Additional insulation?

[Moira Niedzwiecka]

We have a closed panel timber frame system that includes the roof structure, specification as below:

300mm Ibeams

external sheathing 9mm OSB 

external membrane Proclima Solitex plus lapped & taped with Tescon Vana 

25x 50mm treated battens

300mm warmcell 

internal sheathing & racking 12.5mm smartply taped at joints with Tescon vana

internal service zone 45x47mm battens to internal face.

Gives approx 0.15w/m2k

The roof covering is a sarking membrane with 25x50mm counter battens & a natural slate covering.

Question is should we add further internal insulation between the battens of the service zone before the plasterboard?

The roof is part vaulted & part with ceilings.

I was thinking 50mm of rockwool or similar.

Any thoughts/suggestions welcome

 

 

[PeterW]

I’ve got to ask the question why .... The heat loss is more via air tightness and ventilation loss - 40% has been bandied about - and having the membrane to hold the insulation in is pretty much helping you achieve that. 

 

I would expect that it would take a significant number of years to even get close to a payback for installing any additional insulation. 

[Moira Niedzwiecka]

Thanks Peter,

I was just thinking that at this stage is was an easy & relatively inexpensive thing to do.

I should have added that our house is a chalet style so we have a large area of the roof within the bedrooms.

The theory was to make it doubly snuggly as we were not planning to put any direct heating within the bedrooms.

[gravelld]

With 300mm cellulose it's likely you need to pay attention to air tightness and bridging as Peter suggested. I bring bridging up, because the way you described the build up I'm not sure how the timber frame is encased.

[ProDave]

Are you sure that U value is correct? I though 300mm of Warmcell would be nearer to 0.1?

 

I see no problem with filling the service voids with 50mm rockwool batts, but leave a gap where cables run so the cables are not embedded in insulation. That would be my choice rather than solid insulation.

[Jeremy Harris]

We have 300mm of warmcell in the walls (twin stud, so near-zero thermal bridging) and 400mm in the roof (it's a room in roof design).  The wall U value is about 0.12 the roof is very slightly better than 0.1.

[A_L]

You will benefit from the extra thermal resistance of the mineral wool but will lose the thermal resistance of the unventilated cavity (service zone). This will reduce the added thermal resistance by about 13%. Assuming a lambda of 0.036 for the mineral wool the U=0.15 will fall to about 0.13, saving around 1 to 2kWh/m² /yr (Full SAP model)

[ProDave]

6 minutes ago, A_L said:

You will benefit from the extra thermal resistance of the mineral wool but will lose the thermal resistance of the unventilated cavity (service zone). This will reduce the added thermal resistance by about 13%. Assuming a lambda of 0.036 for the mineral wool the U=0.15 will fall to about 0.13, saving around 1 to 2kWh/m² /yr (Full SAP model)

Can I hijack your knowledge here.

 

We have one bit of wall, between the house and the garage that has a worse U value because unlike the rest of the house, it does not have the wood fibre exterior insulation.  I was going to improve that by insulating the service void as this post suggests.  But it sounds like from what you are saying, I would be better leaving the air gap service void, and adding any extra insulation on the garage side (before the 2 layers of plasterboad) Is that correct?

[Moira Niedzwiecka]

Thank you for your replies.

Sorry I made a mistake our roof & walls are both 0.13.

The smartply is the airtightness layer.

The photo is the large dormer that sticks out the front of the house & will remain vaulted.

This is where I was thinking of putting the additional insulation between the battens before the plasterboard or birch ply (not decided which one yet)

[A_L]

20 hours ago, ProDave said:

But it sounds like from what you are saying, I would be better leaving the air gap service void, and adding any extra insulation on the garage side (before the 2 layers of plasterboad) Is that correct?

 

@ProDave , in theory yes but in practice it is possibly more complicated. Your current garage wall has a thermal resistance of about 4.5K.m²/W (15mmOSB 190mm Frametherm/timber), the unventilated service void only contributes 0.18K.m²/W,  or about 4%.  82mm of Frametherm 35 has the same thermal resistance as 100mm of Pavatex, so 100mm in practice. Putting this on the outside will 1) lose floor area in garage and 2) require larger timbers to attach plasterboard?
If the service void is 47/50mm I would put 50mm Frametherm here and 50mm on the outside, adding 2.68K.m²/W (2.86-0.18) to the thermal resistance where Pavatex would have added 2.33. If the service void is only 25mm I would put 25mm of PIR/PUR/Phenolic in there with 50mm Frametherm on the outside, adding about 2.25K.m²/W  (1.0+1.43-0.18)