External insulation by building timber frame wall?

MichaelR

Conventional external insulation involves insulating boards fixed onto a wall, then reinforced with mesh and finished. All the materials are expensive and the result might not be very resistant to external impact (and I expect a lot of external inlact with two kids in the house and a wall facing the bakc garden).

I just thought of another method of external insulation that sounds remarkably simple. I wonder if there is a reason people don't do this...

Basically:

- Remove external render (has to be done for anyexternal insulation)
- Fix usual internal-style thick board, Kingspan or similar, to the outside of the existing wall.
- Build a timber frame wall around the wall. Just a timber frame and fire-retardant-treated external-grade plywood sheets on it, with the top covered by a sloped piece of plywood and some roofing felt.

As the new wall protects the insulation, the insulation itself can be internal-type, not very expensive. Also, the space for the frame creates a cavity around the insulation, which is probably also good?

Is there some catch that I don't see here?

EDIT: realized that the cavity might attract rodents. In that case, what if I do a timber sheet flush to the insulation slab, no cavity? Not considering a block/brick wall because there is no proper foundation for it, but a timber structure can be fixed to existing wall, above ceiling level, and anchored in the ground.

EDIT2: more googling finally revealed that the approach does seem to exist, as "insulated timber cladding" and the version with the cavity as "rainscreen insulation", but is apparently not very popular, or at least has very little online exposure?

BryanF

You don't have to hack of render

The money in ewi is the labour what you're proposing is more labour intensive

Rainscreen systems tend to allow air movement in cavity and between insulation reducing thermal effectiveness

The recent London tower fire was a rainscreen system

Lumen

I've been pondering the rainscreen approach myself for a retrofit of a 1980s block-built house, as I really hate the look of EWI and also don't like the fragility (it's a three-little-pigs mentality).

It seems a lot of effort for an ordinary render finish; I think it makes more sense when you want timber cladding or a panel system.

http://www.greenspec.co.uk/building-design/rainscreen-cladding/

I did have one of those "sounds great at 3am" ideas of a "bell jar" retrofit whereby the wall sections would be pulled out to 1m depth and Dali-eque staircases could be put into the outer fabric weaving around full height walk-in windows, but once the sun rose it seemed a little ambitious.

Lumen

This is a pretty cool example of rainscreen system.

http://www.archdaily.com/600843/golden-view-residence-workshop-ad

It's in Anchorage (Alaska), I wonder how the timber cladding is holding up.

sydthebeat

Ewi are generally sold as "systems" which are certified for use and warranted as such.

If you disassemble the system and introduce you own methods it's highly likely your system would not be certified for use (which would make what your doing illegal) and not warranted (which would make it uninsureable)

If you want to go down the line of timber cladded Ewi then you'll need to find a system that's tested and certified for use in our climate.

MichaelR

https://www.kingspan.com/irl/en-ie/products/insulation/insulation-boards/kooltherm/kooltherm-k5-external-wall-board this Kingspan board is advertised as working with timber cladding (see Brochure link). I will ask around the builder shops for it today.

Calahonda52

MichaelR wrote: »

https://www.kingspan.com/irl/en-ie/products/insulation/insulation-boards/kooltherm/kooltherm-k5-external-wall-board this Kingspan board is advertised as working with timber cladding (see Brochure link). I will ask around the builder shops for it today.

The Devil is in the detail here
from the link

1. Unaffected by air infiltration
2. Resistant to the passage of water vapour

I take 1 to mean airtight, duck like
2 bothers me as it might mean water, i.e. rain which is fine, but being
"Resistant to the passage of water vapour" is something that needs closer consideration: google Solar drive ...

As we seem to be linking stuff here both unabashedly and unashamedly,
http://www.ecologicalbuildingsystems.com/Ireland is always worth a look, and a visit to the show room is recommended.
As for the rain shield, cement panels are the norm here in this damp, drizzly, wet best little country to get wet in!
http://www.greenspan.ie

Calahonda52

BryanF wrote: »

Rainscreen systems tend to allow air movement in cavity and between insulation reducing thermal effectiveness

Yes, but this can be addressed in the math.

BryanF wrote: »

The recent London tower fire was a rainscreen system

Not so sure and I think we need to be crystal on the exact set up here and not rely on incomplete details by journo=heads who have no interest in the detail, or by others who just want the max exposure to their delivery of vacuous blame game sound bites

Two links that are worth a perusal
https://www.reuters.com/article/brit...-idUSL8N1JK2TZ
and
https://www.ft.com/content/0106b7af-...ef7b2?mhq5j=e1

stimpson

MichaelR wrote: »

Conventional external insulation involves insulating boards fixed onto a wall, then reinforced with mesh and finished. All the materials are expensive and the result might not be very resistant to external impact (and I expect a lot of external inlact with two kids in the house and a wall facing the bakc garden).

We had this done 6 years ago. We've had no issues with damage to the finish and we have kids too! The house looks as good as it did the day the work was done.

As for cost, the grant brought the costs down , and having previously been quoted over a grand just to paint the house it made even more economic sense. The finish is maintenance free, never needs painting and can be cleaned with a power washer. What little you might save with a DIY solution you may lose with decreased thermal efficiency when compared to a professionally installed system.

MichaelR

We are not eligible for the grant. Only a small section needs this work (external back walls of converted garage). The house itself has cavity walls and it is warm enough; we are not planning to pump the walls, to avoid moisture bridging.