Attic insulation - Cost vs effectiveness

kdouglas

Hi All,

Looking for some advice/thoughts on insulating our attic.

We've a 3-bed semi-detached house, normal pre-fabricated truss roof.
The centre section (where there's standing room) has been floored for storage with boards screwed directly on top of the joists.

There is the original 100mm or so of insulation between the joists and it's about time I added some top-up insulation to that.

My question is, what to do about the section that's floored, the simplest thing is as per attached image, just roll out top-up insulation on the sections that are not floored, but will this be a waste of money as I'm not increasing the insulation of the floored section?

The options that I see are;
a) Insulate the rafters, creating a "warm roof", which is a fair bit more expensive
b) Remove the existing floored section, add insulation and put the floor back 4-6 inches higher. This is quite a bit more work and reduces head-room for any potential future conversion of the attic space.
c) Just do as pictured and put down top-up insulation across the sections that are accessible. This is the cheapest and easiest option.

Alternative options or advice would be appreciated.

Cheers

anto9

Do you have a swirling roof vent (dont know if thats the right name ?) .It could help a lot if not .Ups ,sorry i forgot that this is an Irish Forum ,so no problems of attic heat lol

homer911

When I added 200mm to my attic I had the same question. Raising the floor as a you say is a lot of work. I decided to place battons across the roof support trusses and I removed most of the existing flooring and placed it on top of the battons - basically a 4 sided shelf area created around the attic access point. I left enough flooring to provide access to the space and room to stand and access the shelves. A bit of a compromise but it was a lot less bother than raising the floor (and I dont think most attic ladders are long enough to cope with the extra height anyway)

kdouglas

homer911 wrote: »

When I added 200mm to my attic I had the same question. Raising the floor as a you say is a lot of work. I decided to place battons across the roof support trusses and I removed most of the existing flooring and placed it on top of the battons - basically a 4 sided shelf area created around the attic access point. I left enough flooring to provide access to the space and room to stand and access the shelves. A bit of a compromise but it was a lot less bother than raising the floor (and I dont think most attic ladders are long enough to cope with the extra height anyway)

Took me a minute to figure out what you meant, interesting approach, but wouldn't work for me, there's actually quite a bit of stuff stored up there, so I do need to keep the floored storage section one way or the other.

dathi

remove the fiber glass from the floored area and replace with 100mm kingspan / xtratherm ( assuming joist is 100mm if deeper fulfill thickness) and add 50mm kingspan above the joists then screw down flooring through the 50 mm kingspan into the joists. this will give you the same u value as the 300mm fiber glass in the non floored area.

kdouglas

dathi wrote: »

remove the fiber glass from the floored area and replace with 100mm kingspan / xtratherm ( assuming joist is 100mm if deeper fulfill thickness) and add 50mm kingspan above the joists then screw down flooring through the 50 mm kingspan into the joists. this will give you the same u value as the 300mm fiber glass in the non floored area.

Hadn't thought of that, although I'm not sure if it would be similar cost to just insulate the rafters...

Calibos

Someone posted in the construction forum that insulation efficiency isn't linear basically.

If one uses the figure of 20% of a rooms heat is lost through the ceiling into the attic.....

Of the recommended 300mm insulation...

The first 100mm of insulation gets you 89% of the heat loss reduction
The next 100mm (ie 200mm) gets you another 6% and brings you to 95%
The final 100mm (ie 300mm) gets you another 2% and brings you to 97%

Maybe the reason they only put in 100mm in the past was the law of diminishing returns and cost of insulation etc

ie. When insulation was more expensive you had a choice. Put in 300mm at triple the cost and lose the ability to use the attic for storage without a lot of extra effort all for a paltry 8% increase in efficiency from 89%-97%.

Or just put in 100mm which costs 1/3rd the amount but still gets you the bulk of the heat retention gains and you can still use the attic for storage.

kdouglas

Calibos wrote: »

The first 100mm of insulation gets you 89% of the heat loss reduction
The next 100mm (ie 200mm) gets you another 6% and brings you to 95%
The final 100mm (ie 300mm) gets you another 2% and brings you to 97%

I guess that assumes the insulation is all equal in terms of U-value, if the 100mm insulation I have right now is older and not as effective at retaining heat, then adding more is going to help.

On that basis, I could top-up where there is space to do so, lift the floor and put down more effective insulation and then put the floor back in place....

dathi

Calibos wrote: »

Someone posted in the construction forum that insulation efficiency isn't linear basically.


Of the recommended 300mm insulation...

The first 100mm of insulation gets you 89% of the heat loss reduction
The next 100mm (ie 200mm) gets you another 6% and brings you to 95%
The final 100mm (ie 300mm) gets you another 2% and brings you to 97%

the actual figures are
zero insulation =2.44w/m2/k
100mm insulation =0.45w/m2/k which is 81% reduction on no insulation
200mm insulation =0.22w/m2/k which is 91% reduction on no insulation
300mm insulation =0.15w/m2/k which is 94% reduction on no insulation

Calibos

Gonna use the flat over our shop that we let out for my example.

The attic is uninsulated and the flat has storage heaters. The winter bimonthly lecky bill is about €350 and the Summer bimonthly lecky bill is about €150. Thus the heating portion of the bill is about €200 per bimonthly bill and thus €100 per month.

If what they say is true and about 20% of heat is lost through an uninsulated attic that means €20. If installing 100mm of attic insulation prevents 81% of that loss then that means €16.20 is saved per month. If by installing 300mm 94% of the heat loss is prevented then that means €18.80 is saved on the monthly bill. A difference of €2.60 per month.

If 100mm costs me €100 to buy and DIY install then at €16.20 saved per month, it'll have paid for itself over a single winter in about 6 months.

