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Old 12-17-12, 11:05 AM   #1
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Default Calculated heat load. Have you done it?

I feel like disclosing this and asking others where they are.

My calculated heat load for my 2160 sq ft is 285BTU/hr per degree of difference outside without sun, overnight heat load. Some thermal inertia from the day may fall into this calculation but this is based on measurements with outdoor temperatures below -10f in heating design load conditions and house temperature was without setback for a substantial period of time.

Structure configuration:
1080 sq ft foundation in a 4 story split configuration with basement undergrade, 2nd level half undergrade, 3rd and 4th stories above grade.

Infiltration:
Decently tight but haven't sealed sill plates with foam board yet and I have three smallish windows downstairs that are seeping cold air in through their thin 1/16" crack from expansion and contraction as I haven't broken the 1/4-1/2" of drywall filler and spray foamed them. I have resolved all of the above grade window surround air leaks, which were substantial gaps with fiberglass that were open-mouth breathers. I measured a gap in the upstairs bedroom window that was 1 1/4 or 1 1/2" inches wide, don't remember at the moment.

Image of convection washdown that has since been resolved. The attic framing goes straight down from inside the attic and the fiberglass ended just above it and some cellulose fell into the cavity but it was basically open allowing heat to rise up and out and for cold air to wash back down into it.


Sill plate leakage(not resolved yet)


What a 1/16" crack looks like at 50 pascals


Insulation:
2x4 construction with R13 in the walls with 3/4" foam exterior. Energy auditor somehow called this an R19 wall since that was code when the house was built, the math doesn't line up though as that would be about R17. During the load calculation period there is 7" of settled cellulose, my neighbor says our houses had 12" when they were built, I don't think it settled that much so I think the original buyer and probably my neighbor was short-changed. House has a significant amount of double-pane glass to my standards, front window has a broken seal and is dirty between the panes from the air and moisture that has passed into it.

Image of average window construction that has been sprayed foamed

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Old 12-17-12, 06:13 PM   #2
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IR cam + blower door + winter time is tons of fun! As it's been getting colder here I've been doing this several times a week.

For the record, my house is a conditioned 2,200 Sq. Ft. with a heat load of just under 19,000 btu/hr at -5. I haven't done much work on the top side of the place yet but there is just enough cellulose on top of balsam wool bags in the attic to cover the joists and some balsam wool bags in the walls. I have done some air sealing in the attic. Ha! Last week I found a wet wall chase that went plain from the attic to the basement. When I was closing it up some cellulose fell down the chute and ended up all over some of my children's stuffed animals down there.

Oh, and there is no record of anything like 12" of cellulose ever settling to 7". The manufacturer we use out here, National Fiber, lists the installed and settled depth and the settled depth is just above the densest recorded density. See: http://nationalfiber.com/docs/CelPakSpecSheet1209.pdf
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Old 12-18-12, 08:10 AM   #3
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Yeah, I looked at the GreenFiber blown vs. settled thickness too and it's clear that they didn't put as much as my neighbor claims is at his house.

What is the indoor temperature to your 19,000btu/hr at -5f outdoors? If it is 70 degrees, that would be 253btu/hr per degree. If it is 65 degrees that would be 271btu/hr per degree of difference between outside and inside. You are probably a little tighter and better insulated. Also are your walls 2x6 or 2x4? I think I can catch up as I don't feel too far away, we'll see.

I just think it's crazy that most homes aren't equipped with a 40,000BTU/hr furnace. I tell people that my heat load is low enough for it and nobody believes me. A well built and sealed 2x6 house with R19 in the walls and R10 foam clad around the outside and R60 in the ceiling, which to me is modest and not really a lot could probably be nearly 4000sq ft before a 40,000BTU/hr furnace is too small even when it gets to be -20f outside as long as the house doesn't have too much glass.

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Old 12-18-12, 04:37 PM   #4
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A note about GreenFiber. I believe this is what's sold at Home Depot? One Sunday I had to get a boat load of that stuff (over 100 bags) because the lumber yards were closed. We ran out of National Fiber cellulose part way through an attic open blow. It's dusty! And it stinks! It smells like horse urine when exposed to moisture (ammonium sulphate). The dustiness alone makes me question it. All of the other brands I have used like NF (100% boron borax) and Green Seal (Also contains ammonium sulphate) are more like fuzz and less like dust. So I have my doubts about the quality of Green Fiber.

The design temp for the calcs I gave were -5 and 70. The problem with a manual j, even one done by a very intelligent, informed, patient and observant individual is that it still doesn't cover all bases properly. By the sounds of it your house should probably out preform mine. My walls are 2x4 with little insulation at all. My windows aren't worth a.... well, they're not good. My walls are board sheathed. I have some things going for me though. The house is a ranch which is not only the easiest type of house to work on, it is also inherently the tightest due to simple geometry and lack of a band joist and so on.

