Thickest possible insulated plasterboard.....

Southern Comfort · 25-03-2015 11:59pm #1

I don't want to piggyback on BarneyMc's thread. I've read that the insulation on the inside of a cavity wall should not exceed half the thickness of the insulation in the cavity.

Does this change if the 2 types of insulation have different thermal conductivities? Ecobead etc. has a thermal conductivity of 0.033 and insulated plasterboard has a thermal conductivity of 0.022.

What is the thickest insulated plasterboard that could be used with a 150 mm pumped cavity?

Strolling Bones · 26-03-2015 12:16am

Southern Comfort · 26-03-2015 12:43am

Strolling Bones wrote: »

0

Just go for thermal mass and u value of 0.2?

Strolling Bones · 26-03-2015 8:07am

You really should. If you dry line internally then you get thermal bridges at each internal wall/external wall junction same at external wall/upper floor same at wall / roof. You also suffer the perenial curtian pole falling down ever 6 months. Most importantly your external walls lose thermal mass. As soon as a cold breeze runs through the internal out goes the heat none of it stored in your external walls.

PROJECT K · 26-03-2015 9:50am

rule of thumb is that the board on the inside is at least half the thermal resistance of the material in the cavity (this has been simplified over the years to half the thickness but this isnt correct).

Thermal resistance = thickness in metres divided by the materials conductivity

therefore EPS at 33mW (150mm thick) has a resistance of 4.545m2k/w

So your 'guide limit' for internal insulation is half of this, say 2.3m2k/W

Working back with PIR at 22mW this allows a thickness of 50mm + 12.5mm plasterboard (i.e. R=T/L therefore T=R*L where L is lambda)

This rule of thumb is to minimise the risk of interstitial condensation and you could prpbably get away with a thicker board, get someone to run the risk analysis for you.

However i would agree with the general principle of the previous post in that insulation should ideally be kept in the same zone and not divided by other high conductity materials such as blockwork...better to widen the cavity to 200-300mm if thats an option at this stage?

sheff the ref · 04-03-2016 4:25pm

I have been advised through my engineer that a point of diminishing returns exists as regards wall insulation and that widening a cavity beyond a certain point will only provide miniscule benefit in comparison to the investment required to widen. The reason for this is that heat loss occurs through many mediums in a house and essentially there is only so much wall insulation can do if a door is open or if heat can be lost through other areas.

My understanding is that this point of diminishing returns is around the 200mm mark for a pumped cavity wall. Would be interested in views on this?

PROJECT K wrote: »

rule of thumb is that the board on the inside is at least half the thermal resistance of the material in the cavity (this has been simplified over the years to half the thickness but this isnt correct).

Thermal resistance = thickness in metres divided by the materials conductivity

therefore EPS at 33mW (150mm thick) has a resistance of 4.545m2k/w

So your 'guide limit' for internal insulation is half of this, say 2.3m2k/W

Working back with PIR at 22mW this allows a thickness of 50mm + 12.5mm plasterboard (i.e. R=T/L therefore T=R*L where L is lambda)

This rule of thumb is to minimise the risk of interstitial condensation and you could prpbably get away with a thicker board, get someone to run the risk analysis for you.

However i would agree with the general principle of the previous post in that insulation should ideally be kept in the same zone and not divided by other high conductity materials such as blockwork...better to widen the cavity to 200-300mm if thats an option at this stage?

JimmyMW · 04-03-2016 4:35pm

sheff the ref wrote: »

I have been advised through my engineer that a point of diminishing returns exists as regards wall insulation and that widening a cavity beyond a certain point will only provide miniscule benefit in comparison to the investment required to widen. The reason for this is that heat loss occurs through many mediums in a house and essentially there is only so much wall insulation can do if a door is open or if heat can be lost through other areas.

My understanding is that this point of diminishing returns is around the 200mm mark for a pumped cavity wall. Would be interested in views on this?

