Full fill or Partial fill insulation boards

pajosjunkbox

As the title suggests, does anyone have an opinion on what's best. Engineer is recommending 120mm full fill but a few people I've mentioned it to including the blocklayers are not so keen.

What would you go for ?

sydthebeat

pajosjunkbox wrote: »

As the title suggests, does anyone have an opinion on what's best. Engineer is recommending 120mm full fill but a few people I've mentioned it to including the blocklayers are not so keen.

What would you go for ?

What alternative do the blocklayers suggest? And does their alternative offer the same u value?

pajosjunkbox

Blocklayer suggesting leaving an air gap to protect from weathering. I'm just really wondering is one method better than the other or is it more or less even.

Gumbo

I'm seeing 150mm full fill on most new builds now.

sydthebeat

pajosjunkbox wrote: »

Blocklayer suggesting leaving an air gap to protect from weathering. I'm just really wondering is one method better than the other or is it more or less even.

So is the blocklayer suggesting 120 partial fill with a 40mm gap? And if so do they realise the consequences of building a 160mm cavity?

Is this a new house build?

B-D-P--

Blocklayers do this as its an easier job for them.

Engineers(those with degrees and research done) suggest 150mm because its a better job.



@Syd, whats so bad about 160mm cavity btw? I'd be more inclined to be getting the better insulation, but no so worried about the cavity gap.

pajosjunkbox

This is a new house build. Engineer specced 125mm full fill Xtratherm. When I was talking to blocklayer he didn't give an alternative spec , just said he would go for leaving a gap to allow for weathering.

I know if the cavity is too wide special ties are needed and and there is more input from engineer needed.

From reading above I see 150mm full fill being used. I wonder if 125 mm is enough. How much does the law of diminishing returns work here ? When does that extra bit of insulation become redundant in terms of payback ?

Gumbo

B-D-P-- wrote: »

Blocklayers do this as its an easier job for them.

Engineers(those with degrees and research done) suggest 150mm because its a better job.



@Syd, whats so bad about 160mm cavity btw? I'd be more inclined to be getting the better insulation, but no so worried about the cavity gap.

Nothing wrong with 160mm Cavity as such, but once you go above 150mm, the 2 leaves of the wall could be considered separate walls and need to be structurally designed by your Engineer. You then also need specialist wall ties as opposed to the standard ties.

Eurocode 6 and SR325 I believe are the relevant codes.

Metric Tensor

There's a few competing issues at play here:

1. Weather protection.
There is no doubt that a cavity void is hugely beneficial in this regard. Any moisture that gets through the render* and the outer leaf of blocks cannot cross a cavity void so the water ingress stops there. If the cavity is full of insulation there is the possibility of this insulation acting as a route to transmit moisture across the cavity to the inner leaf. Different types of "full fill" insulation have different ways of addressing this potential.
(* The external render is not designed to be a waterproof layer but often is at least partially and I'd wager is doing this job in many full fill cavity houses even though it hasn't been designed for this purpose. The problem is what happens once it cracks?!)


2. Insulation and thermal performance
As the requirements for insulation have increased year on year in order to meet the building regulations thicker and thicker insulation is needed and hence the cavity gets filled up** and needs to be increased in width to accommodate the amount of insulation required - and increased even further if a cavity void is required also. As well as the increased cost of building a wider cavity - point 3 below comes into play. The insulation is further complicated by a process called "thermal looping" - if there is cold air movement over the face of the insulation layer, as there would be with a cavity void, then the heat that does get through the insulation is dragged away from it thus slightly increase the rate of heat loss through the insulation.
(**I am talking about insulation in the cavity of a masonry wall here - there are other ways of doing things but that's a discussion for another day!)

3. Structure
kceire has put it very eloquently above. There is a point at which a two leaf cavity wall becomes two separate thin walls rather than one thick one. This depends on the cavity width, wall tie type and installation, relative thicknesses of both leaves and the materials used. As cavity widths increase the cost of making the entire build-up work as one unit go upwards and at a certain point the outer leaf becomes little more than a rain screen with the inner leaf being the "wall."


So everyone doing a wall design has to balance 1, 2 and 3 and the balance varies depending on the type of building, quality of the builder, location, etc.

pajosjunkbox

Thanks for the detailed response. I think you bring up a good point with balancing it all. For my new build I am looking for a balance between well insulated , budget, practicality when building etc. A happy medium of some sort.

Some of my outer block wall will be clad in timber. How does this compare to render in keeping out rain ?

Metric Tensor

If there's a gap between the timber and the wall then it's better than the render (It's essentially a rain screen provided whatever supports it on the wall does not transmit the water across).

If there is no gap then it's a non-runner.

Would you consider rendering behind the timber cladding?

pajosjunkbox

I would consider rendering behind. I would have assumed we would have attached the cladding to latts so that would mean there would be a small gap between cladding and blockwork .

Metric Tensor

A small gap would be fine but depending on the way the laths are installed they (and their fixings to the wall!) might provide a path for moisture movement. Think of the way wall ties have a kink in the middle to stop exactly that sort of thing.

BryanF

Op is this an exposed site?

pajosjunkbox

Not overly exposed. In a bit of a hollow and I planted approx 600 trees to the south of site 3 years ago to screen off house from the road.

pajosjunkbox

Had a kingspan rep just bring me out of the blue. Kingspan obviously dont do full fill boards as he was steering me towards 100 mm partial fill board.

I'm guessing 125 mm full fill is better than 100mm partial ?

BryanF

150 full fill is better than 125mm

250 pumped beed cavity is safer than full fill, less conc snots and less continuous gaps in insulation

225 timber frame (4x2 gap 4x2 with full width sole and header) cellulose fill

...... and so on