However, if the 300mm costs me €200+ extra (and I lose storage ability) but only saves me another €2.60 per month over and above he 100mm then the extra insulation takes....wait for it....76 months to pay for itself. That's 12 years worth of Winters!

dathi

most estimates put the figure at between 25 and 30% again these are only estimates. and you have to take all heat losses from the building into account, as the heat losses from the other elements can make this percentage higher or lower eg. if you have single glazed windows then the high heat loss from them would drive down the percentage heat loss from the roof even though the roof was still losing the same amount of heat.
the other problem you have is alterations to existing buildings now have to comply with part L of the building regs which stipulate 0.16w/m2/k ( 300mm insulation)

roscodub

homer911 wrote: »

When I added 200mm to my attic I had the same question. Raising the floor as a you say is a lot of work. I decided to place battons across the roof support trusses and I removed most of the existing flooring and placed it on top of the battons - basically a 4 sided shelf area created around the attic access point. I left enough flooring to provide access to the space and room to stand and access the shelves. A bit of a compromise but it was a lot less bother than raising the floor (and I dont think most attic ladders are long enough to cope with the extra height anyway)

What did you use for battons? Want to do the same thing, but concerned about the exta weight.
Has anyone come across lightweight batons, made from plastic for instance ?

Calibos

Still don't really understand why anyone would be bothered going to the expense and hassle given the payback period unless mandated by regulation.

ie. spend several hundred euro and all that hassle that only saves you 1-5 euro per month that'll take donkey's years to pay for itself. The bulk of the heat retention efficiency gains (81%) are in the first 100mm of insulation.

Sure, if you've 100mm of old fibreglass insulation that has been permanently compressed to 50mm or something by years of boxes and is thus massively compromised, by all means replace with 100-150mm of new insulation. However given this new information about the depth/efficiency that I only heard mentioned for the first time recently, personally, I've changed my attic insulation plans accordingly which will make my DIY job much cheaper and easier.

If your attic hatch is poorly draught sealed and not insulated then for a tiny fraction of the cost and hassle you'd probably save significantly more on the heating bill than the couple of euro a month (winter months only) you'd get by spending hundreds on more loft roll and battens etc.

roscodub

dathi wrote: »

most estimates put the figure at between 25 and 30% again these are only estimates. and you have to take all heat losses from the building into account, as the heat losses from the other elements can make this percentage higher or lower eg. if you have single glazed windows then the high heat loss from them would drive down the percentage heat loss from the roof even though the roof was still losing the same amount of heat.
the other problem you have is alterations to existing buildings now have to comply with part L of the building regs which stipulate 0.16w/m2/k ( 300mm insulation)

Does anyone recommend
Knauf Earthwool vs Rockwool from a cost and efficiency point of view?

Markcheese

Most builders providers sell insulation bonded to chip board that your install over your joists (and existing insulation)
So extra insulation and storage in one -

homer911

roscodub wrote: »

What did you use for battons?

Just some 2x1 bolted to the supports with roofing bolts. I actually ended up using some leftover laminate flooring laid across the batons as shelving, which was pretty light.

poker--addict

Old thread, but information is timeless.

If the first 100M is so critical, then is topping up attic a complete waste of time and would it be very important to remove older insulation before adding newer/better quality stuff?

TheDoc

I just finished insulating and flooring my attic, and it was a pretty large attic, over 60m2.

• I first laid 100mm earthwool insulation between the existing joints, over the old firebglass.
• Fibreglass is nasty ****, just lay the new stuff over it, makes no odds.
• Where the fibreglass was in proper rag order I did take it up and bagged it. Wear glasses and a mask for this, my eyes were in tatters handling this stuff.
• I then laid 2x4's as batons running different direction to existing joists. Obviously stood them up on their highest side, which was 100mm.
• Screwed into existing joists with 50x100mm screws.
• Laid another layer of 100mm insulation, in the 'runs' between the new batons.
• When done laid OSB boards across, screwing into the new batons using 5x40mm screws.

I floored more than I originally intended. For areas without flooring(which also has insulating properties remember) I just lashed additional insulation in.

Important to remember not to compress the insulation as it basically looses all its properties.

Can give me a shout with any questions, was handy enough to do in the end. Materials cost me 1000, which when you consider 4500 was the best quote I got I saved a fortune, plus I'd 4 rolls of insulations left over and boards and 2x4s as I bought too much :D

TheDoc

https://www.boards.ie/discussion/comment/121647921#Comment_121647921

Nah, not sure where this myth has come from.

If your old stuff is in rag order and bumpy then maybe remove it. The gaps for heat to rise is what insulation is dealing with. If it lays flush in the joists it will be doing what it should be doing. if you notice new stuff coming up over your joists then remove the stuff below.

I just did this in an attic with like 30-40+yr old fibreglass. I'd say I removed half of the fibreglass cause it was clumped and a mess, other half was fine to leave in place and just add onto

TheDoc

https://www.boards.ie/discussion/comment/93166024#Comment_93166024

I bought 2x4's from TJ O'Mahony. I also had concerns about weight but had someone proffesional (granted handy they are a family member) and they said good to go, there is very rare scenarios where using battons isn't suitable.

From the checks I did before I did mine, using 2x4's or something similar is actually advised. The plastic legs or raises are seemingly shite and crumble over time. Your also focusing load onto a smaller surface area which is definitly not good for potentially older joists.

I just made sure my 2x4's were 100mm tall, as that was perfect to add the second layer of insulation

Tombo2001

Sorry for jumping in we have 200m in about most of our attic and 400m in about half. I notice that around the eves there is a bit of a gap, it hasnt been stuff right down, should it be? Cant find this out....

bfclancy2

https://www.boards.ie/discussion/comment/121665009#Comment_121665009

need a gap at the eaves, leave it alone, ventilation