Insulation R Value Comparison

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Rushmere

354 posts

Ultimate Geek

#315414 14-Jul-2024 09:44

Can anyone offer a simple explanation of how to evaluate the effectiveness of wall insulation R values please?

For example, would R4.0 wall insulation be twice as effective as R2.0 insulation in terms of heat retention, or is it a non-linear scale?

I can buy R2.8 wall insulation from Bunnings, which is about 56% more expensive than R2.6. That makes me assume it's a non-linear scale, but the question is, how much "more" am I getting from R2.8 than R2.6. Is the effectiveness of the R2.8 insulation 56% "better" than the R2.6, or is it more complicated than that?

Is it possible to easily compare, or am I being too simplistic?

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649 posts

Ultimate Geek

#3259240 14-Jul-2024 10:17

We've recently been through this thought process for reinsulating our interior walls and did quite a bit of research

The short answer is that it's a diminishing return, so just go with the R2.6 and put the savings aside for something else (e.g. ceiling insulation, double glazing etc.) We used the Knauf Earthwool product which didn't cause irritation like some other products, and was really easy to cut and install. I'd still use gloves and a mask though.

Just make sure the thickness & width of the wall insulation matches the cavity as much as possible.

I know some people who tried to use ceiling insulation in the walls, but the wall insulation is much stiffer and won't sag when mounted vertically.

HTH

Jase2985

13466 posts

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#3259290 14-Jul-2024 10:21

R-Value = Thickness (m) / Thermal conductivity (W/mK)

"Doubling the thickness of insulation will double the insulation’s R-value, cutting heat loss in half. Each time the insulation layer is doubled in thickness, this rule applies. The energy saved per year by doubling insulation from R-10 to R-20, however, will be considerably more than the energy saved by doubling insulation from R-20 to R-40, because of the law of diminishing returns. In some cases, like an attic, it’s worth piling on more insulation because there is plenty of room. It’s much more expensive to add that much insulation to exterior walls."
Those are US values but same thing applies

tweake

2398 posts

Uber Geek

#3259297 14-Jul-2024 10:46

Rushmere:

Can anyone offer a simple explanation of how to evaluate the effectiveness of wall insulation R values please?

For example, would R4.0 wall insulation be twice as effective as R2.0 insulation in terms of heat retention, or is it a non-linear scale?

I can buy R2.8 wall insulation from Bunnings, which is about 56% more expensive than R2.6. That makes me assume it's a non-linear scale, but the question is, how much "more" am I getting from R2.8 than R2.6. Is the effectiveness of the R2.8 insulation 56% "better" than the R2.6, or is it more complicated than that?

Is it possible to easily compare, or am I being too simplistic?

you need to look at the overall system.

for eg if you doubled the wall insulation will you get double the heat retention? no. thats because a wall is made up of timber/steel as well. this is the same trick that building industry pulled on home owners with windows. the windows where rated by glass insulation rating without factoring in the frame. the frame could make double glazing worse than an old wooden single glazing.

a "standard" r2 wall requires r2.2 insulation. however a lot of walls are not "standard".

the other factor is you need insulation to be continuous for it to work properly. having a non insulated spot or low insulated spot lowers the overall insulation value a lot more than what would think. trying to make up for low insulation in one spot by increasing insulation in another does not work well in practice. think of it this way, heat pours out the holes in the insulation. then there is things like wind washing because most walls are open to the outside and wind gets around the edges of the insulation. this is why how insulation is installed makes a big difference in actual real world performance.

is the r2.8 insulation standard or acoustic? acoustic is a lot more expensive and frankly doesn't work that well for acoustics. just standard insulation will do the job.

if you want a well insulated house you need to look at the overall system and how its done.

Rushmere

354 posts

Ultimate Geek

#3259299 14-Jul-2024 11:04

Thanks for the replies.

This is part of new building work, so the wall insulation its just one element of the total design. The insulation all appears to be the same specification (e.g. the same acoustic level) apart from the R Value.

The question really is by how much it is worth exceeding the requirements of my building consent, and it sounds from the above that a 56% increase in cost for a 7.7% increase in R level isn't worthwhile. I'm a bit puzzled why the cost increases by such a high proportion if it doesn't actually deliver that much additional insulation level. Perhaps they just don't sell much of the higher R insulation, so it's disproportionately expensive.

tweake

2398 posts

Uber Geek

#3259309 14-Jul-2024 11:21

Rushmere:

Thanks for the replies.

This is part of new building work, so the wall insulation its just one element of the total design. The insulation all appears to be the same specification (e.g. the same acoustic level) apart from the R Value.

The question really is by how much it is worth exceeding the requirements of my building consent, and it sounds from the above that a 56% increase in cost for a 7.7% increase in R level isn't worthwhile. I'm a bit puzzled why the cost increases by such a high proportion if it doesn't actually deliver that much additional insulation level. Perhaps they just don't sell much of the higher R insulation, so it's disproportionately expensive.

i think you need to post what spec is designed for your consent and what size walls you have.

the cost between the two is most likely because its a low volume product and its a high density product that has higher manufacturing costs. the r2.6 acoustic is $13/m while the normal r3.2 is $10/m. acoustic insulation is simply higher density of fibers.

k1w1k1d

1530 posts

Uber Geek

#3259362 14-Jul-2024 13:29

Here is the product list for Earthwool wall insulation.

