Soffit Vents - too much ventilation?

[berty3000]

Hi all,

I had my bungalow re-roofed in the spring. It has breathable membrane. However, I noticed when I was up there recently messing about with the insulation that the timbers felt a little damp in places. Not soaked, but just a bit damp. I also noticed a couple of small areas where the membrane had a bit of damp on it and not to do with moisture coming in from the outside. There is plenty of insulation up there but it isn't in contact with the membrane, or not significantly so, meaning I don't think this is the problem.

It is possible that this moisture was caused by me being up there for ten hours with the loft hatch open the entire time, and humidity coming up from inside the house, however, as a precaution, I am thinking to install those 70mm round soffit vents around the house. Perhaps one every metre, to help move air around up there and complement the breathable membrane.

Question: is there any reason why I should not do this? Is it possible to have too much ventilation and cause problems that way?

Thanks!

 

[Deluks]

Is the roof space used as a room or is it just a loft?
Do you have insulation on the ceiling/loft floor?

What type of insulation is in contact with the membrane?

Your problem could be that as you've insulated the roof, then the roof space is warmer than the outside, the 'cold' side of the insulation is just beneath the membrane, that's where the warm air meets cold. This will need a continuous, clear air gap and ventilating.

 

[berty3000]

Thanks for the response.

The roof space is just a loft.

It is insulated up there with a base layer of loft insulation, and then more on top of that - the usual soft stuff that comes in rolls. The insulation isn't generally in contact with the membrane. The deeper, top layer of insulation doesn't go to the edges so that there is typically a gap down to the eaves. The underside of the roof tiles has no insulation, just the membrane.

Does this help?

 

[Ivor Windybottom]

Check your humidity levels in the living space for starters. If you're drying washing inside, cooking and showering without extraction and ironing then you are the problem!

Attack the cause, not the symptoms. Your humidity should be 60% or under, any higher and you'll get all sorts of issues including a damp loft.

A breathable membrane should normally be good enough.

 

[berty3000]

Check your humidity levels in the living space for starters. If you're drying washing inside, cooking and showering without extraction and ironing then you are the problem!

Attack the cause, not the symptoms. Your humidity should be 60% or under, any higher and you'll get all sorts of issues including a damp loft.

A breathable membrane should normally be good enough.

Yeah, I have a strict routine about humidity in the home. Generally keep it around 60% (although, what temperature are you basing that on? I seldom have the house above 18C.)

I don't have mold or damp spots, or issues with condensation on windows, apart from a little on bedroom window first thing and bottom of metal patio door which is a cold spot. No issues otherwise. No drying clothes indoors, bathroom kept closed at all times and extractor fan used, window kept open. You get the picture.

Besides which, how would the humidity get into the attic from below, in amounts that would be noticeable? Other than the time I left the hatch open all day recently, I don't see what other opportunities there are for it to get in. Tiny holes above ceiling lights, where insulation is stuffed in above? Seems unlikely. Is the suggestion that the humidity will make its way through veiling plasterboard, then through insulation above into the loft?

 

[Ivor Windybottom]

Water vapour goes through plasterboard. It resists more than a gaping hole, but it's not a vapour barrier. That's why the foil-backed version is used in loft conversions and/or a polythene vapour barrier is used.

But if you're at 60% RH then you're probably fine anyway. I'm assuming a typical indoor temperature of 20 degC or similar.

Perhaps having a look when you haven't had the hatch open for hours would be a good start?

Also check if there are any holes, e.g. around pipes cables, the loft hatch itself. That 60% humidity air will condense into water if it gets up there.

 

[berty3000]

Thanks for the info. So even with plenty of insulation above, the moisture will go through the ceiling as humidity? The ceilings feel bone dry.

This begs the question: why the loft wasn't rotten and caked in mold before I got the roof done. The house will have spent 60 years, with little loft insulation at all for much of that time, and a bitumen membrane. No vents in the soffits or anywhere else and almost certainly people living here who take less care about moisture than I do.

