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Insulation thickness difference


Russell griffiths

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So in my original plans I was going to have 50mm of pir insulation under my ceiling joists all taped up, now it looks like I have plenty of room to fit 70mm pir instead. 

The extra cost would be £600, would this extra 20mm save me much on my heating, how long is it going to pay back £600 

or should I not look at it like that and stuff in as much as I have space for. 

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Do not underestimate the cost of additional fixings as you’re going to need longer screws, potentially battens too as PB screws don’t come that long. 
 

Will probably take you about 15 years to pay that back... give or take a decade ..!

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16 minutes ago, Russell griffiths said:

So in my original plans I was going to have 50mm of pir insulation under my ceiling joists all taped up, now it looks like I have plenty of room to fit 70mm pir instead. 

The extra cost would be £600, would this extra 20mm save me much on my heating, how long is it going to pay back £600 

or should I not look at it like that and stuff in as much as I have space for. 

It depends how you will heat your home!

 

Have a look at the specification for the product you are planning and you will see the difference in R value, it will probably be about 2.27m2K/W vs 3.18m2K/W. 

 

 

 

 

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20 minutes ago, Carrerahill said:

It depends how you will heat your home!

 

Have a look at the specification for the product you are planning and you will see the difference in R value, it will probably be about 2.27m2K/W vs 3.18m2K/W. 

 

 

 

 

Can you explain to a numpty how I use those figures to turn it into something I can understand, will the extra 20mm save me £20 per year on my heating or £100. 

I wish I could just stick to hacking up bits of wood and nailing them together. 

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I don't think there's any quick and dirty rules of thumb so many variables. Area you're insulating,  volume your heating, heating source etc etc Jeremy did a spread sheet I've had a play with but still then you have to start working out energy needed it's like black magic to me. What size is your batten then 45 or could you swap down to 25mm lath? Do you think the price difference is more pronounced due to uncommon our size? Perhaps price based on 75 or maybe even  100

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The 70 mm is just down to the specific make I have found, most are 75mm

i would like to go down to a 25mm batten but a lot of downlights need s deeper batten. 

It is like black magic I really hate all the bloody figures, I’m just trying to build a nice house. 

 

I might just say fook it and stick a woodburner in ??

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2 hours ago, Russell griffiths said:

So in my original plans I was going to have 50mm of pir insulation under my ceiling joists all taped up, now it looks like I have plenty of room to fit 70mm pir instead. 

The extra cost would be £600, would this extra 20mm save me much on my heating, how long is it going to pay back £600 

or should I not look at it like that and stuff in as much as I have space for. 

 

Hmm.

 

70-80mm £22.50 at Seconds and Co. if the suitable version.

https://www.secondsandco.co.uk/product-page/kingspan-ecotherm-xtratherm

 

Presumably cheaper if any seconds come up.

 

And they are advertising a Black Friday Sale in a fortnight.

 

F

 

Edited by Ferdinand
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3 hours ago, Russell griffiths said:

So in my original plans I was going to have 50mm of pir insulation under my ceiling joists all taped up, now it looks like I have plenty of room to fit 70mm pir instead. 

The extra cost would be £600, would this extra 20mm save me much on my heating, how long is it going to pay back £600 

or should I not look at it like that and stuff in as much as I have space for. 

 

Do you have the thermal resistance (R-value)/U-value of the original build up?

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14 hours ago, Russell griffiths said:

Can you explain to a numpty how I use those figures to turn it into something I can understand, will the extra 20mm save me £20 per year on my heating or £100. 

I wish I could just stick to hacking up bits of wood and nailing them together. 

 

 

220 m2 ceiling = big house = (guess) £1200 a year for space heating.

 

Assume 25% of heat loss is through the roof = £400 per year.

 

70mm / 50mm = 40% more insulation.

 

40% of £400 = £160 saving per year.

 

= pay back period of 4 years = a lot less than the @PeterW= there must be a big error in my calculations.

