Closing cavity at the top for blown beads

[Oz07]

Always use it Regardless and always put over fascia vents on under eaves support tray. For what it costs it's not worth emitting. 

[MortarThePoint]

3 hours ago, PeterW said:

Only required if you don’t use breather membrane as airflow is not required otherwise. 

 

There's some debate on this as I understand it. I spoke to my architect and he said that NHBC now mandate ridge ventilation and that people are starting to doubt the breather membranes and install eave ventilation as well. He speculated that NHBC may mandate eave ventilation in due cause but was going to check LABC and NHBC for details.

[MortarThePoint]

3 hours ago, Oz07 said:

Always use it Regardless and always put over fascia vents on under eaves support tray. For what it costs it's not worth emitting. 

 

I'm leave a 50mm gap between insulation in insulated pitches and the membrane for drape of the membrane (otherwise need counter battens). Does this rafter roll not prevent the required drape?

 

The drape is need to allow water to pass between the outside of the membrane and the battens.

[PeterW]

29 minutes ago, MortarThePoint said:

He speculated that NHBC may mandate eave ventilation in due cause but was going to check LABC and NHBC for details.

NHBC don’t set the rules - they are in the Building Regulations so unless you are having an NHBC warranty they are the last people I would check with. 
 

28 minutes ago, MortarThePoint said:

The drape is need to allow water to pass between the outside of the membrane and the battens.

If you have water on your breather membrane then you have bigger issues with a roof. That is the last line of defence. 

[Oz07]

Why wouldn't you put the eggbox crap in? It's literally cheap as chips. Also the over fascia vents I think finish a roof off nicely.

 

I always have

over fascia vents

roll out rafter tray

Breathable membrane

vented dry ridge.

 

I was always to understand more ventilation is better than none in a roof space. Last place I put bedded ridge on with fascia vents years ago got condensation in winter in roof space. We went back and put 3 vent tiles on the cold side of the roof 2/3 up and it fixed it

[Oz07]

@MortarThePoint the rafter roll maintains the 50mm vent. You fix it on a bit loose like your membrane and it follows the drape. Then you just make sure you don't ram your insulation in too tight

[epsilonGreedy]

5 hours ago, PeterW said:

Only required if you don’t use breather membrane as airflow is not required otherwise. 

 

 

My roofer (30 years in the trade and nearing retirement) wants me to use a breather membrane plus the eggcrate breather roll at the eaves. He says the actual performance of "breather" membranes does not match expectations. Think I will follow his advice in the absence of a ventilated ridge.

[JohnB]

A series of holes are drilled in the wall, to an approved pattern, through the mortar joints. A specially designed injection gun is used to pressure- blow the insulating bead into the gap between the two walls. When the installation is complete the holes are made good with colour matched mortar and any dust from the drilling will be cleaned away

[Mr Blobby]

To reserrect this old thread, we are going to use 150mm isover metac glass wool to full fill between rafters and up to the breather membrane. 

(Metac seems about twice the price of knauf frametherm which looks like the same product so may need to review this 🤔)

 

The plan is for the glass mineral wool to be pushed all the way down to close the cavity and then for the blown beads to go up to meet the mineral wool.

 

Here's a pic from earlier today:

 

 

Wall plate to be attached to top of outer leaf.

250 cavity.

 

So the roofers have asked if the insulatoin is suitable to close the cavity and also suggested to close the cavity before the breather membrane is installed on top of rafters.  Which seems like sensible advice.

 

So, just to reassure me here, is it ok to close the cavity with between-rafter glass mineral wool?  It seems sensible enough to me.  Do I need to do anything else?

To close the cavity I assume I can just squeeze the Metac/frametherm between the rafters without any need to cut it (to accomodate the bump at the inner leaf wall plate) or anything. 

 

Am I missing something?  I welcome any advice here from those who actually know what they're doing.😬

 

 

Edited October 12, 2023 by Mr Blobby
crap spelling and even worse grammar

[Iceverge]

More info please.

 

Do you have any cross sectional drawings/sketches?

 

Will there be insulation on the pitch or the roof or on a flat ceiling level?

 

Will there be insulation below or above the rafters? 

 

Have you considered blown cellulose for the roof? 

 

 

[Mr Blobby]

28 minutes ago, Iceverge said:

More info please.

 

Do you have any cross sectional drawings/sketches?

 

Will there be insulation on the pitch or the roof or on a flat ceiling level?

 

Sorry, roof is ventilated cold roof / warm loft with insulation between and below rafters.  Like this:

 

 

 

The only change from the above is the removal of the cement board and PIR as the cavity closer, replacing it with stuffing the Metac further down rafters to the top of the cavity.

 

Its a detached house and general approach here in NI (and it seems on this forum) is to close the cavity with insulation.

 

So my only question here really is whether the Metac is ok to close the cavity.  My roofers asked the question so thought I'd best check 🤔

 

Edited October 12, 2023 by Mr Blobby

[Iceverge]

It will be fine as is. Some beads may make their way up into the first few mm of metac but that won't make any difference. The place I'd be concerned about is the facia. They'll spill out any holes bigger than about a 20p coin.

 

Have you considered a warm roof with the PIR above the rafters? It'd really improve the junction at the eaves? 

 

[Mr Blobby]

On 13/10/2023 at 13:13, Iceverge said:

Have you considered a warm roof with the PIR above the rafters? It'd really improve the junction at the eaves? 

 

Yes we went back and forth a few times looking at the warm roof option, which was originally my first choice.  An extra problem for us is the top covering is standing seam, and this requires a ventilation gap anyway, plus an additional layer of ply/osb for a warm roof added extra cost.  And then to get the required u-values we needed really long fasteners for the PIR.

 

Taken together a warm roof didnt really work for us.

 

On 13/10/2023 at 13:13, Iceverge said:

The place I'd be concerned about is the facia. They'll spill out any holes bigger than about a 20p coin.

 

Our roofer would agree, he suggested pushing some insulation into the cavity below the rafters because he said the balls may come through otherwise.

My latest worry is the insulation falling into the cavity, but as long as that doesn't happen it looks pretty good I think.

(although being typically obsessive compulsive I'll check those small gaps under the rafters)