No air gap at eaves!

My pitched roof does not have any air gap at the eaves. Not sure if they were ever there since the fascia soffits have been covered over with newer PVC types which have vents but this is not brought through the old timber to the attic. Pitch is not insulated neither is the floor joists (work in progress). There is an old asbestos vent in the roof from the previous heating system. There is practically no air movement in the attic at all. Timbers are in good nick. If I insulate between the floor joists will i get problems with condensation etc?

I also had similiar trouble with airflow through the attic. Like you the timber was covered over with PVC. My solution was to install circular vents every few feet or so along underside in the soffit. It just required drilling a 75mm hole with a hole saw (through both the PVC and timber) and clipping in a plastic vent. I got the vents from my local builder provider and installed about 30 or so if I recall. I think they were a Glidevale product. I also had to reposition some insulation so as to make sure that these airways were not blocked.

thanks a million for this. I didn't know you could buy such circular plug in vents.

Franm said:

I also had similiar trouble with airflow through the attic. Like you the timber was covered over with PVC. My solution was to install circular vents every few feet or so along underside in the soffit. It just required drilling a 75mm hole with a hole saw (through both the PVC and timber) and clipping in a plastic vent. I got the vents from my local builder provider and installed about 30 or so if I recall. I think they were a Glidevale product. I also had to reposition some insulation so as to make sure that these airways were not blocked.

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Would it be sufficient to put these vents along just two adjoining sides of the house, e.g. the back and left hand side?

Thanks.

the area of the vents should be comparable to a continuous 10mm gap at the eaves...

so say the length of the house is 10 meters... thats 0.1 m2 on either side...

if the glidevents are 75mm diameter thats 0.0370 m2 each area.... therefore 27 on either side...

the ventilation must be cross ventilation so they should be on opposite sides... preferably on the two longest sides.....

sydthebeat said:

the ventilation must be cross ventilation so they should be on opposite sides... preferably on the two longest sides.....

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I was hoping that you wouldn't say that.....

Is 'opposite sides' also a requirement for roof vent tiles?

It's a high number of vents in the soffit isn't it.

Thanks for reverting on this.

Im assuming that this requirement excludes a passive build with mechanical ventilation where there is no airgap at the eaves?

normandy said:

Im assuming that this requirement excludes a passive build with mechanical ventilation where there is no airgap at the eaves?

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nothing to do with mechanical ventilation...

the requirement for attic ventilation is to dissipate condensation that forms when warm meets cold...

if the roof is designed so that the condensation point does not form within the roof structure / attic, (ie a warm deck flat roof).. then ventilation is not required....

But if you are mechanically ventilating the attic in a passive house should you close the air gap at the eaves?

sydthebeat said:

nothing to do with mechanical ventilation...

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the requirement for attic ventilation is to dissipate condensation that forms when warm meets cold...

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<nods>

Warm air can meet cold air all you want unless there is moisture involved.
That's where the problem arises: in Ireland there is very little warm dry air in terms of weather - its usually moist. In houses they're fine assuming no leaks internally until occupation. Then, human bodies being what they are, and the business of living, all generate moisture vapour, which tends to rise and migrate through materials until it meets a cold surface on which it can condense, or a cold material or construction inside which it can condense.

if the roof is designed so that the condensation point does not form within the roof structure / attic, (ie a warm deck flat roof).. then ventilation is not required....

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Yes and no. Dry rot forms in conditions of 80-90% relative humidity and 18-22 degrees. Sound like an unventilated warm roof? I don't subscribe to the no ventilation AT ALL theory, and I think there is always some dissipation by default, simply due to movement and building tolerances. I think we ignroe this micro-ventilation when moving to sealed building at our peril. I think when we *really* seal buildings up tight, new problems with Toxic Mould etc will rear their heads. As well as a whole new method requried to seal buildings while permitting movements. As well as a way to ensure that flexible seals stay flexible after 20 years.

ONQ

normandy said:

But if you are mechanically ventilating the attic in a passive house should you close the air gap at the eaves?

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If you mean a converted attic, this actually raises more problems than you might think. Instead of the interstitial space being the whole attic volume it now consists of the spaces between the roof covering and the ceiling palsterboard, maybe 10-15% of the attic volume dealing with the same or more moisture vapour trying to find its way out.

Even with warm deck flat roofs, we used to ventilate the rooms by opening windows occassionally. Now with mech venting we don't intend to do this any more. Does anyone foresee problems with this way of living in the name of energy efficiency?

Did you know that the PI cover in some countries specifically excludes things like Toxic Mould? Has there been a debate on how this might effect the building industry and public health in a country prone to humid summers and wet winters like Ireland?

ONQ.

normandy said:

But if you are mechanically ventilating the attic in a passive house should you close the air gap at the eaves?

