This summer we did aerogel dry-lining of our living room in our timber-frame house. In conjunction with other measures gas consumption was down ~7% on last year while HDD was up by ~30%, and overall I think we've halved the heating input per HDD over the last 3-ish years. All well and good, but I'm challenged by a friend in the bleak north of the US to do better still. He's cut his heating per HDD something like 75% (including measures such as 1m of insulation in his loft).

Now, ignoring all my previous misgivings before about EWI and a partly-ventilated structure, I'm thinking of EWI (eg 100mm) on the windowless brick-faced north wall of our house, just up from the A in this pic:

http://maps.google.com/maps?oe=UTF-8&q=KT1+3JA&ie=UTF8&hq=&hnear=Kingston+Upon+Thames+KT1+3JA,+United+Kingdom&ll=51.406735,-0.288781&spn=0.000443,0.000848&t=h&z=20

I was previously concerned that:

a) EWI would be ineffective because the ventilation of the void would take heat away from inside the EWI

b) The EWI might retain moisture and encourage rot in the timber frame

Do either of those worries still hold water?

Running next to the north wall is a (barely used) footpath, so I'd have to get some sort of special permission I imagine. Our ground floor level is several feet above the path level, so if I could go right right down to path level I'd be usefully starting to trap some heat in the thermal mass on the warm side of the insulation.

I imagine I'd need planning permission anyway for EWI, yes? (IE no GPDOs apply.)

We're not in any sort of conservation area!

What flavour of EWI should I consider?

Note, BTW, that the bottom west quarter of that north wall is the one that we've drylined internally, so it would have a double dose!

Note also that some kind of tapering would be needed at the top since the gable/tiling hardly extends beyond the wall I think (I'd have to look again in daylight).

Rgds

Damon

Sincerest apologies for the 62min delay Damon I know you are used to better service than this and I don't have a detail, Sorry! But I think you should pump the cavity between the brick and the TF and then externally insulate with EPS.

NOOOOOOOOOO!!!

EWI is pointless on timber frame external cavity wall. Air movement in the cavity will reduce, if not eliminate any usefull contribution from the insulation,

DO NOT blow insulation into the external wall cavity. As much as anything else, if you try and sell in future, you might find trouble shifting it. I have seen this a few times recently. Home owners (unaware that thier property was timber frame) have cavity insulation installed. Come sale time a survey is conducted and this issue raised. The property then becomes un-saleable.

Moisture ingress might be minimal due to lack of windows on that elevation, but also, being a north facing wall the chances for it to dry out will also be minimal.

What insulation is between the studs currently?

In north america, a lot of the cladding systems are 'siding' (i.e weatherboarding (quite often vynal)) so it can be removed and insulation installed. E.g. installing 100 mm eps, new breather membrane and then battens (or strapping as they call it) and new cladding.

Timber

I hear you about filling the 'void'; that was my feeling. And VH might be winding us up! B^>

We only did minimal exploration and we still don't know, but the construction (where we didn't dry-line) may be:

"Early on we made a couple of exploratory holes in the plasterboard of the exterior walls to check the construction, and rather than a void, behind the 12.5mm plaster is a ~0.5" fibre matt enclosed in a plastic lining, which seems in fairly good nick, eg no obvious sagging."

Rgds

Damon

At the moment your bricks are wet/porus/damp and any moisture getting through the brick can theoretically cross the cavity along any ties installed sloping down towards the timber frame or along any mortar that fell on the wall ties. This is why its not recommended to pump the cavities in a timber frame house to allow the ventilation to dry out any moisture that might travel along the wall ties. I don't agree with this as timber frame wall ties are L shaped and the timber frame has a membrane to shed water.

If you externally insulate, the brick now becomes bone dry, the risk of moisture ingress is eliminated, you can then pump the cavity to stop air movement between the 2 layers of insulation because the moisture that could jump the cavity has been eliminated.
The timber frame now becomes similar technically to a timber stud when drylining the inside of a partial filled cavity wall .

You'd better go with the guys advise above with regards house insurance and resale but I can see nothing wrong with it technically!

Water Vapour coming from the inside the house can condense on the cold side of the timber frame, the ventilated cavity is necessary to dry this moisture. If the cavity is insulated and the brick externally insulated then the timber frame is too warm for condensation to occur on the cold side of the timber frame.

Water Vapour won't rot your timber frame, condensation will, by warming the timber frame you bring it away from the Dew Point risk zone and it then becomes similar to your wooden kitchen table, its then no longer subjected to condensation or temperature differences of minus 10 outside and plus 20 inside which makes it move constantly.

We don't see any condensation at all in the newly dry-lined living room except possibly a tiny bit on the glass with the heavy double-lined curtains open when it's very cold out. And I don't think we're anything like airtight enough for whole house MHRV.

I'd be slightly concerned now that we do have the aerogel in that corner of the house that the natural dew point would still be somewhere inside the wall because 40mm of aerogel inside beats 100mm EPS for example.

ST: where were you working? At the secret MI5 lair? Oh, wait...

And gimme a clue how to do that risk assessment: I'm game to try it. (Yes, the Bill people did ask if they could film in our house, but as I had a server room full of computers at the time I declined!)

Rgds

Damon

Weather station I use for HDD calcs is EGLL, ie Heathrow Airport, if that helps.

We use the nursery next to Arrow Plastics! B^>

Rgds

Damon

Hi ST, I wouldn't mind a copy of that spreadsheet if possible please

Great thanks, Cardiff will do: 51°29'N: 03°10'W

Seems that Rockwool is somewhere between 21 and 25 kgm^-3.
So I would have though that you coudl just lay it down.

So where do you want the air to flow?

The bad bit is where moisture laden air is in contact with a surface that is below the dew point temperature. Insulating the roof to the rafters should remove this problem, but then that is what the debate on the 'insulation solid walls' thread is about.
Seems that most people on here like is to leave an air gap below the roof that is also open to the atmosphere somewhere, allowing air movement.
I think, and I am not trying to be controversial here, that to stop condensation being a problem you have to look at the temperature gradient across each component of a wall or roof construction and calculate the risk at each interface.
Reducing RH within the insulated envelope is probably the best way to deal with it really though.

Timber: could you illuminate me further please?

Rgds

Damon

OK, read it, thanks.

1) It says that I shouldn't have done my drylining quite the way I did it.

2) It says that I have to take down the brick outer leaf before cladding, and does note that retaining a drained and ventilated cavity to prevent rot will undermine the performance of any EWI.

Rgds

Damon

We didn't know there was a VCL in the existing wall until the last minute, when everything had been ordered and so we went ahead and attached the aerogel-lined plasterboard over the existing plasterboard (and fastened through it), doing our best to make the inner surface an VCL. So now we have a 40-year-old outer one and a new inner one very much on the warm side, with an overall wall U-value of a little over 0.2W/Km^2.

As the EWI: well, I don't think we want that much fun! But I'll keep thinking about it.

Rgds

Damon