What's in your walls? Have you dense packed them? You have R-10 on the sheathing? Nice! I'm jealous. How did you make R-10? Sounds like 2" of XPS. A 4,000 Sq. Ft. house could certainly be 40,000 btu/hr if the air sealing if handled properly. If it's passivehaus like it could probably be 1/2 that. People are nuts when it comes to building envelopes. I want to cry every day when I go to work and see these houses, many of which are less than 20 years old.
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Old 12-19-12, 06:41 AM   #5
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"What's in your walls? Have you dense packed them? You have R-10 on the sheathing? Nice! I'm jealous. How did you make R-10? Sounds like 2" of XPS. A 4,000 Sq. Ft. house could certainly be 40,000 btu/hr if the air sealing if handled properly. If it's passivehaus like it could probably be 1/2 that. People are nuts when it comes to building envelopes. I want to cry every day when I go to work and see these houses, many of which are less than 20 years old."

No, The original post was mine but I was trying to do math for what I thought would extrapolate with higher levels of insulation that would still work with the smallest condensing furnace out there with the 2x6 construction with 2" exterior foam, close attention sealing during the build, etc. My exterior foam is R-Max and is 3/4" inch thick, 2x4 walls with fiberglass all installed when it was built.

My top floor has a shared wall with the attic where the studs go straight down for the lower floors with 2x4 framing, but the attic rafters are separate 2x4 2 foot OC with only the interior 2x4s with insulation. I'm adding insulation to the outer 2x6s but I can't find the insulweb stuff, I don't need much, just enough to cover 11 feet wide by 4.5 feet tall. The other half of the upstairs I put fiberglass there but I can't get a roll of fiberglass across the 2 feet of headroom that I have when I need to traverse the vault that is between the two halves of the attic and I'm not about to cut 7 or 8 sections and drag them across that section unless I really need to.

The hard part to get cross, smaller than it looks.


Looking at the area that was completely open from the attic to an interior lower level wall. You can see the 2 foot OC framing if you look to the right of this image.


2 inch foam spray foamed into one of the openings shown above, this was the first one and I was a bit sloppy, the others look better but I didn't take a picture.


The opposite side of one of the upper level walls, this is part of the section that I haven't gotten the insulation into. Ideally I'd like to dense pack this with cellulose. You can see what damage that opening to the attic does from the second picture.


...I also have a skylight where the drywall surround is sheathed but not insulated yet and I imagine that is the biggest energy sieve of the attic. I need to get up there, planning to wrap that with 2" foam, should I put two layers and have 4"? I really actually want to remove that skylight but I don't know how much work that would be because I'd need to cut out the drywall surround for it, cut it out of the roof and patch the roof and the ceiling. Not sure how hard that would be but I think it is out of the cards for me to do that job, especially the roof part.

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Old 12-19-12, 07:07 AM   #6
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For my insulation I bought just over 1000 pounds of it for about $300. I actually think I have enough for about R60 to R75 blowing it over the 7 inches settled that I have now.

If I can get insulweb stuff(not sure where to buy it, not seeing at the hardware stores and the people at the store are clueless when I ask for it or describe what its for), I'll probably fill the walls that face into the attic to R40 putting that over all of the existing fiberglass since if I were able to and it were cost effective, I'd have all the walls at R40. Unfortunately for the other exterior walls, I'm not good at drywall and I don't think it would make sense to tear them out, frame it for more insulation and add more so they will be R13 plus the 3/4" exterior foam for the life of the house.

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Old 12-19-12, 07:09 AM   #7
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You should be able to get cellulose netting at any decent lumber yard. If not they can order it for you. Worst case is you order a roll yourself. I will respond later with more details so we can continue this interesting discussion.
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Old 01-08-13, 08:52 AM   #8
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I'm not sure if you have any in your area, but Menards carries insulation mesh / web. When I got the mesh for my wall it was sold in 5' x 300' rolls and was $50.
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Old 01-09-13, 07:26 AM   #9
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Calculated heat loads (typically Manual J) and actual measured heat loads are often way, way off. I have done manual J for decades now and use this only as a wild guesstimate.

I have found that air infiltration is a FAR better predictor of heat load. Some examples:

Manual J says to put in a 4 tonn (output) heating unit for a maximum design temp of -10 F (1800 sq foot home). Builder did a tremendous job sealing home and blower door test shows less than two air exchanges per day! Air to air HRV provides necessary ventilation.

Standard 2x6 walls (probably net R15), R 40 in attic, R 20 below floor and double pane gas filled windows.

I suggested a 2 tonn unit. First HVAC installer would not do this as it was incompetance on my part. Had to find another installer that would do it

Actual field tests show unit running 100% at design temp with a 2 tonn unit maintaining an interior temp of 75 F.

Another similar sized home with poor air sealing (and yet more insulation) required a 4 tonn unit. No ERV, poor sealing around windows/doors. Blower door showed air exchange of once per hour . . .

Personally, I would take a less than adequately insulated home, but one with very low air infiltration over a higly insulated home any day.

Insulation is easy to measure, sealing cracks is hard to quantify, but sealing air leaks has a FAR larger effect.

Think about this . . . what is the point if insulating a long box with rolls and rolls of insulation on the outside - if both ends are left opened.
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Old 01-09-13, 05:46 PM   #10
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I should clarify something. I'm not talking about a manual J load calculation. I'm talking about the load calculation based on furnace runtime when we had multiple instances of -10 to -20f temperatures outside and then calculated to a 'per degree' value. These are actual load values. This is why I'm so confident that a 40k condensing furnace could do the job in my 2100 sq ft house because the 75k 76% furnace(57k output) doesn't even run half the time when it is at design temp.

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