Insulate attic at Rafter or Floor Level?

Blisterman

Doing some refurbishment work on a house to improve air tightness and insulation throughout, as well as installing MHRV The attic will just be used for storage, though there will be two lightwells bringing light from a rooflight to first floor level, built as stud partitions.
Trying to decide which is better, to insulate at attic floor level or rafter level.

Here's how I currently see it:

Advantages to floor level:
Easier and cheaper
Reduces the volume of air to be heated throughout the house.
Less tricky to ensure sufficient ventilation to roof timbers.

Advantages to Rafter Level:
Water tank won't have to worry about freezing.
Heat isn't lost through the attic stair hatch and below the water tank.
Don't need to insulate the stud walls to the lightwells.
Items stored in attic won't have to contend with temperature and humidity fluctuations.

Is there anything I've missed? And I'm not all that sure which is better for air tightness and MHRV, and which carries greater risk of mould and damp.

The lightwells mean that the total surface area to be insulated is roughly the same.

Calahonda52

ideally the MHRV unit should be within the conditioned space so where is the unit being located?
If doing ceiling, are light well walls insulated?
What is directly under the slates/roof tiles?

Blisterman

Spoke to the MHRV supplier and they said the difference between it being in a heated or unheated space was negligible.
The light wells will be insulated if insulating at floor level. That's one reason I think it'd be easier to form a continuity of air tightness and insulation by having rafter level insulation, as the interfaces become a bit simpler.
The roof is non original cement slate tiles on a sarking felt on rafters.
Edit: It'll have to be a cold roof insulation as replacing the tiles is not an option.

Calahonda52

Blisterman wrote: »

Spoke to the MHRV supplier and they said the difference between it being in a heated or unheated space was negligible.
The light wells will be insulated if insulating at floor level. That's one reason I think it'd be easier to form a continuity of air tightness and insulation by having rafter level insulation, as the interfaces become a bit simpler.
The roof is non original cement slate tiles on a sarking felt on rafters.
Edit: It'll have to be a cold roof insulation as replacing the tiles is not an option.

.
I posted something on it many years ago here.

this is one article
https://www.greenbuildingstore.co.uk/mvhr-dos-donts-where-to-locate-mvhr-unit/

Will need to leave 50 mm behind rafter insulation and vent at or close to eaves

BryanF

OP,
Would you mind doing an internal volume calculation for us,

What is the

1. House (l * w * h) = X m3

2. attic (l * w * (h/2) = X M3

What is delta/ % extra volume your heating and recovering air from?

Will this increase the size of your mvhr system?

Blisterman

1. Excluding attic, roughly 400m^3

2. Attic is roughly 50m^3

So an increase of 12.5%.

I assume, since the attic will be better insulated than most of the rest of the house, that it doesn't necessarily mean a 12.5% increase in heating bills? Especially since I won't have any radiators inside the attic space.

The MHRV was originally priced to include an attic, so it's more a question of whether I can downgrade.

Blisterman

One additional thing: The rafters are only 70mm deep, so having spoken to Kingspan, they recommend leaving the entire rafter depth uninsulated to allow for ventilation, and to insulate 120mm K7 Board on the underside only.

Would I be setting myself up for all sorts of condensation and damp problems in doing this?

BryanF

Where is the vapour barrier/AT layer with the KS proposal?

Blisterman

Vapour barrier is on the warm side between the insulation and plasterboard.

BryanF

Blisterman wrote: »

Vapour barrier is on the warm side between the insulation and plasterboard.

You mean sandwiched between the plasterboard and insulation?

Blisterman

Yes, since there isn't an insulated plasterboard which would achieve the U value required.