Ceiling Insulation R53 vs R45?

[Angelfire]

Hiya Folks,
Starting to price out my insulation (mineral wool) for my detached garage. This is located near ABQ NM which falls into the 4B climate zone. The IRC requires R49 in ceilings of homes it appears. Interesting considering just a few years ago, it was R38 but I digress. Anyway, as many of you already know, the mineral wool approach isn't cheap so I'm looking at two options. One gives me R45 and the other would be R53. The difference in cost is about $600 or so (not huge but money counts!). So I'm curious as to whether I will really notice a difference between R45 and R53? I know as the R values climb, the effective return drops but I really don't have a good feel for what real world differences I'd see between the two options. I should note this is a flat roofed structure and blowing anything in is not an option I'm willing to consider (requires significant destruction to a pretty new roof) and I'm pretty well set on mineral wool just due to the "other" benefits that come with it. So just looking for any advice from those in the know or with experience between my two options. Thanks much.
Cheers.

 

[DC73]

So I'm curious as to whether I will really notice a difference between R45 and R53?

I don't think you'll notice a difference between R45 and R53. Proper air sealing will make more of a difference. I used both mineral wool and fiberglass in my shop to get a good bang for the buck envelope. R15 mineral wool in the walls and next to the ceiling, then R19 fiberglass perpendicular to and above the mineral wool in the attic. You could use one layer of mineral wool and R30 unfaced fiberglass batts above to get to your R45 and save a few bucks.

DC

 

[86turbodsl]

law of diminishing returns on that, you get not a lot of more bang for the buck on the difference. Go R48 for cheaper.

 

[matt_i]

17% difference there going from R45 to R53.

My take is more of a concern on energy prices going forward. Without going into a great speculative discussion, if you are fairly certain you are going to be there a long time then I'd invest the extra bucks.

 

[Voi]

Anyway, as many of you already know, the mineral wool approach isn't cheap so I'm looking at two options. One gives me R45 and the other would be R53.

If it were me I'd focus on air sealing first. This is time consuming but some of the value of the extra R8 can go towards caulk for junction between sheathing and roof rafters and canned spray foam for sealing electric boxes, electric pass through points, etc.

If there is money left over and your flat roof is framed with solid lumber or metal trusses then I'd also consider a layer of rigid foam the reduce thermal bridging. But since you have a new roof the only place for this would be on the interior which will eat up ceiling height. With tall ceilings it might not be a problem - that's up to you.

The thermal bridging issue is not as significant with I-joists or open web wood trusses. But R-8 or more of foam is still more effective here (in your case, on the interior side of the roof framing) than up in the rafters.

 

[Angelfire]

Thanks folks for the inputs.

I'm pretty much done with the air sealing at this point. All cavities in the walls have been caulked, all holes for electrical etc....have been foamed, around windows, doors, etc....just need to finish up the receptacle/switch boxes (am using duct seal on those) so I'm pretty confident it'll be pretty tight.

I have web trusses and since the slope is nil, it makes it a bit difficult to just roll out another layer on it....everything has to be done from underneath. I'm sure it can be done however and it's something I will reconsider. I had simply planned to put R30 up (Mineral wool) between the trusses, then push another piece of Rxx (Mineral wool again) up under it. But the mention of trying to get the batts to run perpendicular has me reconsidering this approach. And yeah, as much as I hate working with fiberglass, that may be an option as well (I must be somewhat allergic to the stuff....I will itch for days after messing with it even when properly suited up) although I'd rather not mess with it.

Thanks for the advice and it's given me a few things to go ponder!
Cheers.

 

[Dragfluid]

How about a little more info on your building? What is the height of the inside of the roof? What is the desired head space?

 

[mrpizza]

Cellulose would likely be cheaper and easier if you can get above the ceiling.

 

[Dragfluid]

Cellulose would likely be cheaper and easier if you can get above the ceiling.

Yes, cellulose is more cost effective. That's why I wanted the info that I asked for. If the inside roof height is high enough, he could frame up a ceiling and blow in 16" (or more) of cellulose and have a nicely insulated ceiling.

 

[Angelfire]

Hiya.
I need to get a tape on the underside of the roof to the bottom of the trusses but if memory serves, it's about 22" at the walls and peaks at around 24" or so. But like I said, I'll get a tape on it to be sure. I'm looking to maintain a minimum of 6" of headspace for ventilation above the insulation.

Your suggestions earlier about a mix of MW and FG have got me thinking I may actually kill two birds with one stone here. I'm working up numbers now that would give me an R60 ceiling for less than I was looking for in the R45 situation. This would be a combination of MW and Fiberglass and would give the 6" of headspace I'm looking for. So at this moment, I think that's my plan unless any new nuggets of wisdom come flying in.

I'm not entertaining anything being blown in (too messy if you ever plan to open the ceiling up) or spray foam (way too expensive). I realize I'm probably sacrificing some performance/efficiency/costs but I need to stick with something I can do myself with limited space to work (long story but we just relocated back from Ireland and the garage is a heap of stuff.....I'm getting by but a sub would say no way).
Cheers and thanks for the advice.

 

[ctfjr]

17% difference there going from R45 to R53.

My take is more of a concern on energy prices going forward. Without going into a great speculative discussion, if you are fairly certain you are going to be there a long time then I'd invest the extra bucks.

Well, that is true but not the whole story. Assuming your % savings is correct (its not, close but its not because it doesn't compare the 'total' R factor of the ceiling, that would include air spaces and other structural elements that provide some insulation. That would change the % difference).

More to the point is that the savings ONLY effects the heat loss of the ceiling. Let's assume a guesstimate that the heat loss of the ceiling represents 20% of the entire heat loss of the building (its probably much less). The the 'savings' would be 17% of 20% or 3.4% of the heat loss of the building. If he spent $2000 per year heating his garage (a pretty impossibly high number) he would save $68. So with the most generous numbers it would take 9 years to pay this back. With realistic numbers probably 15 or more.

 

[Firebrick43]

Well, that is true but not the whole story. Assuming your % savings is correct (its not, close but its not because it doesn't compare the 'total' R factor of the ceiling, that would include air spaces and other structural elements that provide some insulation. That would change the % difference).

More to the point is that the savings ONLY effects the heat loss of the ceiling. Let's assume a guesstimate that the heat loss of the ceiling represents 20% of the entire heat loss of the building (its probably much less). The the 'savings' would be 17% of 20% or 3.4% of the heat loss of the building. If he spent $2000 per year heating his garage (a pretty impossibly high number) he would save $68. So with the most generous numbers it would take 9 years to pay this back. With realistic numbers probably 15 or more.

I don't know where you got the 20% number, it's more like 35%.

They consider r60 the optimum range. R60 is the point where going past is a diminishing return and hard to make a ROI.

Fiberglass suffers r value loss the greater the delta t between inside/outside. It's loose construction allows convection air currents to form that degrade performance. Cellulose and rock wool don't suffer from this. I would personally use cellulose in the ceiling and rock wool in the wall(or dense pack cellulose)