Roof insulation

[deuce22]

Hi.

What are the problems I could face for doing this.

I have vaulted ceilings in my Bungalow and on the original drawings I was told to put 100mm in between the rafters and 40mm on the face.

Over the course of this project I have had 3 different BCO out and I can remember one of them saying that 25mm on the face is sufficient.

Is 25mm sufficient. If not what would I have to do, other than remove everything and re-fit thicker boards.

Thanks.

 

[freddiemercurystwin]

I normally do 100mm between the rafters and 25mm beneath. The only definitive way to prove what insulation is required is to contact Kingspan Technical department and ask for a U value calc (they do about 1000 a day). They'll ask you the rafter sizes and spacings and work out exactly what you need to comply. Takes about 3 minutes in total and they'll email you the calc to shove under the inspectors nose if one kicks up.

 

[deuce22]

Thanks very much mate.

I'll get onto that today.

I'm pretty new to the building industry, but have learned quite a lot from some decent tradesmen and forums like this one.

I understand the importance of insulation now. Most of the old school builders that I've worked with don't seem to care and cut corners whenever they can.

When I do my next project I will go overboard with insulating the property.

Thanks.

 

[meadowhog]

I put in 100mm between and 50mm under just in case BC objected too much with me having more glass than the 25% floor area etc. It sounds a bit like yours and the drawings that were passed are for 25mm under. You could always stick another layer under what you've got if you have to.

Untitled

• meadowhog
• 22 Jun 2009
• 2

 

[^woody^]

Thicknesses depend on the actual insulation type

You only need 50mm under the rafters if using 75mm (Kingspan TP 10) or 70mm (K7) between. For 100mm between it is 20 or 25mm below

The thickness between the rafters can depend on rafter depth, as you may need to maintain a minimum air void

Just as important as inserting the insulation, is ensuring airtightness - so fit everything snugly and block all gaps - use flexible material as necessary to allow for shrinkage of timbers

 

[deuce22]

Hi.

I have just spoken to someone from technical at Kingspan and although he has told me what to do I'm not sure whether to listen to him.

After telling him the thickness of the timbers he said to just put 150mm in between the rafters, but I've been told be someone else that, that was the old way of doing it and it now doesn't conform to updated regs.

I then told him that one BCO said 40mm under and one said 25mm under.

He asked if I was using breathable felt or if the roof was vented.

I'm using breathable felt, but have no vents, so he then said 25mm on the face is fine.

Does this sound about right.

Thanks.

 

[JoeMar]

hi

i've recently completed an extension project with a vaulted ceiling. i have used 150mm Celotex (FR4000) between the rafters. the roof is vented at the ridge, breather felt, 50mm air gap, insulation, 500 gauge VCL, plasterboard

According to the calcs on the celotex website the U-Value of my roof is 0.19W.

unless i am mistaken i believe the new regs coming into effect in October sets the minimum at 0.2W.

 

[jeds]

You should not be building a new build house based on elemental U-values. The values should have been calculated into a SAP TER which accounts for all the other heat loss facters. It may be that your roof was slightly uprated to account for something somewhere else. If you change that now you might find that your house does not pass the SAP DER later.

For the record; 100mm between rafters and 20mm beneath, with rafters at 400c/s, gives a U-value of 0.23, which would comply as a target for a new build.

Also note that a lot of people (including a lot of builders) get confused between extensions and new build houses. The target U-values are different. For example, the above would not comply for an extension.

 

[meadowhog]

As you are using breathable felt you dont need an air gap so unvented is fine.

If you put Celotex under it can be taped up and used as the vapour barrier instead of fitting a seperate VB.

"The old way"? as long as it does the job.

The two questions I would ask myself are, is the head room affected too much by going under in which case fit it all between and is it going to pass regs. I would ring the BC and explain your concern. They may ask for the calcs. 100 between and 25 under passes but BCO may have other ideas. If they say 25mm under is Ok your drawing may need an amendment.