too much insulation?
farmgirl3
| Posted in General Questions on
I am building a very simple ADU, 20 x 24 with a simple gable roof and double stud walls. One level, slab on grade. Southern Maine, zone 6
I am planning on rockwool insulation batts in 2 x 6 exterior wall with a 3 inch gap filled with rockwool batts and an interior 2 x 4 wall no insulation. This will give me an r38 wall.
The ceiling will have r53.
Yes, smart vapor barrier, yes vented rain screen.
Is this too much insulation?
Should I use less in the gap between walls? The ceiling?
Asking because when you do enough research you get very turned around sometimes!!
thanx
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Replies
Overall yes.
You can calculate the heat loss for any surace and see where you stand.
For your ~500sqft ADU will about about that many sqft of external wall area. Zone 6 means somewhere around 0F design temperature so the heat loss would be:
Walls
500sqft*(70F-0F)/R38=921BTU
Roof
500sqft*(70-0F)/R52=631BTU
So overall so small I rather have that go up a bit and save on the build cost and a bit more floor area.
You can try different R values in the above formula to see where your heat loss/build cost works out the best. You are also at the point that your overall heat loss would be dominated by your windows/doors and air leakage, so those also need a lot of attention.
These higher R values only make sense if you are using cheap insulation like cellulose for both walls and roof. In that case you'll end up with an ADU you can heat with a couple of 100W light bulbs.
Thanx for the reply but I have to admit to not understanding the formulas.
I didnt say before I will be using energy star doors and mathews bros double pane windows.
Does a 2 x 4 exterior wall make more sense and still have a gap with another layer of rockwool? Giving an R30 wall?
And just do the ceiling with R 38 rockwool batts?
Heating with a heat pump sized by a local heat pump contractor
thanx
What level foundation insulation, what type windows and doors?
I would think the only way to 'overinsulate' would be to waste money on insulation that isn't saving you more money/fuel/carbon when there is a way to better spend that money.
Your 60-100 sq ft of windows may lose more heat than all the walls, and if you chose a simpler wall assembly you could use that money to go to the next better grade of windows.
Also many places tax you on sq ft of footprint, so if you have a more slender wall assembly you are getting more space for your tax dollar.
energy star windows and doors, mathews bros double pane windows
I dont have the foundation r number
The more relevant question, for most of us, is how to achieve the greatest value for energy efficiency? gusfhb addressed this - shifting some of your costs to better windows, doors, and seals may provide more benefit than wall or ceiling insulation. With such a small and presumably tight structure, ventilation and hot water may turn out to be (relatively) large sources of heat loss.
too much insulation?
The answer depends on your point of view and goals.
If you have the money to spend and it make you feel good. Then No
For a production builder that builds the code minimum house Then Yes.
If you are likely to sell the house in 6 years like most people do. Then Yes.
From a return on investment point of view assuming energy prices go up less than 10% per year. Then Yes.
If you have drunken the Passive house Kool Aid My guess is NO.
If you are preparing for Armageddon. Then NO.
If you are 18 years old and are positive you will be living in this house when you die on your 115 birthday. Then NO.
If you are convinced the earth will warm to the point that NY will have a tropical climate in the next few years. Then Yes.
Walta
Thanx
Succinct and very funny and I appreciate the kind words.
You must include the cost of the different heating fuels in the ROI calculation for insulation. This is part of a sound building plan. A well designed home will feel roomy but have a minimum of building envelope for the square footage. Avoid fancy rooflines and appendages to the main structure, they do not add value. Build better in terms of energy efficiency, we have no idea of the consequences coming from unlimited fossil fuel usage.
A good building envelope efficiency goal for a heating climate would be 1 Btu per square foot per heating degree day. This will put you in the Net Zero range for any heating dominated area.
Doug