Note the thickness column (90/140). 90mm R1.3 - R2.8. 140mm R3.2 - R4.4.

neb

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#3259376 14-Jul-2024 14:07

Rushmere: I'm a bit puzzled why the cost increases by such a high proportion if it doesn't actually deliver that much additional insulation level. Perhaps they just don't sell much of the higher R insulation, so it's disproportionately expensive.

It could be Goldilocks pricing, where you make something artificially expensive in order to sell more of the superficially-cheaper stuff that you're actually trying to sell. Certainly the last time I was at M10 the R2.2 section was almost empty while there was plenty of R2.6/2.8 just sitting there gathering dust.

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Rushmere

354 posts

Ultimate Geek

#3259383 14-Jul-2024 14:15

k1w1k1d:

Here is the product list for Earthwool wall insulation.

Note the thickness column (90/140). 90mm R1.3 - R2.8. 140mm R3.2 - R4.4.

Yes, thanks, I also just found that table after a bit of digging around, and it does offer a bit of additional information.

It's 90mm insulation I'm looking at currently (the deliberations on the 140mm cavities will wait for another day), and I'm using Earthwool. The R2.6 (which is specified as the minimum to be used in my building consent) is listed as "SoundShield" whereas the R2.8 is listed as "SoundShield Plus", so there there does appear to be some difference in acoustic properties as well as the small difference in R value.

Presumably the R2.8 is somewhat denser, but I can't find anything that really quantifies the difference, so it's difficult to make any meaningful judgement as to how much you're actually getting for the 56% extra cost per square meter. Although some extra sound insulation would be of value, in the absence of any other meaningful information to quantify it, I guess I'l stick with the R2.6, as it seems that the difference in thermal performance is very small.

tweake

2398 posts

Uber Geek

#3259397 14-Jul-2024 15:03

Rushmere:

The R2.6 (which is specified as the minimum to be used in my building consent)

why is r2.6 required?

is it for sound? if so what other sounds measures have been specified?

if its insulation then i would be looking as to why thats the case. it could well be the architect has made it borderline to pass and required an increase to pass minimum spec. however thats only theory spec, not real world conditions. if its that borderline you may find that it will be bellow minimum spec in real life. going to r2.8 may make no difference if they are not using enough insulation in the first place. this is where you need to look at the plans and work out why thats the case.

Rushmere

354 posts

Ultimate Geek

#3259403 14-Jul-2024 15:20

The building code requires a minimum of R2.0.

The architect suggested using at least R2.6 to help achieve better than minimum energy efficiency, which I was happy with at the time.

Now that even higher R value insulation is readily available since the building code increased the minimum requirements last year, as I stated in my original post, I was trying to figure out how much benefit I could get from that, as I didn’t understand how the R scale was calculated. Now that I know, I don’t think the even higher R value (2.8) is worth the extra cost.

tweake

2398 posts

Uber Geek

#3259408 14-Jul-2024 15:37

Rushmere:

The building code requires a minimum of R2.0.

The architect suggested using at least R2.6 to help achieve better than minimum energy efficiency, which I was happy with at the time.

Now that even higher R value insulation is readily available since the building code increased the minimum requirements last year, as I stated in my original post, I was trying to figure out how much benefit I could get from that, as I didn’t understand how the R scale was calculated. Now that I know, I don’t think the even higher R value (2.8) is worth the extra cost.

the architect can easily tell you the numbers if you want to know. they have done the calcs they should know exactly what the difference it makes. it really depends on how the house is designed in the first place.

if you really want a cheap easy way to improve thermal efficiency, reduce your window size. window to wall ratio is the number one factor. remove or downsize a window can reduce building cost and greatly improve insulation value.

k1w1k1d

1530 posts

Uber Geek

#3259410 14-Jul-2024 15:59

My understanding is that R2.0 is the minimum insulation value for the total area of all external walls.

Insulation values of R2.6/R2.8 are to make up for the areas that are less well insulated, eg framing, windows, doors, etc. Your architect will have done the calculations of all these items and come up with the minimum insulation R value to meet the R2.0 requirement.

tweake

2398 posts

Uber Geek

#3259411 14-Jul-2024 16:10

k1w1k1d:

My understanding is that R2.0 is the minimum insulation value for the total area of all external walls.

Insulation values of R2.6/R2.8 are to make up for the areas that are less well insulated, eg framing, windows, doors, etc. Your architect will have done the calculations of all these items and come up with the minimum insulation R value to meet the R2.0 requirement.

sort of. what matters, for compliance, is the overall insulation value of the house. so you often get poor insulation in one room being offset by better insulation in another. or more commonly poor wall insulation and increased ceiling insulation.

however thats a poor method. you get variations per room etc, and makes whole home heating/cooling more difficult. once you start getting into decent insulation levels it doesn't really work as heat goes in all directions and insulation needs to be continuous.

RobGisMe

1 post

Wannabe Geek

#3260020 15-Jul-2024 21:34

30 odd years ago when I took off the GIB and put in R1.8 in one room and R2.2 in another room. The improvement with R2.2 was totally noticeable over the R1.8. Glad the standards have increased now to better match what I learned through trial and error.

elpenguino

3427 posts

Uber Geek

#3260096 16-Jul-2024 11:05

Wall insulation is a (usually) one time chance to maximise the comfort of the place as well as decrease the energy requirement.

When doing a reno, I always max out what a wall will physically take.

Most of the posters in this thread are just like chimpanzees on MDMA, full of feelings of bonhomie, joy, and optimism. Fred99 8/4/21

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