Yep from my looking at that very topic i found the same, 200mm is the mark, thats exactly what I am currently doing on my new build

sydthebeat · 04-03-2016 6:35pm

from my own professional views... the 2011 regulations go beyond the point of reasonable diminishing returns if you do not use MHRV.

for example a recent assessment i have carried out on a 2250 single storey build requires:
500mm quilt in attic,
150 PIR in cavity with 62.5 Insulated plasterboard,
200 PIR under floor,
220 PIR on roof slopes
0.74 U Value windows

i would much prefer if the regulations focused better on thermal bridges and air tightness as the ACDs are pitiful.

sheff the ref · 04-03-2016 10:58pm

Somewhere or other though, we have to accept that it is a house with multiple occupants, not a spacesuit. Doors will be opened and left open. Windows will be opened and left open. You wonder about the benefit of extreme air tightness in that context

sydthebeat wrote: »

from my own professional views... the 2011 regulations go beyond the point of reasonable diminishing returns if you do not use MHRV.

for example a recent assessment i have carried out on a 2250 single storey build requires:
500mm quilt in attic,
150 PIR in cavity with 62.5 Insulated plasterboard,
200 PIR under floor,
220 PIR on roof slopes
0.74 U Value windows

i would much prefer if the regulations focused better on thermal bridges and air tightness as the ACDs are pitiful.

BryanF · 05-03-2016 8:23am

sheff the ref wrote: »

Somewhere or other though, we have to accept that it is a house with multiple occupants, not a spacesuit. Doors will be opened and left open. Windows will be opened and left open. You wonder about the benefit of extreme air tightness in that context

No drAfts at your feet,is 1

sheff the ref · 05-03-2016 5:17pm

At a house at the moment. Door wide open for 4 minutes 33 seconds while the weekly shopping was brought in. A number of internal doors also open so external airflow was not restricted. Chilly enough outside today.

Realistically the financial outlay on air tightness wouldn't compensate for that. Bathroom window was open for a good while this morning too.

Taping around windows etc. is just basic common sense in terms of air tightness but the value of MHRV is highly questionable and possibly amounts to spending €000s to save cents if even that in the average practical house. Really and truly you would want double automatic sliding doors like a shopping centre to justify having MHRV

Would love to see some research facts on it to be honest. Are there any such facts available on the heat loss through open doors and windows in comparison with heat loss through non airtight parts of a house

BryanF wrote: »

No drAfts at your feet,is 1

Calibos · 05-03-2016 5:47pm

There was a recent thread which linked to an a article about a renovation and MHRV retrofit of a 70's extended semi-D to passiv standards. The archi got one of his masters students to work out the cost component of the work for the passiv standard removing all 'do anyway' costs. Came to €100,000 incl Vat. The owners were thrilled that their heating bill was €50 a year. Amazing Huh?

Our €200,000 2400sqft Victorian Reno with 3 open fireplaces leaks air like a sieve with French doors open for dogs for probably an cumulative hour a day winter or summer. Wrecks my head but howandever. Cost to heat for the year is €1900. It would take us 52 (2068) years for that passive spend of 100,000 to start paying back.

But hey, €50 heating bill a year guys!! And I wouldnt have to wait 20 minutes for the house to heat up and I would have the eternal gratitude of my grandchildren who might inherit the house or some future owners!!

sheff the ref · 05-03-2016 5:58pm

1.For 50 quid a year they must be keeping all doors and windows closed at all times

2.There must be a happy medium somewhere of less spend vs less heating costs.

Calibos wrote: »

There was a recent thread which linked to an a article about a renovation and MHRV retrofit of a 70's extended semi-D to passiv standards. The archi got one of his masters students to work out the cost component of the work for the passiv standard removing all 'do anyway' costs. Came to €100,000 incl Vat. The owners were thrilled that their heating bill was €50 a year. Amazing Huh?

Our €200,000 2400sqft Victorian Reno with 3 open fireplaces leaks air like a sieve with French doors open for dogs for probably an cumulative hour a day winter or summer. Wrecks my head but howandever. Cost to heat for the year is €1900. It would take us 50 years for that passive spend of 100,000 to start paying back.

