Which Has Lower U Value?

Southern Comfort

Can anyone tell me which has a lower U Value, 100mm of Rockwool Flexi or 100mm of Aerodorm Platinum?

sas

The thermal conductivities for each are as follows:

0.038 Flexi
0.031 Aerodorm Platinum

A lower value is better.

The u-value is calculated using the thermal conductivity.

Aerodorm is better (on paper).

Things you might also consider:

1. Flexi will be alot easier fit. All the marketing in the world for those concertina type insulations won't change the fact that they are not as easy to install as they say. They are easier than cutting rigid boards to fit alright. The rigid board manufacturers only state the advantages of one of their products over one of their other products.

2. The flexi will allow your roof to breathe alot more than the aerodorm. This is important for the long term protection of your roof structure. However, if you get the your vapour barrier installed to a high standard this becomes less of an issue. I do feel strongly about it however.

3. Price. I'm reasonably sure flexi is cheaper. For a roof however, you may need more than 100mm to comply with building regs. That likely applies to both materials actually.

Hope this is of use to you.

Southern Comfort

sas wrote: »

The thermal conductivities for each are as follows:

0.038 Flexi
0.031 Aerodorm Platinum

A lower value is better.

The u-value is calculated using the thermal conductivity.

Aerodorm is better (on paper).

Things you might also consider:

1. Flexi will be alot easier fit. All the marketing in the world for those concertina type insulations won't change the fact that they are not as easy to install as they say. They are easier than cutting rigid boards to fit alright. The rigid board manufacturers only state the advantages of one of their products over one of their other products.

2. The flexi will allow your roof to breathe alot more than the aerodorm. This is important for the long term protection of your roof structure. However, if you get the your vapour barrier installed to a high standard this becomes less of an issue. I do feel strongly about it however.

3. Price. I'm reasonably sure flexi is cheaper. For a roof however, you may need more than 100mm to comply with building regs. That likely applies to both materials actually.

Hope this is of use to you.

Thanks for the reply, SAS. Your points are very well made.

My rafters are only 95mm deep (600mm span) but I think the Aerodorm Platinum can be cut to my specifications in the factory, i.e., width and depth. If you've read some of my other posts you'll have seen that my converted attic has a plasterboard ceiling and a low pitch so I can't really afford to lose any more head height inside. My plan is to have the tiles and felt removed, have the insulation put in from outside and then cover it with a breathable membrane before replacing the tiles.

I've also discovered that Knauf Rafter Roll 32, which is very similar to Rockwool, has a U value of 0.32. http://www.knaufinsulation.co.uk/products/glass_mineral_wool_rolls/loft_insulation_rolls/rafter_roll_32.aspx

Thanks to you, I now know that that's roughly the same as the Aerodorm Platinum so maybe I should go that route, bearing in mind all the other legitimate concerns you've raised as well.

And one more thing. I visited a builders providers yesterday and discovered Xtratherm Thermal Liner, an insulated plasterboard. The thinnest one is 27mm, made up of 9.5mm plasterboard with 17mm of insulation. Maybe I could sacrifice that much head room......

Could you possibly figure out what U value I'd achieve if I used the Knauf Rafter Roll 32 and attached the 27mm insulated plasterboardboard to the existing ceiling? (I really don't want the hassle of taking down the existing ceiling).

Thanks in advance.

S.C.

sas

What I would do if I was you is as follows:

1. Remove tiles and felt as you've suggested.
2. Insulated over your rafters. This is a pic of my garage where I insulated over the rafters with softboard. You can see the board just about the felt line. IMG_7124.JPG. This is only a 35mm board but you get the idea.

Note: this stuff is very pricy but I like it's breathability. All the major insulations manufacturers supply a board that can be used in a similar manner. They're called sarking boards. As you mentioned Aerodorm, google for Warmsark. You could stick 100mm of insulation there and loose no head height.
3. Fully fill your rafters with an insulant of your choice.
4. This is important I'm afraid. Take down your plasterboard and fit a vapour membrane. With a "warm roof" construction as I've described I feel this is a necessity.
5. After membrane fit 2*1 to create a service cavity and install your plasterboard again. This wil result in no more loss of head height than your proposed 27mm plasterboard option.

I would recommend you talk to an interested Arch technician. etc. An hour of their time wouldn't cost a whole lot and would be well worth it.

Disclaimer: I don't work in construction nor do I have any construction qualifications. My advice comes from my experiences to date as I manage my own build and come could be complete nonsense.

Southern Comfort

sas wrote: »

What I would do if I was you is as follows:

1. Remove tiles and felt as you've suggested.
2. Insulated over your rafters. This is a pic of my garage where I insulated over the rafters with softboard. You can see the board just about the felt line. IMG_7124.JPG. This is only a 35mm board but you get the idea.

Note: this stuff is very pricy but I like it's breathability. All the major insulations manufacturers supply a board that can be used in a similar manner. They're called sarking boards. As you mentioned Aerodorm, google for Warmsark. You could stick 100mm of insulation there and loose no head height.
3. Fully fill your rafters with an insulant of your choice.
4. This is important I'm afraid. Take down your plasterboard and fit a vapour membrane. With a "warm roof" construction as I've described I feel this is a necessity.
5. After membrane fit 2*1 to create a service cavity and install your plasterboard again. This wil result in no more loss of head height than your proposed 27mm plasterboard option.

I would recommend you talk to an interested Arch technician. etc. An hour of their time wouldn't cost a whole lot and would be well worth it.

Disclaimer: I don't work in construction nor do I have any construction qualifications. My advice comes from my experiences to date as I manage my own build and come could be complete nonsense.

Thanks again, SAS, for taking the time to give a detailed and considered reply.

Insulating over the rafters and raising it 35 mm probably isn't an option for me, I think, because I have a semi detached house and my side of the roof would be that little bit higher. (I should have given you that info at the start).

I'm veering towards using the Rafter Roll and the 27 mm insulated plasterboard. It'd give me a U value of about 0.28 which I'd be happy enough with in my circumstances.............