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Old 02-18-15, 03:59 PM   #31
Mikesolar
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So, MN, what is your target heat loss?

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Old 02-19-15, 08:13 AM   #32
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Quote:
Originally Posted by Mikesolar View Post
So, MN, what is your target heat loss?
Current manual J calculates to just above 26000 but my actual heat loss is just under 18000 at 70f and -13 outside(83 degree difference). ..although I never keep it at 70f when it is that cold outside and I'm comfortable colder than that normally anyway.

Here are my figures
Walls R13 fiberglass+R5 polyiso+R20 XPS=R38(maybe a point or two less because of bridging but in reality its very small when it is behind R25 of foam).

R38 walls 4596
R60 roof 1494 (I'm actually planning for more but these are the figured I sketched into notepad)
R5 glass 3602
9692 BTUhr at 83 degree difference
116.8 BTUhr per F

Add some for slab losses and air infiltration but since I'm well below the heat loss for manual J in regards to my actual measurements, I have a feeling that I could be below 10,000BTUhr either way. The odd thing is that my house configured this way with a 60 degree difference inside versus outside would probably hit the passive heat load figure which would be 70 inside and 10 outside or 60 inside with 0 outside. If I keep natural gas heat, I can use aggressive setbacks with a quick return to warmth. With my current 57,000 BTUhr output furance I can go from 40 degrees to 70 degrees in under 3 hours. I don't normally keep that large of a setback and if I stick with natural gas, the next furnace will be a 40,000 BTUhr condensing furnace. With better insulation I wouldn't sink as low into the setback, although recovery would be a similar time. When the house has warm air flowing, it feels a good 10 degrees warmer inside and once the furnace is running for a good 15 minutes, even 50 degrees feels acceptable with 55 comfortable with a t-shirt, subtract 5 degrees with a hoodie.
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Old 02-19-15, 09:38 AM   #33
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MN REn, will you be following the exterior insultn retro fit model of removing interior vapor barrier and installing a new one on the exterior of the R13 stud wall before spacing out impermeable exterior insulation with new exterior sheathing and siding? The R25 you plan to add should than put your new VPBR well inside the non condensing side of your new total insulation. It is important to remove old interior vapor barrier to not create a vapor pocket(if you believe it to be functional). When I reinsulated my current home the original interior vapor barrier was as nonfunctional as the wall insulation and the preventing of air infiltration around doors/windows.

You will be insulating to the point where doors, windows, air infiltration, fresh air needs will be far bigger factors for heat demand than more insulation. My house(modest sized 50's doll house) is about 25% lower R than your goal but it is high enough to really notice first the lose of two kids off to college and than even more the almost total conversion to LED lighting. The extra bodies and use of almost constant incandescent lighting did a lot of space heating that is now gone.
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Old 02-19-15, 12:43 PM   #34
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Drake, your:

" If I keep natural gas heat, I can use aggressive setbacks with a quick return to warmth."

won't work well with a radiant slab.
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Old 02-19-15, 06:47 PM   #35
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I think it was MN Ren that made that statement. I am quite aware of the opposite of that with a high mass slab and is exactly why we want it. NO more setback temp swings.
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Old 02-19-15, 08:04 PM   #36
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Sorry Drake indeed it was.
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Old 02-20-15, 10:27 AM   #37
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No offense taken, I appreciate all your comments.

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