Attic trusses, warm loft, internal PIR location

[Thorfun]

hello, starting the internal PIR in our roof which has attic trusses. my plan is to simply screw the PIR to the bottom of the horizontals at the top of the attic trusses leaving a gap between the PIR and the mineral wool between the trusses. like the photo below:

 

 

is that air gap going to cause an issue with my U-values? the loft space will only be used for storage so I'm not worried about the loss of head height I just think it's easier to attach to the bottom of those horizontals rather than trying to fit PIR above them in the small gap!

 

there is 190mm of Frametherm 32 between the trusses and the PIR is 100mm.

 

PIR will be covered in Intello Plus and taped where required to continue the airtightness layer from the rooms below.

[ProDave]

 Cut the PIR edges at an angle to match the roof pitch and there will be no gap?

[Thorfun]

1 minute ago, ProDave said:

 Cut the PIR edges at an angle to match the roof pitch and there will be no gap?

I will be doing that as shown by the rectangles in red below. my question is to the gap above the horizontals as circled in red below?

 

 

[ProDave]

If the frametherm goes right to the apex I don't see an issue.

 

Put any cables in e.g. for lights before you close it all up.

[Thorfun]

11 minutes ago, ProDave said:

If the frametherm goes right to the apex I don't see an issue.

 

Put any cables in e.g. for lights before you close it all up.

yep, the frametherm goes to the apex. re: cables, I was just going to fit a service cavity under the PIR and airtightness layer as in the rest of the house for lights/sockets etc. will again result in a lower ceiling height but as it's just going to be storage I'm not worried about that. we'll end up with about 2.1m ceiling height once boarded so it's not that bad anyway.

[Temp]

1 hour ago, Thorfun said:

I will be doing that as shown by the rectangles in red below. my question is to the gap above the horizontals as circled in red below?

 

 

 

Got any left over Frametherm to put on top of the PIR before you put it in place?

 

 

 

Edited June 7, 2022 by Temp

[Thorfun]

4 minutes ago, Temp said:

 

Got any left over Frametherm to put on top of the PIR before you put it in place?

 

 

 

might have some offcuts left but not enough to fill that void along the length of the roof! what are you thinking/worried/concerned about with the air gap?

[crispy_wafer]

I'd think if the gap in the area indicated is unventilated then I could not see why there'd be a problem, if ventilated or outside air can bypass the wool then that would affect your U values.  I've just had a butchers at your blog - Well done, good going!

[Thorfun]

2 minutes ago, crispy_wafer said:

I'd think if the gap in the area indicated is unventilated then I could not see why there'd be a problem, if ventilated or outside air can bypass the wool then that would affect your U values.  I've just had a butchers at your blog - Well done, good going!

well.....air could bypass the wool as there is a breathable membrane above it and below the slates. I remember a post a little while back where this was discussed in a wall and the suggestion was to break up the air gap. so I might simply chuck all of the offcuts of mineral wool I have above the PIR to reduce the airflow within that 'cavity' to reduce the impact on the U-values.

 

thanks for the comment about our blog. we're getting there! it's just a little slow when you're having to do a lot of the work yourselves.

[Temp]

4 hours ago, Thorfun said:

might have some offcuts left but not enough to fill that void along the length of the roof! what are you thinking/worried/concerned about with the air gap?

 

Not worried but hate to see a gap that could be filled with insulation wasted.

[Iceverge]

Jam it full of offcuts, better than going in the skip. 

 

It'd be a good place to hide any secret documents...

[Thorfun]

14 minutes ago, Iceverge said:

Jam it full of offcuts, better than going in the skip. 

that is exactly the conclusion we came to last night! will need a big long pokey stick to get them to the other end of a 2.4m sheet of PIR though.

[crispy_wafer]

18 hours ago, Thorfun said:

thanks for the comment about our blog. we're getting there! it's just a little slow when you're having to do a lot of the work yourselves.

Keep at it.

 

Time I found so far is the biggest factor at play, and not necessarily having the prior knowledge of how to do something, I was adamant I'd do the donkey work like cutting fitting + taping , two layers of PIR between and under + Airguard membrane of all layers, it's taken us the best part of 6 months.  Not a long job per se, but getting the actual time to spend doing the job is the limiting factor.  I reckon in hindsight it's a bit belt and braces but nevertheless, I'll be happy knowing it's bang on...

 

As this is your loft space, What have you got planned at foot level?  Would you insulate here as well?  We've got a loft where attic trusses were used, similar PIR'd between and under. I'm unsure if I really need to insulate at foot level, and if so, how much, considering how much insulation is between and under the rafters

[Thorfun]

1 hour ago, crispy_wafer said:

Keep at it.

 

Time I found so far is the biggest factor at play, and not necessarily having the prior knowledge of how to do something, I was adamant I'd do the donkey work like cutting fitting + taping , two layers of PIR between and under + Airguard membrane of all layers, it's taken us the best part of 6 months.  Not a long job per se, but getting the actual time to spend doing the job is the limiting factor.  I reckon in hindsight it's a bit belt and braces but nevertheless, I'll be happy knowing it's bang on...

 

As this is your loft space, What have you got planned at foot level?  Would you insulate here as well?  We've got a loft where attic trusses were used, similar PIR'd between and under. I'm unsure if I really need to insulate at foot level, and if so, how much, considering how much insulation is between and under the rafters

just thinking some rockwool underneath to assist with sound attenuation for if/when someone is walking in the loft. I see no benefit in putting lots of insulation for any other purpose under foot in the loft. 

 

I agree that doing it ourselves is probably more than belt and braces and wonder how a 'professional' would be doing it. but it's things like this, i.e. the fabric, that I'm happy to belt and braces as it's not something I want to try and redo after the house is finished! most of the house walls are finished with insulation now, just got the loft to finish PIR under the rafters and then I can do the AVCL, tape and service cavity. I have a fixed date of the plumber starting first fix August 1st so I need to be done by then. it's nice to have a deadline to work to as I find it focuses the mind.