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if you are mechanically ventilating the attic then its considered a warm area thus its within the insulation envelope, thus again "if the roof is designed so that the condensation point does not form within the roof structure / attic, (ie a warm deck flat roof).. then ventilation is not required...."

if the roof isnt breathable then you are in trouble....

to onq...
rot in roof timbers is down to a lack of knowledge of how materials work together....
in the warm roof i describe above, where no ventilation would be required, i would be specifically talking about a designed breathable construction... .intelligent vapour control membranes, breathable insulation with a sd value less than that of the roof timbers (thus directing the bulk of the vapour through the insulation and not the timbers,, ie rockwool or cellulose)...

trouble occurs when the construction is made non-breathable.... ie with bitumous felt or polyethlyene vapour barriers, or insulations that have sd values worse than timbers ie polyurethanes, pheonlics, polyiso's, even blown soy based foams and some polystyrenes... when these are used they act like a dam in the construction and the vapour is directed into the areas where it should not go ie the structural timbers....

Thanks Ong and Syd, Im sorry for taking up your time on an item but Im not sure if I still know what to do.
Is it that if the roof construction is breathable you can seal the air gap at the eaves?
Or is it that you should never seal the gap?
Im asking because the SEI passive guidelines are not clear to me or my builder and while the drawing does look as if there is no gap, the accompanying text says 'rafter continuous vent to eaves'. I've had trouble getting anyone to clarify properly and the advice is conflicting.
Incidentally this roof is pitched, Im using a proclima vapour barrier with all joints overlapped and penetrations sealed, Im fixing 400mm mineral woll between the rafters, Im fixing that with chicken wire and Im then going to go across the rafters with a kingspan insulation board.
The attic is a living space so inside the envelope which is why it will be mechanically ventilated.
The gap is now sealed so am I looking at putting in glidevents like Bobo and Franm?
Thanks very much all

ps, what does sd stand for?

Forget that I asked the sd question!
Got it.

normandy said:

Thanks Ong and Syd, Im sorry for taking up your time on an item but Im not sure if I still know what to do.
Is it that if the roof construction is breathable you can seal the air gap at the eaves?
Or is it that you should never seal the gap?
Im asking because the SEI passive guidelines are not clear to me or my builder and while the drawing does look as if there is no gap, the accompanying text says 'rafter continuous vent to eaves'. I've had trouble getting anyone to clarify properly and the advice is conflicting.
Incidentally this roof is pitched, Im using a proclima vapour barrier with all joints overlapped and penetrations sealed, Im fixing 400mm mineral woll between the rafters, Im fixing that with chicken wire and Im then going to go across the rafters with a kingspan insulation board.
The attic is a living space so inside the envelope which is why it will be mechanically ventilated.
The gap is now sealed so am I looking at putting in glidevents like Bobo and Franm?
Thanks very much all

ps, what does sd stand for?

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Ok, firstly i wouldnt seal the air gap at the eaves. Its a good idea to allow ventilation up the slope of the roof to ridge vents if possible... its not a big issue.
Its hard to envisage without knowing the wall construction as well...

Regarding your specification... everything else is fine apart from the kingspan board insulation.. every other material as youve described is breathable apart from the kingspan. Substitute the kingspan board for rockwool flexi.
Tests have shown that when a non-breathable material is used where condensation may occur, it increases the volume of moisture through gaps in this material by 1600 times...!! thats not what you want...

i would envisage your construction as being, from external to internal:

roof finish on
battens
counterbattens
breathable felt
100mm rockwool flexi between 100 x 44 battens
200mm rockwool flexi between rafters
intello proclima + VCL
service cavity created by 100 x 44 battens filled with rockwool flexi.
non-foilbacked plasterboard as ceiling finish with skim

also, remember that "breathable" means a two way action.. not just internal to external.... during warm summer days moisture (humid air) will be pressure pushed from external to the internal, and the construction needs to be able to allow this action as well to prevent problems. thus the proclima + membrane is used to facilitate this...

if your construction is designed to allow this then, theoretically, you do not require ventilation.
However, current building regulations state that ventilation is required, but thats because it doesnt legislate for a designed roof construction that is breathable.... you would need your certifier to understand and be happy with the construction....

normandy, has your engineer / architect / specifier carried out a condensation risk analysis on the construction?? If they had this problem should have been highlighted.

If you are fixing 400mm between rafters, what depth of rafter have you???
remember, if you squeeze 400mm of a quilted material into a 200mm space, you only actually have the performance of 200mm of insulation....!!

whats your target u value for your roof?? are you going down the certified passiv institute route (as thats the only way to truely know whether your build is passive or not....

sydthebeat said:

<snip>
to onq...
rot in roof timbers is down to a lack of knowledge of how materials work together....
in the warm roof i describe above, where no ventilation would be required, i would be specifically talking about a designed breathable construction... .intelligent vapour control membranes, breathable insulation with a sd value less than that of the roof timbers (thus directing the bulk of the vapour through the insulation and not the timbers,, ie rockwool or cellulose)...

trouble occurs when the construction is made non-breathable.... ie with bitumous felt or polyethlyene vapour barriers, or insulations that have sd values worse than timbers ie polyurethanes, pheonlics, polyiso's, even blown soy based foams and some polystyrenes... when these are used they act like a dam in the construction and the vapour is directed into the areas where it should not go ie the structural timbers....