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11 minutes ago, epsilonGreedy said:

I must be a big error in my calculations.

 

Yes, adding insulation suffers from diminishing returns. e.g. doubling the thermal resistance of a building element halves the heat loss, doubling again to 4x original thermal resistance reduces heatloss to 0.25 of original value. So adding twice as much insulation in the second doubling than in the first doubling saves only half as much.

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Hi @A_L I intend to go back to the guy who did my sap assessment and check a few things as I’m not sure if I have found the right documents. 

I think my roof buildup came out at 1.4 does that look about right, I’m not entirely happy with that and think it needs to be better what’s your thoughts. 

Cheers. 

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24 minutes ago, Russell griffiths said:

Hi @A_L I intend to go back to the guy who did my sap assessment and check a few things as I’m not sure if I have found the right documents. 

I think my roof buildup came out at 1.4 does that look about right, I’m not entirely happy with that and think it needs to be better what’s your thoughts. 

Cheers. 

I’ve been where you are now Rus 

The initial sap is a waste of paper

I went elsewhere for the final sap 

As Built and still scored 89 without MVRH solar PV 

I was truthful about everything 

Others perhaps arnt 

 

We had a planning condition that we had to score 24% higher the the national average We had water harvesting and a massive array of solar panels as part of our planning 

With decided not to fit either and exceed all insulation spec 

I thought Em we may have problems at the end 

Paid my 100 quid and received my my detailed report the following day 

The Sap is at best a rough guide 

 

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Have you checked seconds and co for the insulation? You may be able to get phenolic insulation at a much cheaper price than PIR, and it tends to come in pretty much perfect condition, could save you a small fortune.

 

I recently bought 21 100mm boards for a little over £400, retail these would have been astronomical.

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15 hours ago, Russell griffiths said:

Can you explain to a numpty how I use those figures to turn it into something I can understand, will the extra 20mm save me £20 per year on my heating or £100. 

I wish I could just stick to hacking up bits of wood and nailing them together. 

Well it depends how you heat your home - it costs me about £60 in gas a year and maybe 2 weekends, a couple of litres of petrol and a lot of sweat to process fire wood, so if I insulate my home it takes quite a long time to get a return from my investment. 

 

However, if I heated from gas 100% I reckon my gas bill would be about £1000 a year - then it would make more sense certainly, but also, I suppose future proofing and all that!

 

So it is all interlinked, I am not good at U values and thermal calcs and what not, I hope people on here will help me when I do need this done, but I know that 20mm is not going to save you much relatively but if your buy cost is £600 then it is probably affordable enough (if funds are still OK of course) just to do it and have benefit of knowing it is there.

 

I have just added 25mm sheets of PIR to the underside of my rafters (vaulted ceilings), it is not a lot 25mm, but knowing it is there, fully taped makes me feel better about it and it only cost another £400 or something. The "main" insulation was already in between the rafters - but this fully sealed layer seems good in my eyes! It will help a bit, but I am not expecting to see a saving on energy.

 

 

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Does anyone know the CO2 generation from making a sheet of PIR and transporting it? Or indeed generation of any other nasties?

 

If I had heaps of solar power, or wind etc. and therefore didn't give a stuff about burning power I would just invest in generation capacity, I would not insulate much I don't think - so there will become a tipping point that clean generation returns us to just pumping lots of heat into our homes and not worrying too much about it. Every time I cut a PIR sheet it stinks and off-gasses, it really makes me wonder. 

 

I'd love to have the time to fully research this, I may get the chance next year to do a paper on it but it depends. 

 

 

 

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Doing some calcs the saving in heat loss would be 35W over the whole roof with a temperature difference of 10 degrees. Maybe you will save £15 a year on mains gas, £45 a year with full rate electricity, so between 3% and 8% or a payback of between 40 and 13 years.

 

@Russell griffiths Are you costing in labour with your £600?

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