Earth Notes: On Super-Insulating Our Smallest Bedroom and a Half

Retrofitting interior aerogel wall insulation (IWI) to our smallest bedroom in October 2011...
aerogel magnesium board in window reveal

Following on from the apparent success of our project to superinsulate our living room with aerogel in 2010, we took the opportunity presented by converting our boy's nursery to something a little more grown up to superinsulate its external wall, which is only about 4m^2.

We'd already insulated better above it by topping up the loft insulation, and below it by insulating abover the living room below and reducing air infiltration from outside with foam.

Doing just this one small wall shouldn't require building control (with October 2011 part L reg applying) especially as our target was a U-value of 0.3W/m^2K or lower, though I did exchange emails with building control (BC) at the council back in July to make sure.

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Magnaline Superslim

This time, since my aim is in part to provide a case study to help others plan (or plan to avoid, I suppose) aerogel-based IWI/drylining, I went with a different brand of board, Magnaline Superslim magnesium board from Enviroform Solutions.

(Magnesium board aka Magnesium oxide wallboard aka MgO board, used in place of traditional plasterboard / gypsum drywall.)

To reduce the chance of thermal bridging by the partition wall between the boy's room and the girl's (larger) bedroom next to it to the outside, and because it would require only 50% more drylining material to do, a slot was cut in the partition wall between the rooms next to the exterior wall and the aerogel was continued though that along the west wall to the edge of the north wall in the girl's bedroom. In that way both those small rooms had their west-facing exterior walls drylined at the same time. At some point in the future we may dryline that (larger) north wall to vastly improve overall thermal performance of the girl's room.

This time, as our bedrooms are heated less of the time, and to a lower temperature, I went for 30mm of aerogel rather than the 40mm in the living room. According to our calculations based on the assumed external wall construction and its implied U-value of ~0.8W/m^2K, that should bring us well under the regs' 0.3 U-value.

Wall Buildup

On exploration we discovered that there was less mineral fibre than we had believed, 25mm rather than 50mm, giving a U-value more like 1W/m^2K from this construction:

INSIDE
12.5mm plasterboard @ 0.140W/mK
Plastic membrane (assumed to be VCL)
~25mm mineral fibre @ 0.040W/mK
~12mm? (Ply)wood outer skin @ 0.140W/mK
Tiles hung on wooden battens
OUTSIDE

Removal of the existing plasterboard (to save space in a very small room) gets the wall to pretty-much spot on 0.3W/m^2K, >3x better:

INSIDE
5mm plaster bonding layer and skim @ 0.160W/mK
9mm magnesium board @ 0.140W/mK
30mm aerogel @ 0.013W/mK
Plastic membrane (assumed to be VCL)
~25mm mineral fibre @ 0.040W/mK
~12mm? (Ply)wood outer skin @ 0.140W/mK
Tiles hung on wooden battens
OUTSIDE
finished room

The board has a foil VCL in front of the aerogel (behind the magnesium board). All gaps were foamed, then the wall primed with Gyproc Drywall Sealer to form an inner VCL, all with the aim of preventing moisture migration through the board and condensing in the woodwork with the risk of rot.

The builders reported the board to be easier to cut and drill (less snagging) than the Spacetherm, but more difficult to fix/level because the reduced 'give' of the magnesium board caused screws to pop and not lie flush.

(I believe that the aerogel in each case is the same Aspen Spaceloft blanket, though presumably slightly reformulated as this stuff was blue/grey whereas the Spacetherm was white.)

We also avoided penetrations to the outer wall by removing a socket in the larger bedroom, surface-moutning power sockets in the both rooms, and moving the radiators to the internal partition wall (also to avoid pushing heat out through the external wall quite so fast).

Curtain rails were mounted on wooden battens to minimise point loads on the magnesium board.

Magnaline insulated window reveals (containing 10mm of aerogel) were fitted in both bedrooms, with a pine architrave/reveal fitted over that.

There was quite a lot of air inflitration visible around the windows when work was in progress, so non-expanding squirty foam was applied to fix.

Monitoring

Using iButton miniature temperature monitors to check thermal performance revealed a severe air-infiltration problem under the floor in the room before the work had even finished, tying up with evidence of infiltration above the living room ceiling (ie the same void) in thermal imaging. Foam was applied to reduce that issue.

As of 2014 the house is being monitored in real-time, and for example a sample from overnight 2014/11/13 showed that this bedroom's temperature (2b green line in the graph) stayed steady overnight ie was at equilibrium with one occupant (~100W heat input) and a temperature differential of ~6.5°C from outside (ignoring heat flows to the rest of the house).

Update 2017/01/08: I just happen to be looking at the data for overnight... With an external temperature of ~8°C, and the kids in their bedrooms, the temperatures in the rooms were steady or even slightly rising at ~18°C, which is the insulation doing its work, ie ~5W/K loss for each entire room.

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