Insulation for dormer stud wall

johnnyboy08

Would anyone have any recommendations on the type of insulation I could get for a dormer stud wall?
The wall separates bedrooms from a cold attic space and there is no insulated plasterboard (just ordinary type), no vapour control membrane present and current insulation is 100mm or fibreglass batts (see attached image).

A guy in Kingspan said I could use K7 perpendicular to the stud over the existing fibreglass batts but I'm a bit worried about condensation forming on the warm face of the board as I couldn't guarantee the fix would be airtight and there is no VCL.

Could something like rockwool slabs be used with the long edge horizontally so the studs are covered, cold bridging reduced but yet minimising any chance of vapour buildup?

In the wind

I am looking into a similar situation. Had an expert come out & he propose to install ALUthermo Quattro (not a typo). I felt the materials & manpower was too much so now I am looking at installing a lower performance alternative myself called "YBS AIRTEC REFLECTIVE DOUBLE INSULATION 25 X 1.2M".

It basically bubble wrap coated with foil on both sides that you staple gun over the Fibreglass batts & seal the seams with foil insulators tape.

MicktheMan

In the wind wrote: »

I am looking into a similar situation. Had an expert come out & he propose to install ALUthermo Quattro (not a typo). I felt the materials & manpower was too much so now I am looking at installing a lower performance alternative myself called "YBS AIRTEC REFLECTIVE DOUBLE INSULATION 25 X 1.2M".

It basically bubble wrap coated with foil on both sides that you staple gun over the Fibreglass batts & seal the seams with foil insulators tape.

Imo, a waste of time, money and effort.

celticbhoy27

MicktheMan wrote: »

Imo, a waste of time, money and effort.

Half way through a dormer job myself. Pretty much same as you've described. Insulation batts and then the reflective ybs over them. Difference its made is ridiculous. Used to have stove lit all day (backboiler) and boost heating for couple hours to help get rads piping hot. Could never get temp over 19 degrees. House wouod be 16/17 in morning. Since doing insulation we've lit fire three times since September. Heating goes on for hour and half in evening and the thermostats at 21 degrees.

Seem to have quoted wrong post. Apologies

Dr_Colossus

Good informative video and pertinent to the situation with a dormer:

Outkast_IRE

Did this in my parents house . Rockwool flexislab is specifically made for this , no sag , flexi edge on one side, excellent fire resistance , good acoustically, easy to diy

In the wind

Hi Mick, I'd be interested to why you think that. I don't want to do it if it is ineffective. I'd like to hear more...

MicktheMan

I test buildings for heat loss and regularly enough come across this type of product especially in roof / dormer situations. Their ineffectiveness really comes to the fore during the air tight test unless every seam, junction and penetration has been sufficiently dealt with. In my experience, using a proper air tight membrane instead would yield a better result and would be way cheaper too. Also, I have yet to see any certification (IAB or equivalent) of these products for use in buildings.

In the wind

MicktheMan wrote: »

I test buildings for heat loss and regularly enough come across this type of product especially in roof / dormer situations. Their ineffectiveness really comes to the fore during the air tight test unless every seam, junction and penetration has been sufficiently dealt with. In my experience, using a proper air tight membrane instead would yield a better result and would be way cheaper too. Also, I have yet to see any certification (IAB or equivalent) of these products for use in buildings.

That's great, thanks for the expert point of view Mick. The absence of the certificates you mentioned is telling a tale in itself.

Regarding the installation of the airtight membrane; Do I understand you correctly that the intent would be to make the attic airtight from the outside world with the airtight membrane or were you proposing to install an airtight membrane on the attic side of the dormer stud wall (where the shiny bubble wrap layer would have otherwise gone)?

I hope that question makes sense?

MicktheMan

Not sure if I understand your question but generally you need to first define where your thermal envelope is and then insulate and air tighten that envelope. The air tight barrier needs to be on the warm side of the insulation and needs to be continuous i.e. fully sealed to other element airtight barriers such as external walls / windows etc. This may seem difficult to achieve in an existing dormer but by having (i.e. experiencing) the air tight test done first, the solutions tend to be self explanatory and readily achievable for a competent diy'er.

In the wind

MicktheMan wrote: »

Not sure if I understand your question but generally you need to first define where your thermal envelope is and then insulate and air tighten that envelope. The air tight barrier needs to be on the warm side of the insulation and needs to be continuous i.e. fully sealed to other element airtight barriers such as external walls / windows etc. This may seem difficult to achieve in an existing dormer but by having (i.e. experiencing) the air tight test done first, the solutions tend to be self explanatory and readily achievable for a competent diy'er.

OK, got it. So I can decide where the envelope boundary is. That helps thank you.

I would tend to leave the dormer attics open to air and Form the airtight envelope on the stud wall of the dormer attic.

Lumen

In the wind wrote: »

OK, got it. So I can decide where the envelope boundary is. That helps thank you.

I would tend to leave the dormer attics open to air and Form the airtight envelope on the stud wall of the dormer attic.

What about the ceiling under the void formed between the stud wall and the roof?

In the wind

Lumen wrote: »

What about the ceiling under the void formed between the stud wall and the roof?

Currently 80-100mm of fibreglass roll. I'll add another 100 after I have finished some planned electrics works.

i'll also set up a raised platform above the 200mm of fibreglass for christmas tree & all the other sh!73 we have accumulated.

there are loft stilts available for this purpose on screwfix. they're called "diall-loft-storage-stilts-210mm-12-pack"

great idea in my opinion.

In the wind

Outkast_IRE wrote: »

Did this in my parents house . Rockwool flexislab is specifically made for this , no sag , flexi edge on one side, excellent fire resistance , good acoustically, easy to diy

never heard of it. I'll take a look. thank you.