I bought an old National Home <1000 s.f. that I have been working on for an elderly relative to live in and I am wondering how much insulation is enough before I spend money foolishly. The house is one story on a slab in IL.
I have replaced all the windows and doors, foamed all the exterior walls and have 3.5 fiberglass batts in the ceiling. I am insulating the attic now and plan to include it in the conditioned space.
The rafters are only 2 x 4s and are going to stay that way – I did sister an additional 2 x 4 to each one the full length. I put 1.5 inches of foil-covered polyisocyanurate rigid insulation between the rafters, leaving an air space between the foil and the roof deck.
Now I am putting 3.25 inch polyiso inder that, covering the entire attic and gable end walls and sealing it all very carefully so it will be a very tight house. My question comes down to – should I leave it at the 1.5 inch plus the 3.25 or should I add another layer of 3.25 under the first?
I was told the Polyiso R value is 6.5/inch – I thought it was 6.0/inch -, but either way, I want the house to stay warm and not be drafty and the gas bill low. Do you think the second layer of 3.25 inch is worth it?
Thanks
Replies
R43 is enough.
I would add a little more than the 4.75 inches which is about R31.
R43 is enough accodring to the state of Illinois, see here:
http://www.illinois.gov/KeepWarm/Documents/insulation.pdf
I would use enough foam to get around r50, which is about 8" myself, but any way, the state says you need 6.7".
most of IL is at about 6500 total degree days.
= 156K degree hours.
If R=1, you lose 156,000 BTUs per sq foot of ceiling
Guess at IL energy prices at $1.00 per 100,000 BTU, thus at R1 it cost $1.56 per sq ft per year of heat loss thru ceiling.
You have a R value of about R30, so your energy cost is 5.2 cents per year per square foot.
Another 3" will boost you to about R50, and your cost will drop to 3.1 cents per year per sq foot.
Say a sq ft of insulation is 60 cents - you spend 60 cents and save 2.1 cents per year, ROI = 3.5%.
If your investments return > 3.5% or you expect them to be better than that fafter inflation in future years, no added insulation, if your return less or you want to "lock in" that 3.5% return, then add the insulation.
Add another layer on top of that any your return for that last layer drops to 1.5%, etc..
Me, I'd go ahead and add the layer, as even with fracking and high nat gas supplies, energy costs increases could easily outstrip inflation which drives the ROI above higher in future years if energy costs rise fast.
most of IL is at about 6500 total degree days.
= 156K degree hours.
If R=1, you lose 156,000 BTUs per sq foot of ceiling
Guess at IL energy prices at $1.00 per 100,000 BTU, thus at R1 it cost $1.56 per sq ft per year of heat loss thru ceiling.
You have a R value of about R30, so your energy cost is 5.2 cents per year per square foot.
Another 3" will boost you to about R50, and your cost will drop to 3.1 cents per year per sq foot.
Say a sq ft of insulation is 60 cents - you spend 60 cents and save 2.1 cents per year, ROI = 3.5%.
If your investments return > 3.5% or you expect them to be better than that fafter inflation in future years, no added insulation, if your return less or you want to "lock in" that 3.5% return, then add the insulation.
Add another layer on top of that any your return for that last layer drops to 1.5%, etc..
Me, I'd go ahead and add the layer, as even with fracking and high nat gas supplies, energy costs increases could easily outstrip inflation which drives the ROI above higher in future years if energy costs rise fast.
How much is enough?
Thanks for the replys. I am going to put both layers on. The insulation boards are actually 3.25 inches, so that will give me 8 inches of Polyiso under the roof with the foil radiant barrier under the ventilation channel, plus the 3 inches of fiberglass on top of the ceiling in the living space.
It may be overkill, but I have the gable end open now and I can get the 4 x 8 sheets in easily - an option I lose when I side it, so I figured better to err on the side of more. I'm hoping it stays temperate in the attic through leaks in the attic floor, but I will add a small vent if I have to.
I am foaming and taping the joints where the boards meet and foaming any voids where I have to cut around obstacles like plumbing vents, supports, etc. I just wonder if I should put some form of barrier on the underside to reflect heat down and ensure the vapor barrier?