Click to expand...

Thanks sydthebeat.

Its was precisely to get this kind of learned response that I made my post.

I knew in theory you were right, but I have seen too many practical examples of theoretically correst warm flat rooves going awry that I had to prompt you for clarification.

Not that many people seem to know about intelligent membranes and I think there was an article referred to somewhere recently in, was it Archiseek about internal vapur checks that allow vapour migrate back into rooms when its appropriate to allow the ventilation system deal with it.
You might recall it or have that link to add to the discussion here.

ONQ

onq said:

Thanks sydthebeat.

Its was precisely to get this kind of learned response that I made my post.

I knew in theory you were right, but I have seen too many practical examples of theoretically correst warm flat rooves going awry that I had to prompt you for clarification.

Not that many people seem to know about intelligent membranes and I think there was an article referred to somewhere recently in, was it Archiseek about internal vapur checks that allow vapour migrate back into rooms when its appropriate to allow the ventilation system deal with it.
You might recall it or have that link to add to the discussion here.

ONQ

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there has been some good articles in construct ireland by joe little architect about future mould problems in single leaf buildings....

i have often posted about this over on boards... id be surprised if they knew or understood it on archiseek, theyre more interested in the aesthetics over there than the technical...

sydthebeat said:

there has been some good articles in construct ireland by joe little architect about future mould problems in single leaf buildings....

i have often posted about this over on boards... id be surprised if they knew or understood it on archiseek, theyre more interested in the aesthetics over there than the technical...

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Nope, you're bang on - it was Joe Little and I think I was reading both forums at the time, if you follow me.

Very interesting article - assuming its the same one - about inter alia "intelligent" vapour checks, and it seemed to open up the whole "sealed" high-tech construction to great debate.

ONQ

· Ok, I'll retrofit the vents
· 2 sides of the house have; natural stone cladding...planning requirement
100mm block
200mm cavity with full fill ecobead (structural calcs on the way)
(300mm teplo magmatec basalt ties undergoing BBA cert)
100mm block
60mm insulated plasterboard (not up yet, propose K18 kingspan kooltherm)
One side of the house has; 300mm poured concrete retaining wall
150mm cavity with ecobead
215mm block
125mm insulated plasterboard (not up yet)

There are 3 courses of quinnlite above the dpc on the inner leaf of all four sides which means that they’re not in line with the 200mm overslab EPS but I’ll bring the EPS up on the warm side to meet the insulated wall slab. I know the mosart house went below the dpc with the insulating block but my site isn’t bone dry so I didn’t.

· Will use rockwoll flexi instead of the Kingspan board in attic
· Re roof construction; we have .slates
.battens
.No counterbattens
.Breathable felt
.150mm rockwoll betw 170mm rafters
.150mm rockwoll vertical run supported with 50x25mm battens
.150mm rockwoll horizontal run supported by 50mmX25mm battens
. Siga membrane and tapes have been used instead of proclima as specified (I need to check the details of the siga memb. My understanding is that Proclima airtight membranes do have humidity variable vapour diff resistance properties so can you explain why you specified proclima + VCL)
Service cavity not created yet
Plasterboard not on yet (is this where I should use rockwoll flexi?)
Does it make a difference to thermal performance whether you put the extra insulation above or below the rafters if internal space is not an issue?
· There is no condensation risk analysis done.
· I don’t have an architect working on the construction phase (the architect who did the design made too many technical mistakes while drawing up the plans)
· The engineer contracted isn’t familiar with a passive build.
· I’m the specifier and flying by the seat of my pants.
· I have a good builder but he also has never done a passive build … we’re on a steep learning curve with the obvious cost implications like retrofitting vents.
· The phpp is being done by an engineer who has a master’s degree in passive housing. This has been very helpful but we’re 5 counties away and as the passive construction was a retrograde decision and not made at the design stage it’s not ideal as I seem to need advice on a continuum.
SEI gave me a list of 10 firms when I was looking for someone to run the software but most knew nothing at all about it and some had heard of it but at the end of the day I couldn’t find someone locally who was competent to use it.
· Target value for the roof is probably .11
· The SH demand is currently at 10kw so it looks as if it might meet the standard. It’s the airtightedness that will be critical and if Easton Mews in Kildare achieved only 1.6airchanges with a similar construction with very good technique I’m not confident of the prospect of getting to 0.6! If the miracle happens and I’m not dead first I’ll apply for the cert!
If I could ask one more question…with regard to interior/party walls breaking the continuity of insulating plasterboard on the inner leaf of the peripheral walls, do you just live with the thermal break and factor it in to the phpp figures?
I’m looking in particular at a stud wall in the converted attic space that is up before the slabbing. What I’m wondering is if it’s better practice to fix a stud wall through the slab?

Thanks very much all …I feel as if a load is being lifted. I hope this isn’t too much advice to expect!