Using 70mm Insulated Slabs

iamlegend2008

Hi

Im at first fix stage of an 1800 sqft west facing dormer overlooking the atlantic (so its quite windy). Im considering using 70+12.5mm insulated slabs on all external walls but have received conflicting advice from the iinsulation salesmen, one says 38mm boards are fine and know need for more and the other says of course more is better.

There is already 60mm xtratherm in the cavity.

I spoke (briefly) to a BER assessor who also felt that there would not be sufficient payback on uysing 70mm boards which would add about 1600 euro to the build.

My goal is to insulate (or possibly overinsulate if thats possible) in the hope of not having to have high heating costs in the future.

My gut feel is that I should use them and I can live with the loss of a few inches in the room size.

I would also be interested in hearing from anyone who has used them to date.

Thanks.

muffler

No such thing really as too much insulation. Id strongly recommend that you use them and without knowing the details of the overall insulation of the house you may need that extra insulation.

Juantorena

I'm considering using a (highly) insulated plasterboard also - someone suggested the thicker you go with these boards - i.e. the more insulation - the more difficult they are to put up. Any comments on this?

sydthebeat

if you have two layers of insulation, the inside one being thicker than the other, then the point of condensation will be on the cold face of the internal one.....leadign to the associated health problems with mould etc.... therefore the thickness of 'dryling' insulation should never exceed the cavity insulation....
and as a cautious approach, i wouldnt even let it equate to 50% of the cavity insulation.... this is because the cavity insulation is hardly ever installed properly thus lowering its insulative properties...

as a matter of practise a condensation risk analysis should be carrie dout on any construction that breaks from the norm.... these 'dryling' constructions show that condensation occurs...

point of reference, if you see ICF units youll notice that the external insulation leaf is always wider than the internal (or at least should be).

also, poroton blocks are created with the insulative property of the block increasing from external to internal, to avoid this very situation....

sinnerboy

better to use the extra funds you are thinking of ( wisely ) spending on getting the building air tight .

There are three systems available - intello , siga and moy isover . Contact them all and appoint a specialist contractor , recommended by the supplier to install the membrane system before you dry line

Especially with your exposed site , what you are think of doing is like putting on a thicker wooly jumper - whereas really you need a wind cheater jacket

Juantorena

sydthebeat wrote: »

if you have two layers of insulation, the inside one being thicker than the other, then the point of condensation will be on the cold face of the internal one.....leadign to the associated health problems with mould etc.... therefore the thickness of 'dryling' insulation should never exceed the cavity insulation....
and as a cautious approach, i wouldnt even let it equate to 50% of the cavity insulation.... this is because the cavity insulation is hardly ever installed properly thus lowering its insulative properties...

as a matter of practise a condensation risk analysis should be carrie dout on any construction that breaks from the norm.... these 'dryling' constructions show that condensation occurs...

point of reference, if you see ICF units youll notice that the external insulation leaf is always wider than the internal (or at least should be).

also, poroton blocks are created with the insulative property of the block increasing from external to internal, to avoid this very situation....

sydthebeat,

So, if my 'most external' insulation is 130mm and the insulation thickness for the plasterboard - i.e. the 'most internal' - is 55mm, I shouldn't be running too much (if any) condensation risk? An on-line u-value calculator I'm using for approximations doesn't indicate this as a risk - but it's always helpful to get another, independent view on these things.

heinbloed

Where did you get a "most external" insulation of 130mm from?
A cold external wall (cavity) is not to be used in the U-value calculation, even though many builders claim that this is the usual way.
"Insulation" is the material that is not ventilated, so not the entire cavity can be considered as "insulation" with an accordingly U-value calculated for. Most on-line U-value calculators take it for granted that the insulation material is airtight. But with our cavities this isn't the case, they're usually punctured at many places all around the building to let air flow/blow through...... The colder it gets outside the more air passes through the cavities.And the more windy it is the more air flows through the cavity as well. And EPS board is not airtight in itself, so an EPS board fitted into a ventilated cavity is by far not as insulating as the EPS manufacturer guarantees.The EPS U-value is only guaranteed for an airtight EPS board, for example plastered or covered on all sides with an airtight material.
Therefore several deductions have to be made on shoddy workmanship and absurd U-value"calculations".
So if the 130mm insulation were calculated on the basis that the cavity itself is insulating then this calculation was wrong. Only if the 130mm insulation are made from "standing air" they can be used according to the manufacturer's guaranteed U/K-value.

iamlegend2008

Thanks Sinnerboy. I like the analogy about the wincheather vs woolly jumper .

I was the impression though that houses could be only made air tight if planned that way from even before construction and Im now about to start first fix. Is it really possible to make a dormer house air tight at this stage (ie after the roof and joists have already been done) ?

sinnerboy

if you have not started plastering ( or plasterboards ) - it is not too late

read through this

http://constructireland.ie/Articles/Planning/Fingal-Council-Housing-pushes-up-energy-standards/Page-4.html

and study these details - esp. p20

http://www.planningportal.gov.uk/uploads/br/masonry_internal_wall_insulation_illustrations.pdf

Juantorena

heinbloed,

Thanks for that reply, yes I was including the cavity and outer leaf into the calculations.

By the same token, should the potential cavity created due to the battens the plasterboard is attached to be taken into account? Could that be deemed to be an 'air-tight' cavity?

trevdahead

hi there,
have you considered externally insulating your house with a
70mm phenolic board,this will eliminate any cold bridging.
it would be far better than than using insulated slab,internally.
stopping the cold at the first instance rather than the last.
it would cost more to do but is a far superior method of insulating

heinbloed

Hi Juantorena!The same principle aplies: standing air can be used in the calculation of U-values. Keep in mind that an air layer of more then 16mm width will start to circulate in itself. At the cold surface-usually that is the at the outer surface- the air will cool and drop therefore and at the warmer surface the air will warm up and rise upwards.The old double glazing window panes did not go beyond this dimension of 16mm. The effect of insulation would be reduced by circulating air because energy(warmth) would be taken away from the warm surface and deposited somewhere else, in simple words.So as long as there are all holes in the wall well sealed and the panel is sealed around at all edges(to avoid the chimney effect!) and the gap is less then 16mm wide you can use this gap as as "standing air" in your U-value calculation. Similar to a double glazing pane, but without the glass(smiley),just the layer of undisturbed air.

Juantorena

Hi trevdahead,

Not sure if your external insulation suggestion was directed at me or the op - if me, as I am using SIPs I am assuming (total assumption tbf as I hadn't really considered it) that external was not an option. Perhaps others can comment/clarify?

YourAverageJoe

Hi everyone,

Just wondering if the condensation risk analysis in Builddesk U software is reliable. I am trying to decided on my cavity wall construction and was planning on external block, air cavity, insulation (thickness undecided), internal wall, insulation (thickness undecided) and then insulated plaster board. Checked different dimensions with the software and none showed condensation risk?

Thanks

sinnerboy

yes and no .

yes because when ( i assume) you include a vapor barrier - this a magic bullet to the software - no condensation is predicted

no because , in real life , the vapour barrier will be compromised during the build by accidental damage , lack of care , failure to make good after services penetrations- it will fail in random spots and condensation will occur locally

don't build cavity walls unless you absolutely have to

use either ( no particular order )

ICF
closed panel timber frame
solid blocks + external insulation
poroton