Block Outer Skin Types

[climbinggeorge]

There are a few restrictions around our design due to budget, skill and lenders.  Before I meet with an architect I'm trying to work out the most info I can and some trying to gain some opinions on blockwork outer skin.

The plan is for myself and my dad to lay the blocks then have someone more skilled render them
 

So we are looking at a 150mm timber frame stuffed with rockwool at the moment with a blockwork skin, the best block summary I've found are here: http://www.greenspec.co.uk/building-design/blocks/

 

Currently a product called Fibolite seems to tick a lot of boxes for being light enough to lay and ok to render on?
Are there any other options I should know about? 

[ProDave]

You will need to work out the U values of the wall, but I think with just a 150mm frame (filled with insulation) a cavity and a blockwrok skin, you will struggle to meed building regulations.  It is now considered normal to put a layer of Kingspan type insulation on the inside of a timber frame of that thickness.

 

Remember the blockwork skin is mostly an expensive rain shield. It adds very little to the insulation as the cavity has to be ventilated, and you are not supposed to fill the cavity between a timber frame and a block skin.

 

What is the motivation for a blockwork skin? is is just appearance or "that's what we do"?  You could consider what I have done, a 195mm timber frame, and 100mm wood fibre external wall insulation board which directly takes render.  Same overall wall thickness but much more insulation. Fitting the wood fibre board was DIY and I just got somebody to render it.

[climbinggeorge]

Cool thank you, I assumed I'd need some further insulation but hoped there was some super-duper blocks that might do it.
Other options include not using rockwool but that choice was driven by it being easier to fit.

Only based on the restriction of lenders, it was down to 1 if we use board or 4 options for block work

[Jeremy Harris]

We have a 300mm thick timber frame, filled with cellulose insulation and battened and clad with larch on the outside.  No blockwork, brick or whatever and no problem with either insurers or mortgage lenders. 

 

It's true the that online quote systems that insurers use, and that some lenders use, can't cope with what they call "non-standard construction", but that doesn't mean that it's either impossible to insure or mortgage, or even that it's more expensive to insure or mortgage, it just mean you have to ignore the online stuff and deal with either a broker or directly with a lender. 

 

The curious thing is that we've found that going through a broker for insurance has been a fair bit cheaper than insuring our old block and brick house using an online direct service, to the tune of around 30%. 

Edited May 3, 2017 by JSHarris

[ProDave]

The thing about a block skin on a timber framed house, is building regs require the cavity to be ventilated, so even if insulationg blocks were used, the cod air able to enter the cavity would negate most of any extra insulation provided by the block skin.  That's why I wanted to avoid it.

 

The other advantage of an EWI system with render is because there is no cavity, there are no vents needed, so you get a crisp clean render, not interrupted by rows of vents you get with the render on a blockwork wall.

[climbinggeorge]

This had been through a broker, also restricted by needing 85% advance

 

Good point on the cavity being required, maybe I need to look at a better alternative to rockwool on the inside then or for that to be thicker

[Jeremy Harris]

There are a few key things about insulation and it's effect on the way the house performs, and, perhaps more importantly, how comfortable it is.  The thing that is very often overlooked is that good insulation on it's own doesn't make a house comfortable, even though on paper it may seem that the heat loss is low.  Having a high decrement delay factor is as important, perhaps even more important, than just having high performance insulation (and our basic building regulations requirements are definitely not high performance!).

 

Decrement delay is explained fairly well here: http://www.greenspec.co.uk/building-design/decrement-delay/

 

If you choose to have a brick, block or other form of masonry external skin, then, although it does next to sod all in terms of decreasing heat loss, it will act to increase the decrement delay.  This then means that very low decrement delay, but high thermal resistance, insulation, like PIR foam, can be used to get a relatively thin, high performance, insulation layer internally, relying on the masonry outer skin to help increase the decrement delay.  It's a far from perfect solution, as the external skin isn't really where you want to the high heat capacity layer at all, but it does make bodging things to use conventional, pretty poor, building methods can be made a bit better.

 

The better solution is to use a reasonably high decrement delay structure, that inherently provides good thermal performance as well.  This then increases the thermal time constant (how long it takes the house to cool down in winter, or how long it takes to over-heat in summer) to the point where normal diurnal temperature changes have little effect; it takes a prolonged period of cold day and night weather for the house to need any heating, and conversely needs prolonged high day and night temperatures for it to over-heat.

 

Rock wool is a pretty average insulation material, as it has average thermal resistance and a pretty short decrement delay.  Wood fibre/cellulose has a similar thermal resistance, but with a much longer decrement delay, so tends to give much better performance in terms of real comfort, rather than the heat loss numbers on a bit of paper.

Edited May 3, 2017 by JSHarris

[climbinggeorge]

Had it confirmed with the broker that only timber frame and block is ok with the lender.
Which in a way makes it easier, that becomes a fixed problem and now I can work out how what the best method within those parametres is

[Jeremy Harris]

That's strange, as we had no problem with all-timber construction, and around here there are lots of all-timber houses, some that are a couple of hundred years old or more, and no one seems to have a problem getting a mortgage on them.

 

I know for sure that the Ecology Building Society have no problem at all with all-timber, or timber with cement board cladding, and two other lenders we asked had no problem either (I think one was the Leeds, but can't be 100% sure).  Even our pretty useless bank were OK with lending on an all-timber house (not that we used them or would recommend them in any way, shape or form).

[climbinggeorge]

It's bounced back and forward due to other restrictions (without trying to go off topic) but now looking at timber frame and board with a different lender, thanks for advice