 

[berty3000]

RH is often in the low 60s but I think that's more to do with the temperature generally not being much above 18C. I don't know what temperature people base it on when they say over 60% is a problem. 64% at 17C and 64% at 20C are totally different things in terms of moisture levels. I'm assuming they mean at a typical room temperature of 20C as you say, in which case, I think RH in my home is at the high end of acceptable. Right now it is at 61% in the room I'm sitting in but the temperature is 18.2C.

 

[HarryC89]

Condensation risk is lower when there is less insulation. So increasing insulation can actually cause condensation problems when they weren't present before. If you have a cold roof (e.g. insulation between and over joists, loft space unconditioned) then it should be ventilated at the soffits. Google 'cold pitched roof detail' and you'll see lots of details showing soffit ventilation and an air gap allowing the outside air to carry away any moist air.

 

[HarryC89]

Also I think there are probably calculations out there to say how much soffit ventilation per m2 of loft space but I don't -think- you can really have too much, at the end of the day the loft is an unconditioned space and outside the heated envelope so you shouldn't be concerned about soffit vents causing heat loss.

 

[berty3000]

Condensation risk is lower when there is less insulation. So increasing insulation can actually cause condensation problems when they weren't present before. If you have a cold roof (e.g. insulation between and over joists, loft space unconditioned) then it should be ventilated at the soffits. Google 'cold pitched roof detail' and you'll see lots of details showing soffit ventilation and an air gap allowing the outside air to carry away any moist air.

But why is this? I don't get it. Insulation should mean warm air is trapped below the ceiling better, and above the insulation is more separate from below, surely? Why would insulation make condensation worse - assuming it isn't stuffed up to the membrane?

Also, are you saying soffit vents are worth doing even with breathable membrane? I'm happy to get them, just don't want to waste my time and money, or end up introducing problems somehow.

 

[HarryC89]

But why is this? I don't get it. Insulation should mean warm air is trapped below the ceiling better, and above the insulation is more separate from below, surely? Why would insulation make condensation worse - assuming it isn't stuffed up to the membrane?

Also, are you saying soffit vents are worth doing even with breathable membrane? I'm happy to get them, just don't want to waste my time and money, or end up introducing problems somehow.

It's not something I've done myself but I do SAP EPCs for a living and can tell you that all new build houses will ventilated the soffits for a cold pitched roof so take from that what you will.

Condensation is worse when insulation is higher because the difference in temperature between the inside and the outside of the wall / ceiling is much higher than a poorly insulated one (because the heat is moving much slower). Moist air escapes through that ceiling and reaches the cold side. If the temperature is at the dew point, it will condense into water. If there is nowhere for that water or moist air to escape, it will stay there and cause damp. It's a well known phenomenon and is a particular problem when insulating solid walls as they are hardest to ventilate but it can occur in any scenario where there isn't enough ventilation. Just google 'Does higher insulation cause condensation' there is loads of material out there.

 

[berty3000]

That all makes perfect sense, thanks for explaining it!

So it seems logical then, as per new builds, to have the added insulation for energy efficency / comfort reasons, but then to ensure the attic is well ventilated to prevent moisture settling up there. The soffit vents will complement the breathable membrane.

Out of interest do new builds need to have tile vents as well, or just the breathable membrane and soffit vents?

Having said all that, it does still seem strange that damp wasn't a serious problem in the past when this house had bitumen membrane and little insulation. There would surely still come a point where the moisture through the ceiling would reach the dew point, and very little moving of air or ability to escape.

 

[HarryC89]

I can't comment on the tile vents sorry! it's usually soffit vents plus ridge vents which allows for cross ventilation.

When a envelope (floor / wall / roof) is poorly insulated more heat escapes through it more quickly. So the difference in surface temp between the inside and the outside of that envelope is much lower. So when moist air escapes through that surface, the chance of it condensing on the cold side is greatly reduced. That vastly reduces the condensation risk. It seems counterintuitive but old houses actually suffer a lot less from condensation issues (interstitial condensation at least) than newer houses.

 

[berty3000]

Interesting!

Just for some extra info - there is a flat roofed extension at part of the house where there are no soffits available for me to put vents in. I'm mindful that the ideal is to have vents on both sides of the roof to allow cross flow of air. Nevertheless, do you reckon this would be ok? Crosses are where I intend to put soffit vents.