How much cellulose is too much?
user-7694189
| Posted in General Questions on
Building a new construction in zone 6, rectangular home approximately 34×50 with R30 walls and a .50 ACH goal. With that in mind, I’m trying to decide how much blown cellulose insulation to put in the roof/attic assembly. Code calls for R60 and while I want to build better than code, there is a point where value to money spent just doesn’t make sense. Any advice from the building science on when to stop insulating or any articles or studies that may have graphed out the ROI for insulating R values?
Thanks
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
I'm not an expert but I did come across this article that shows that there is a point of diminishing returns with insulation. The article linked has a graph too. https://www.greenbuildingadvisor.com/question/diminishing-returns-of-added-insulation
It seems that around R50 - R60 is about as far as it's worth going.
Thank you that article was exactly the kind of information I was looking for
It’s probably right around R-60, possibly under. If you want the complicated answer: you can discount future energy savings of R-60 vs higher R values and compare those savings to their cost and the other fuels. In short, insulation is often not a great investment and even as an investment, has distinct disadvantage vs. other methods of betting on future energy costs.
Run a BEopt model. Won't take long to build and it spits out energy usage. So you can multiply that by your nominal power delivery charge to come up with your own ROI.
Thanks for the advice Matthew. Where would I find a BEopt model? Is that software I have to buy, or is it available online?
The BEopt computer program is paid for with your tax dollars and is free to use.
https://www.nrel.gov/buildings/beopt.html
The program has a steep learning curve. Make the time to watch the 16 training videos.
https://www.youtube.com/watch?v=GdMYCuwp0AY&list=PLHC0xDtkdjgec8QhVt7exJY3tpSLEFk-d&ab_channel=NRELResidential
The program is very powerful but it is still garbage in garbage out. Is has some cost per square foot standard data that inflation has made outdated you should get bids and from the bids calculate and enter your $/SQF.
Building beyond the most resent model codes will not make economic sense as have pushed the limits.
If you are going beyond R60 on your ceiling expect push back due to the weight and consider upgrading to 5/8 drywall ceiling.
Walta
I wouldn’t say steep learning curve. I probably only watched 2-3 videos. You click and drag to build the footprint of the home, and then you can customize the different assemblies instead of relying on the default R values, air tightness, and appliances they suggest. In this case you would not use their cost assumptions at all, all you need from this model is energy usage due to heating and cooling (as opposed to appliances) at different levels of attic insulation. So it doesn’t matter that their costs are outdated.
R60 for 1250 square feet would run about $2000 in material costs. Going to R80 will add about $600. Being that in your home's future you'll likely end up with ASHP (or maybe already) I'd be tempted to just spend the extra $600. That number is likely immaterial considering your overall build costs.
User...189,
I think there are two ways of looking at this.
One is ROI for just that assembly in isolation, which rests on a few assumptions about future energy prices and interest rates.
The other is whether that assembly represents a weak point compared to the others in the house, and the money would be better spent upgrading them instead. Modelling helps with this, but in general improvements in window performance often yield higher gains than walls and roofs
Dennis,
"I'd be tempted to just spend the extra $600. That number is likely immaterial considering your overall build costs."
The problem I find with that reasoning is it can apply to almost any upgrade. It's 30%, and if you do that regularly, building the house will cost 30% more.
@Malcolm, I can't disagree with your logic :-)
The problem is that saving 5000HDD x 24 x (1/60 - 1/80) @ 30 years and 7% interest is only ~ 8MMBtu discounted to today. And you’re paying $600 for those 8 MMBtu ($75/MMBtu) while I can buy that for ~$15/MMBtu.
I'll add to the chorus: R-60 is likely the most that makes sense, based on a lot of BEopt modeling and some Passive House modeling.
Putting it in deeper is cheap insurance! Cellulose Settles! The manufactures and sellers
of cellulose all (to the last man) claim that it doesn't BUT IT DOES. Just how much is open to argument.
Here is a way to guess at it - assume that you place $1000 worth in your
attic and that 50% of that cost is the labor - then assume that it settles by 20% over
some period of time that you live in the house - bringing it back up to original depth
will then cost you another 20% of the material cost (or about $100) but what will the
inflation rate have added to the labor & material costs over that length of time? Who Knows?
But, what we do know is that adding an additional 20% in depth right now will
cost about $100. CHEAP INSURANCE! When it comes to insulation, ROI calculations
aren't all that reliable - going a little overboard is cheap insurance.
Loose-blown cellulose definitely settles; installers typically blow a couple of inches extra so it should end up close to the designated R-value. It's 1.8 lbs/ft³ when settled.
Cellulose is dense-packed to somewhere between 3.0 and 4.2 lbs/ft³. At the lower end of that range, it can settle. At the upper end it can't, unless it gets saturated with water, and even then it usually doesn't.
I had to vacuum out an attic that had been insulated with R60 blow in cellulose, about 15 years after installation. Not only does it settle considerably, but the first 3-4" also shows some cohesion, likely due to higher humidity levels in the attic in summer. For the reno in question, a 3/4 hp shop dust collector with 4" hoses worked quite nicely as a combination vacuum/blower to transfer the cellulose from attic 1 to attic 2 in preparation for a 2" closed cell flash and fill in attic 1. There was some extra care involved in breaking up the surface "crust" to vacuum/blow the material.