GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

What kind of peak load could I achieve for my house?

Alan B | Posted in Energy Efficiency and Durability on

Dana had mentioned on another thread about the peak load sounding high for a house so i’m wondering how much i could do for my house if i could insulate and airseal it better. Hard to do based on my circumstances but for interest sake and something to work towards.

I live near Toronto Ontario, 99% design temp about 0F (-19C) in a house that is about 700sq ft. Poorly insulated about 14.5ACH50. Single story, 2/3 basement, 1/3 crawlspace, balloon framed, no basement floor (concrete) or basement wall insulation (brick) above ground true 2×4 loose fill cellulose walls, conventional attic true 2×8 iirc with old fiberglass paper faced batts between the rafters with the paper mangled up by animals over the decades. 2/3 of the attic is easy to stand up in, 1/3 has enough room to crawl in so i can’t do R50 unless its in contact with the rafters). Much of the air leakage is in the basement crawlspace and the balloon framing allows some gaps that extend from basement to attic and are partially sealed up as well as i can at this point.

Last winter we had a few weeks below the 99% temp and my 40K 95.5 AFUE furnace held up fine but for more then day was almost exactly at the design temp so i did some math based on the furnace run time between first and second stage dividing by efficiency and determined my peak load is about 25K (lets say ±2k). This is with vents closed in a few rooms and doors closed, one being an attached outside storage room.

I have the Hot2000 data for my house and it says the peak load is a bit over 40K so its not very accurate. I can hunt it down and upload it if that will help, let me know.

The basement is unfinished and the head room makes it a bad idea to insulate the floor and being a century plus old house i do not want to insulate the brick basement walls in case the lack of heat could cause them to crack. Perhaps not likely but I’m not wanting to chance it.

I am curious if i could get the air leakage under 5ACH50 and the attic insulated what kind of peak load i could get down to.

TIA

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Expert Member
    Dana Dorsett | | #1

    You can calculate the conducted losses from the U-factors, then the subtract the known indoor heat sources. That should be less than your duty-cycle estimated 25K. The difference would be the air infiltration.

    Alteratatively you can estimate the ACHnatural using the Lawrence Berkeley National Labs factors as outline in this Energy Star document:

    https://www.energystar.gov/ia/home_improvement/home_sealing/ES_HS_Spec_v1_0b.pdf

    See the map & table on p.5. You have LBL factor of 18.5 for a 1 story house. So if the house tested at 14.5 ACH/50 your infiltration rate would come in at about 14.5/18.5 = 0.78 ACHnat. Assuming 10' ceilings upstairs your volume is about 7000 cubic feet upstairs, and assuming average of 7' in the basement/crawl it'll be about 5000' cubic feet below, for 12,000 cubic feet total volume.

    0.78 x 12000= 9360 cubic feet per hour. Assuming 68F/20C indoors, 0F outdoors that's a temperature difference of 68F. Air has a specific heat by volume of about 0.018 BTU per cubic foot per degree F, so the infiltration load is then:

    9360 ft^3/hr x 68F x 0.018= 11, 457 BTU/hr (almost half the estimated total heat load.)

    Cutting that to 5ACH/50 would bring the infiltration losses to 11, 457 x 5/14.5= 3951 BTU/hr, knocking ~7500 BTU/hr off the total heat load. If that model is accurate for your house (big "IF"- the error bars are significant) that would bring the load down from 25K to 17.5K, which is pretty significant. Mind you 17,500/700' = 25 BTU/hr per square foot, which is still on the high side, but a heluva lot better than the 35 BTU/hr per square foot you measured.

    With even more tightening up and basement insulation it's usually possible to hit the 15-20 BTU/hr per square foot range (half where you started) without going nuts on it.

    It's totally safe to insulate the foundation on the exterior side with EPS. If you don't want to cover up all that pretty brick above grade, even digging down 2-3' below grade to accommodate 2-3" of EPS, on up to the first foot or so above grade will make a difference. The above grade portion can be finished with an EIFS or even just a hard-stucco scratch coat on steel lath troweled flat to make it look a bit like concrete. Before finishing the foam, cut a kerf into the mortar for some flashing at the top of the foam, and put a bit of slope to direct the moisture toward the exterior. Of course it'll be a much bigger difference if you can take the foam up as high as the foundation sill plate, and insert the flashing between the plate and the top of the brick.

    With 2" of EPS on the exterior then becomes totally safe to add up to a couple inches of rock wool on the interior- the brick dries toward the interior, and the exterior surface of the brick NEVER takes a hard-freeze the way it does right now. The average temperature of the brick would in fact be slightly higher than it is right now because the basement & crawlspace temps would be higher, with effectively no rain-moisture penetration, and less ground moisture reaching the brick it would be drier.

  2. Alan B | | #2

    "If that model is accurate for your house (big "IF"- the error bars are significant) that would bring the load down from 25K to 17.5K, which is pretty significant"

    Indeed, a very interesting reduction if accurate.

    "Mind you 17,500/700' = 25 BTU/hr per square foot, which is still on the high side, but a heluva lot better than the 35 BTU/hr per square foot you measured.
    With even more tightening up and basement insulation it's usually possible to hit the 15-20 BTU/hr per square foot range (half where you started) without going nuts on it."

    Thats very interesting, though with the challenges with this house i doubt i can pull it off though i would like to.

    "It's totally safe to insulate the foundation on the exterior side with EPS."
    I agree but this can only be done on one side relatively easily. On the one side there is a makeshift concrete riser outside the foundation added much later presumably because that side of the house has a higher grade (house built on a slight hill) and the mudsill was built at grade (not higher) and might even be a bit below grade on that side. On side 3 there is a driveway beside the foundation and a concrete patio butting the foundation on the rest and on side 4 there is a slab on grade and a concrete side walk.

    "If you don't want to cover up all that pretty brick above grade, even digging down 2-3' below grade to accommodate 2-3" of EPS, on up to the first foot or so above grade will make a difference. The above grade portion can be finished with an EIFS or even just a hard-stucco scratch coat on steel lath troweled flat to make it look a bit like concrete. Before finishing the foam, cut a kerf into the mortar for some flashing at the top of the foam, and put a bit of slope to direct the moisture toward the exterior. Of course it'll be a much bigger difference if you can take the foam up as high as the foundation sill plate, and insert the flashing between the plate and the top of the brick.
    With 2" of EPS on the exterior then becomes totally safe to add up to a couple inches of rock wool on the interior- the brick dries toward the interior, and the exterior surface of the brick NEVER takes a hard-freeze the way it does right now. The average temperature of the brick would in fact be slightly higher than it is right now because the basement & crawlspace temps would be higher, with effectively no rain-moisture penetration, and less ground moisture reaching the brick it would be drier."

    I forgot this part, above grade is not brick, the wall stackup is plaster, lath, plaster, 2" barn board, true 2x4 (with loose fill cellulose), painted wood siding (no outside sheathing) a plastic very air permeable "blanket" and vinyl siding.

    Also if i insulate the vented attic what is the best way to deal with the 1/3 part of the roof that can only be crawled in? Can i put insulation up to the rafters or is that a moisture risk?

  3. Alan B | | #3

    bump

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |