What do you think of this cavity wall spec?

T

TheBoy

Guest
Hi all.

This is my 1st post but I have been following certain threads now for a while. I am building a new house & have been considering how to go about it. After considering timber frame & icf I have decided to go with a block cavity wall but to try and do it as best as I can.

Again I have considered putting insulation boards on the inside & have ruled out external insulation as I have heard it is very expensive. Quinnlite blocks have been considered but are again expensive & are hard to work with re plaster etc & have ruled out poroton as I don't know much about them.

Therefore I am thinking of 100mm Alphatherm blocks on the inner leaf, a 150mm cavity with platinum ecobead- I would prefer not to use Kingspan etc due possible thermal looping. I have also sourced wall ties in England that apparently conduct heat 20x less than stainless steel ties & are about the same price & they will ship for free.

I worked out a U value but don't know if I have done it right. Please see what you think. Please let me know if there are any major flaws & any other things I need to consider to improve.

Layer & Resistance (m2K/W)

Ext surface 0.040
Ext render 0.038
Dense conc block 0.074
Platinum ecobead 4.545
Alphatherm lightweight
block (Acheson&Glover) 0.256
Int render (gypsum,sand) 0.023
Plaster skim 0.017
Int surface 0.130
TOTAL 5.123

U value = 0.195?
 
Google Builddesk. You can download a free 30 day trial of a software package that works all this out for you and you can mess around with different materials from their database. It also does a condensation risk analysis although I doubt you will need from your proposed layout.
 
those calcs assume a thermal conductivity of the ecobead at 0.03... have you seen a cert stating this is correct....
any certs ive seen have polystyrene beads at 0.033 lowest...

also... if you are using regular mortar between the alphatherm blocks you need to include this in you calcutions as thermal bridges as well..
if you are not using regular mortar and using a thinbed system, then i would question whether your wall ties will work... the course levels will be off....

have you also included fo rthe thermal bridges of the wall ties???
also, according to the tie datasheet, they do not as yet have a BBA cert....
no cert = you cannot use them and still comply with building regulations...
if theres no cert, thers no proof they do what they say they do....
 
A friend of mine got a sample of those wall ties and showed them to his engineer. His engineer didn't believe that they would sufficiently tie 2 leafs of block work together. He wasn't impressed with them at all I believe.

Be very careful... you are likely to be much better of with regular (albeit larger) wall ties.
 
we were originally thinking timber frame but now are moving towards a block build after the last meeting with architect. What would any of the experts here recommend as a wall spec for a block built, airtight, MHRV house. Is drylining with insulated plasterboard worth doing? or should we just use more insulation in the wall? Also, what size cavity should be left, what type of insulation etc.
 
sman said:
we were originally thinking timber frame but now are moving towards a block build after the last meeting with architect. What would any of the experts here recommend as a wall spec for a block built, airtight, MHRV house. Is drylining with insulated plasterboard worth doing? or should we just use more insulation in the wall? Also, what size cavity should be left, what type of insulation etc.
Click to expand...

sman, you really need to get contruction specifictions written up or drawn up by an architectural technician with input from a BER assessor (or preferrably a technician who is also an assessor.)

a building needs to be organised and planned dynamically with all factors considered as one integral chain of systems...

-the initial design for solar gain and passive cooling
-the element constructions (ie wall, floor, roof)
-the windows and doors
-the ventilation system
-primary heating system and controls
-secondary heating and controls

laterations to one factor will affect others...
therefore it is disingeuous to comment on one without know what the others are...

get professional help on this...
 
Yes the ecobead website gives 0.033.

Will be using regular mortar. How much do I need to allow for this & wall ties? I think I read somewhere that the Swedish use a different calculation for U value which allows for adjustments?

Yes I would be sceptical of the ties without a cert. However the company told me they have been used in my area so must have been passed by Building Control. Apparently the cert is expected in April. I have received samples but to be honest I couldn't tell whether they'd be good or bad- they are certainly made of a weird material. If I can't use them, then what is the best alternative? I've heard that stainless steel helical ties are good due to low cross-sectional area?

As cavity walls go, do you think this is ok?

I am building quite a large sunroom with many windows & high ceiling. My builder has agreed that it will be cold when the potbelly stove is not on. He has recommended UFH for this room & the kitchen area saying that it will make it warmer. Is this likely to be the case? I've heard UFH is very sore on oil. Would there be any alternative ways of increasing the warmth of the sunroom? Use insulating plasterboard on inside?
 
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