But hey, €50 heating bill a year guys!! And I wouldnt have to wait 20 minutes for the house to heat up!!

Calibos · 05-03-2016 6:26pm

With payback periods that long (2068) they'll be sitting pretty if WW3 happens and Russia cuts off the gas or crying into their cornflakes because while they still don't have any heating bills, neither does anyone else because of Nuclear Fusion powered electric heating.

MicktheMan · 05-03-2016 9:52pm

sheff the ref wrote: »

but the value of MHRV is highly questionable

Sheff, do you use a fridge or freezer in your house?

sheff the ref · 05-03-2016 10:47pm

MicktheMan wrote: »

Sheff, do you use a fridge or freezer in your house?

Fridge

MicktheMan · 05-03-2016 11:06pm

sheff the ref wrote: »

Fridge

If you consider that our grandparents didn't have a fridge, but were able to store food, you might think that the value of a fridge is also highly questionable.
The point I'm making is that a ventilation system does a job just like a fridge, in that, neither are really required but both are useful and do a specific job. The same can be said about most modern conveniences.

sheff the ref · 05-03-2016 11:16pm

MicktheMan wrote: »

If you consider that our grandparents didn't have a fridge, but were able to store food, you might think that the value of a fridge is also highly questionable.
The point I'm making is that a ventilation system does a job just like a fridge, in that, neither are really required but both are useful and do a specific job. The same can be said about most modern conveniences.

I don't dispute the function of a MHRV

What I do question is the value of it in say a family situation where various members leave doors and windows open. It does happen, will happen and is unlikely to stop happening.

You could compare it to having air conditioning in the car running with the windows open

MicktheMan · 05-03-2016 11:40pm

sheff the ref wrote: »

You could compare it to having air conditioning in the car running with the windows open

Good analogy.
So, do you buy a car without AC, open the windows and sweat or buy one with AC but close the windows when it is running?

I have mvhr now for the last 8 years in a typical family situation (incl 2 dogs) and simply put, windows don't get opened during the heating season because there is no need to. I wouldn't be without it.

sheff the ref · 05-03-2016 11:41pm

How do you heat your house and what impact does it make on heating costs?

MicktheMan · 05-03-2016 11:59pm

sheff the ref wrote: »

How do you heat your house and what impact does it make on heating costs?

To be honest I don't know the impact on our heating, but I do know the impact on our indoor air quality.

We use a standard oil boiler, 2 heating zones, each controlled by a chronostat. Living area zone internal temp set at 20 degC from 6am to midnight, bedroom zone set to 17degC all the time(I don't like overwarm bedrooms:)).
Heating season is generally mid to late Oct until mid to late March.
This heating regime consumes on average 700 litres of oil per year with no other fuel used (e.g. openfires etc).
House is a 2100 sqft 1977 built bungalow.

Metric Tensor · 06-03-2016 12:33am

The argument for MHRV, in my opinion, is not that it will lower your heating bills by substantial amounts - it is that it will provide a more pleasant, healthy internal environment in modern houses with high levels of air-tightness.

It is designed ventilation delivering balanced flows of air throughout the house compared to the old fashioned open a few windows or drill some holes in the wall and hope for the best!

Calibos · 06-03-2016 10:15am

The real question is whether that extra convenience or comfort is worth up to a 100 grand extra on your mortgage?

Metric Tensor · 06-03-2016 10:39am

That 100K is to bring the house to near "passive" standards.

On a new build getting a reasonable air tightness value and installing MHRV does not cost 100K extra.

BryanF · 06-03-2016 11:21am

Calibos wrote: »

The real question is whether that extra convenience or comfort is worth up to a 100 grand extra on your mortgage?

where this alarmist rubbish is coming from?
Are we comparing like with like?
Part L 2011 (current building regulations) v passive standard?

BryanF · 06-03-2016 11:28am

Ok so having read back through the thread, it's seems at certain point this became about the cost of retrofitting to passive standards and not new build construction.

Calibos, why not find the thread where you mention 100k spend and post there?

Thickest possible insulated plasterboard.....

Comments