External insulation choices

[Richard Owen]

Here at YTC we have a schizophrenic internal climate. The massively insulated timber frame extension is lovely all year round, but the solid walled original cottage does suffer by comparison. Even though it now has 150mm of solid foam in the roof.

The Supreme Commander and I were knocking ideas around regarding internal insulation for the cottage. Then one of us suggested external insulation. And the idea wasn't thrown out.

I've only got four PV installs to get done before the deadline so my mind is turning to what to do with the rest of December. And I was wondering what the collective wisdom was on external insulation?

I could get 50mm of foam on without too much trouble. More than that and I'd have to take the roof off to extend the rafters.

What would others do?

[desperate]

Richard, we have a similar situation here at Cactusville, I am probably going with 5cm of celotex or similar EWI which will reduce the U value to about 0.3 from about 2.4 w/m2/K. Wookey has argued that it would be better and not much more trouble to go for 10 cm and a value of 0.18. My feeling is that the extra 5cm is firmly in the region of diminishing returns, loosing 5cm around all the reveals and various other details is relatively easy, but loosing 10cm is another ball game. Of course it all depends on your specific building detailing. For us 5cm looks preferable, but that is based on me and one of my chaps doing it DIY, if you are paying a contractor to do it, it may put a different slant on your options.

Desp

[Richard Owen]

Paying?

Where would be the fun in that?

[stannn]

Desperate
Of course you are not forced to have the same thickness of insulation at the reveals as there is on the walls.
Richard
I was trying to imagine wall insulation tapering off at the gutter, to avoid the roof being extended, and the wall being tile faced for instance. It's not very practical at the eves though.
Stan

[Bodidly]

I have just been insulating the outside of my parents house. The build up was 3x2 timbers fixed at 400mm centers with 70mm celotex between with slate hanging. I know there is an issue with thermal bridging on 3x2 wood but the results are far better than doing nothing. To overcome the problem with extending the roof I have just lapped a lead flashing over the top of the insulation and under the barge boarding. I did take a few pictures and could try to post them if that would help.

Beau

[wookey]

Bodidly, some pics would be good. A top lead flashing is the normal way of dealing with this where the roof is not extended.

Desp - you are quite right that 50mm is 8 times better than existing, whilst 100mm is 'only' 13 times better (i.e only 60% better than 50mm).

The way to decide is to remember to take a 15-20 year view. In 20 years time will you be really wishing you'd done it better? Will U=0.3 be looking a bit crappy by then and in need of an upgrade? Also if 'doing it properly' means that in fact it's simply not going to get done in the forseeable then that's a good reason to take the easy option. Just so long as you've thought about it and made a choice you can live with and defend in the future.

[dhaslam]

Is there anything left to loose heat  in large amounts  if the walls are  well insulated?    The roof is easy enough but what about floors, windows and ventilation?   

[Bodidly]

Some photos of the slate hanging.

Beau

[dtl]

I know someone who is considering external insulation.

They live in in a old field stone house will rubble infill walls.
The floors are suspended wooden floors with underfloor insulation.

It seems to me that if a house has underfloor ventilation then there is not a lot of point putting insulation on the exterior walls, since the underfloor ventilation allows cold air to circulate under the floor and within the interior wall cavities.
This then negates the external insulation?

[Bodidly]

Second time lucky

[wookey]

Very neat job. Well done.

DTL: yes, houses like that are much harder to treat. You almost certainly want to use vapour-permeable insulation to avoid issues with making the structure all soggy with reduced ventilation. Hemcrete is a good option, and gutex do a fibrous flexible insulation that is very good for conforming to irregular-shaped walls and then rendered over.

The bottom side of a ventilated floor should be treated as outside, but if the floor is insulated then putting EWI on the walls should still work. You have thermal bridging where the wall goes past the floor and you may have thermal bypass if that wall is full of holes. Is it? Exactly what to do depends on the details.

The best thing to do with such building is probably replace the suspended floor with a concrete one so you can then seal the building up (from airtitghtness POV) much more effectively. You still want to leave it vapour-open as the walls won't have any DPC.

[billi]

to go through the whole  idea/work of external insulation  and then just use 50 mm  ?

Still i  would consider  trompe  wall ideas as well  , or   a big  glass extensions and use the natural walls as heat-stores  and the extension  as a new habitat  and circulate warm air through the building  and let the non insulated ones absorb it

But have no idea  how that relates to the U value

Billi

[Richard Owen]

Thanks for the ideas so far.

Those photos were really useful.

We're going to have to have render as the external finish, so I'll need to think about an appropriate backer for that.

Billi, I agree with you that 50mm seems weak beer compared to other insulating things we do (we've got 250mm of Warmcell in the extension walls and 350mm in the roof. Even the floor has 100mm of PU foam.) but the balance is whether I've got the enthusiasm to take the roof off (for the 3rd time) just to extend the rafters 100mm or so and put it all back on again.

I don't like the idea of the lead flashed wall top. It's not the aesthetic, it's more to do with connecting the insulation in the roof to the insulation in the walls to produce a complete shell.

Also Billi, the house is at the North end and the extension is at the South end. There will be a conservatory at the South end but moving sufficient warm air to the North is a bit of a pain.

[davec]

We're renovating a stone farmhouse. EWI will be 180mm wood fibre, giving 0.22 W/m2K. The existing tiles are goosed so we're stripping these and building on top of the existing structure to increase the overlap at the eaves: 140mm wood fibre, then 50mm gap, sarking, slate; with another 125mm between the existing rafters, we should get 0.15 W/m2K for the pitched roof.

[Baz]

Excellent photos of a type of job not (as yet) seen a lot on the forum. Thanks.
In defence of the insulation gap at the top it will possibly help prevent ice dams in bad snow and bearing in mind some people install special electric heaters in the roof to avoid that it might not be such a bad idea.