Standard eaves overhang and insulation query

Curious Geroge

A typical roof cross section attached shows the eave overhang.

This has prompted 2 questions since I'm currently at this stage of the build.

1) wondered if anyone might tell me the common size for this overhang ?

2) I highlighted a section in yellow and wondered if this should have insulation packed into it prior to the eaves framing board been put in place. I don't see any way to do this after as the wall plate would not allow access here again. Thoughts?

DOCARCH

1. Anywhere from about 175mm to 300mm would be pretty much 'standard'.

2. Not much point insulating where you have highlighted? It is not doing anything (beneficial)?

Question would be what is your wall construction and insulation, and, what is your roof construction and insulation? Generally wall insulation should pretty much seamlessly meet/connect with roof insulation.

Curious Geroge

DOCARCH wrote: »

1. Anywhere from about 175mm to 300mm would be pretty much 'standard'.

2. Not much point insulating where you have highlighted? It is not doing anything (beneficial)?

Question would be what is your wall construction and insulation, and, what is your roof construction and insulation? Generally wall insulation should pretty much seamlessly meet/connect with roof insulation.

This will surely receive some feedback but I had more than a couple of talks with the roofer and the engineer. There is a block on flat closing the cavity with my holicore resting on the block on flat and the wall plate on the holicore. The eaves drop about 300mm below the wall plate thus no cold bridge. The block on flat seems to be 2 fold, support the holicore and strengthen the roof. That'd be my reason to fill this area.

DOCARCH

Curious Geroge wrote: »

This will surely receive some feedback but I had more than a couple of talks with the roofer and the engineer. There is a block on flat closing the cavity with my holicore resting on the block on flat and the wall plate on the holicore. The eaves drop about 300mm below the wall plate thus no cold bridge. The block on flat seems to be 2 fold, support the holicore and strengthen the roof. That'd be my reason to fill this area.

There is some logic to your explanation and reason for insulating where have shown (subsequent to you explanation)...but...the drawing you have attached really does not show (at all) the arrangment of your cavity, cavity closer, hollowcore slab, etc., etc...so, not easy to comment?

Curious Geroge

DOCARCH wrote: »

There is some logic to your explanation and reason for insulating where have shown (subsequent to you explanation)...but...the drawing you have attached really does not show (at all) the arrangment of your cavity, cavity closer, hollowcore slab, etc., etc...so, not easy to comment?

In an attempt to replicate what I have in place, I did a bit of an update done and attached to the original. Not to scale of course

fclauson

George

get a THERM/PSI calc done for the junction - you cannot work out what where to place the insulation just by guessing

The weakest line is my arrow on your diag - its will be maximising the insulation across this line which needs to be looked at

MicktheMan

fclauson wrote: »

George

get a THERM/PSI calc done for the junction - you cannot work out what where to place the insulation just by guessing

The weakest line is my arrow on your diag - its will be maximising the insulation across this line which needs to be looked at

+1

I've also added a significant heat leak path (see red line in pic).

eaves-framing-l.gif

If these are not addressed your risk of condensation and mould is quite high

Curious Geroge

Thanks MicktheMan and fclauson. As you have shown, this was my concern with the engineer but structural agreement with the roofer meant we needed the block on flat. To late to change it now, the roof is almost on so I need to concentrate on ensuring or limiting heat loss. Assuming an airtight membrane will be used in the roof, would insulting the highlighted "yellow" area be worthwhile ?

fclauson wrote: »

George

get a THERM/PSI calc done for the junction - you cannot work out what where to place the insulation just by guessing

The weakest line is my arrow on your diag - its will be maximising the insulation across this line which needs to be looked at

I do appreciate the comments but in my circumstances, its not sustainable to continue hiring people for these tests. The PHPP quotes came in at €1400+vat, this THERM/PSI calc would be ~€200 and since I did not go passive from the beginning, it does'nt have the same value.

prewtna

Great thread.

Cannot disagree with the previous comments. I would add that the principal of insulating around the eaves / top of the wall seems to be the only way around the detail that has been constructed.

If we assume you are / can insulate above the hollow-core slab - then be sure you maintain a continuous line of insulation over it and the wall-plate and down into the eaves area. This will eliminate the black line. There seems to be feck all you can do about the red line other than make it as long as possible. But be sure to maintain ventilation up through the eaves into the roof structure above. I assume your air-tightness membranes etc should all be 'living' down below these items anyway ie below the hollow-core? or are we looking at an eaves detail in a dormer situation?

In standard construction, cold bridges are almost inevitable, where you have no choice other than to have a cold bridge (as appears to be the case here) your best bet is try make the path along which the cold travels before it meets the warm air / surfaces as long as possible. Its not great and you cannot guarantee that there will never be condensation given the detailing as it appears at present.

Curious Geroge

prewtna wrote: »

But be sure to maintain ventilation up through the eaves into the roof structure above. I assume your air-tightness membranes etc should all be 'living' down below these items anyway ie below the hollow-core? or are we looking at an eaves detail in a dormer situation?

Its a attic living space so wouldn't the airtight membrane run along the rafters down to the holicore (upside) and I would glue it there. On the holicore (downside), I would use airtight tape or again a membrane to seal that. Air movement through the holicore is limited as I had the ends sealed.

fclauson

George

Can I suggest you ask your engineer how he will sign off given the construction detai. Either he has complied with the ACDs or has does a calculation so as to comply with the requirement of Part L 2011 FRSI calculations

When you get you BER calc done (I assume you already have a provisional one) then the accessor will need to know details of that junction - if its not to the ACDs you will get a big penalty as they will have to use default values which may mean you struggle to get Part L compliance

See slides 12 onwards of this - to see just how important this junction is - if its not done right then you will end up with mould growth around the ceiling line of each room where there is a thermal bridge

see http://www.phai.ie/wp-content/uploads/2013/10/STL_2013_Presentations/faq/Andrew_Lundberg_Thermal_Bridge_detailling.pdf

Curious Geroge

fclauson wrote: »

George

Can I suggest you ask your engineer how he will sign off given the construction detai. Either he has complied with the ACDs or has does a calculation so as to comply with the requirement of Part L 2011 FRSI calculations

When you get you BER calc done (I assume you already have a provisional one) then the accessor will need to know details of that junction - if its not to the ACDs you will get a big penalty as they will have to use default values which may mean you struggle to get Part L compliance

See slides 12 onwards of this - to see just how important this junction is - if its not done right then you will end up with mould growth around the ceiling line of each room where there is a thermal bridge

see http://www.phai.ie/wp-content/uploads/2013/10/STL_2013_Presentations/faq/Andrew_Lundberg_Thermal_Bridge_detailling.pdf

Thanks fclauson, this is the main reason for the thread being created. I'm aware of the detail but it was needed to support the structure for both holicore and roof. So with the potential risk of mould as per the presentation, what steps would be best taken to limit the cold bridging ?

fclauson

Get a detailed psi calc done to show what temps you are getting and the play around with insulation options to try and resolve