroach on an already tight space. My worry here is that the now warm attic will cause condensation to form at the rafters possibly, via a cold bridge - should i install a thin layer of insulation onto the face of the rafters to prevent this or do i need to install a vapour check at some point in the build up?
I've been reading a few sites including the insulationireland.net site which surprisongly seems to differ completely from what i assumed was correct practice in that it calls for builders paper right up the underside of the tile (i.e. negating the ventilation cavity) and filling with insulation completely... i can't post a link to that as not had 15 posts yet... any help would be greatly appreciated!


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