Insulation under subfloor in solar house?
Last Post 19 Mar 2019 07:49 PM by sailawayrb. 6 Replies.
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Pete DronkersUser is Offline
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02 Mar 2019 09:37 PM
Hi all, new to this forum and looking forward to getting involved! Dr. Google hasn't been so helpful in trying to figure this out; I'm currently designing my solar round house in southern Montana, but it's not being built on a concrete slab. It has tons of solar gain but a standard 9 1/2 inch floor joist and subfloor system with 3 foot crawl space and partial unfinished basement. It's using 32 inch ICF foundation walls backfilled around them, plus spray foam for insulation. It will be extremely tight, including the basement. The basement is part of the thermal envelope. I'm trying to select a good flooring option that will both absorb at least some heat, as well as resist damage from being drenched in sunlight much of the year (the low E 180 windows I'm selecting are built specifically for passive solar, but only block about 80% of UV light). In thinking this through, the question also came up about whether or not to insulate under the floor where the sun will be hitting the floor (essentially the southern half) The benefit would seem to be less heat infiltration into the basement where it's not really needed, and more heat in the house. But on the other hand, if that heat does penetrate into the basement area, given how well insulated the crawl space is and how deep it all is within the earth, I wonder if this might actually help to absorb some of that heat and disburse it back into the house at night, much like a slab would. There isn't much literature about this. Most passive solar houses use slabs and so the idea of a passive solar house build on a standard subfloor isn't a common topic. My instinct is to insulate under the area of solar gain, but wanted to see if there's another way to look at it. Thoughts are most appreciated. Thanks! --Pete
Dana1User is Offline
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02 Mar 2019 10:54 PM
Tons of solar gain on a low-mass house isn't necessarily a good thing. Going nuts on solar gain can easily roast you out of the place during the day even in winter, even at -10F.

Simulate the house with at least something like BeOpt (a free downloadable DOE modeling tool) as you tweak the design features. It will tell you whether insulating between the joist helps or hurts your situation. (My gut tells me that insulating between the crawlspace and first floor is more likely to increase energy use and lower comfort.)
Pete DronkersUser is Offline
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02 Mar 2019 11:20 PM
Thanks Dana1. I have designed an awning system that matches the latitude to shade the windows entirely in summer but on the winter solstice the solar penetration will reach pretty far into the house. The amount of window space isn't over the top. On the south side it's between 1/3 and 1/4 of the walls. I wish I could attach a 3d rendering of the house so you could see, but can't seem to embed a pic into this window. I will certainly look into BeOpt. I guess I can always do exterior screens if the gain turns out to be too much, but not much you can do when it's not enough.

rvalueUser is Offline
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19 Mar 2019 12:20 AM
Pete, why not consider eliminating the crawlspace? We build homes on slabs a fair bit here; where everyone says "you have to have a basement/crawlspace". That way you can absorb some of that heat.
Dana1 is right, you can easily overheat in the winter, then you end up having an argument about opening your windows because your wife gets too warm cleaning the home, and you don't want to waste the free energy. Or you could just live in a dirty, messy home.
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sailawayrbUser is Offline
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19 Mar 2019 03:23 PM
Some of the original passive solar designs of the 1960s would move air under the floor. These designs didn’t work well and moving warm moist air in places that can’t be inspected and cleaned led to mold issues too.

Passive solar building designs require the proper amount of interior thermal mass to perform well. Concrete floors and masonry walls are typically used to supply this interior thermal mass. Passive solar buildings should be properly designed (roof overhang geometry, required thermal mass and maximum monthly generated Btu/h heat gain for the building location and climate), aligned with the expected monthly building heat loss (a function of climate and building construction), and fully integrated with the HVAC system to eliminate any risk of over heating the building. We have a suite of free DIY passive solar design calculators on our website that will enable accomplishing this:

Borst Passive Solar Altitude Angle Calculator

Borst Passive Solar Roof Overhang Design Calculator

Borst Passive Solar Fenestration Exposure Calculator

Borst Passive Solar Heat Gain Calculator

Borst Passive Solar Thermal Mass Performance Calculator

Crawl spaces are just nasty and are always avoided by good designers/builders these days. If you doing an ICF foundation, you should consider just floating a slab between the ICF foundation walls to whatever slab elevation you desire. You can use self-compacting pea gravel to fill the ICF foundation walls to achieve the desired slab elevation. Having a concrete slab floor is also the lowest cost approach for having the highest performance hydronic radiant heated floor too.
Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do!
Dana1User is Offline
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19 Mar 2019 05:10 PM
"Crawl spaces are just nasty and are always avoided by good designers/builders these days."

Not necessarily. Sometimes crawl spaces in the form of a conditioned "mini-basement" can be an optimal place to install ducts, drains, water etc where it can be inspected, modified and repaired easily completely within the thermal & pressure boundary of the house, often for less money than solutions for keeping it all indoors for a slab-on-grade house.
sailawayrbUser is Offline
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19 Mar 2019 07:49 PM
Well, I did qualify my statement by saying “good designers/builders”...LOL! Yeah, if you are looking for the easiest GC solution, do the tract home crawl space solution. Or if you already have a bug and rat infested crawl space it might make sense to seal it and condition it...but seal it well as rats will chew into and out of ABS drain lines to gain access to a conditioned crawl space... However, if you are looking for the best long term and highest energy efficiency solution for new construction, never do a crawl space. Duct work is increasing no longer allowed in unconditioned crawl spaces and attics as States step up to the newer energy efficiency codes. Conditioning space that you don’t actually live in reduces energy efficiency and should always be minimized. Duct work is better placed in plenum trusses or soffits and should always be within the conditioned building envelope. Drains lines can be located and placed under slabs such that they can be more easily repaired than in a crawl space. Water lines can be placed in chases such that they can be more easily repaired than in a crawl space. No one likes to work in a crawl space, even if it is conditioned and will charge you accordingly for repairs. And designers/builders tend to do what they always have done...so perhaps it would have been more accurate for me to have said "better informed and more progressive designers/builders"...
Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do!
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