Chooseing HVAC, help me understand thermal mass
Last Post 08 Mar 2009 06:41 AM by jmagill. 13 Replies.
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diyicfindianaUser is Offline
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15 Feb 2009 01:59 PM
We have been dreaming about this home we are about to build for years.  We have the house plans complete.  We are trying to finish the HVAC decisions and are now confused.  We thought we knew what we wanted and now our heads are spinning.

We are building an ICF house with a Lite Deck concrete roof.  It will be on a slab and will be bermed into a hillside.  It will be completely bermed on the rear, one side will have a sloping berm, the front will have four feet of berm, and the garage end will just be level.  But, there will be an ICF wall between the garage (which will be ICF) and the house.  South facing with lots of windows. 

We had planned on radiant floor heating.

Here is where we are not sure about things.  We planned the house this way for a lot of thermal mass.  We would like the house to stay warm in the fall after things have gotten cool outside and we would like the house to stay cool into the spring/early summer when things outside have gotten warm.  Plus, we just want to be half way underground to reap the benefits of the ground.

If we put down a vapor barrier and the XPS that is, say, R-10, aren't we just insulating ourselves out of the thermal mass that we so desire? 

How would you best heat and cool this home?
DonnerwetterUser is Offline
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15 Feb 2009 11:38 PM
Diyicfindinia

This is a hotly debated subject on the Thread "Internal wall surface temperatures". It would be wise (in my humble opinion) if you would read all the posts there. This was started by Bruce Frey; whom I for one respect greatly for his profissionalism and knowlage. Once at this Thread (as you are new here) you can PM (Private E-mail) any of the contributors there; to elaberate on their comments - as well as on any Thread of GBT (Green Building Talk). You are about to make a major financial decision - building your house! May I extend my welcome to this site and hope that you find it very helpful. (no ...I did not get paid for this announcement...LOL)
tdbuilderUser is Offline
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17 Feb 2009 08:53 AM
One thing to keep in mind is that there have been several reports done on xps vs eps. The study done by Foam Control (# 1016) states that after 15 years of being in contact with the soil the eps had 4.8% moisture content and the xps had 18.9%, after 4 weeks in a lab at 72 degrees with 50% humidity the eps had 0.7% and xps had 15.7%. So the eps hold less moisture and get rid of it faster. The eps retained 94% of its stated R value and xps retained 52% of its stated R value. When xps get wet it losses its R value, so you have to keep it dry.
allwallcoUser is Offline
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02 Mar 2009 05:51 PM
You get no thermal mass benefits from a wall system that has 2" of foam between the concrete mass and the interior. If you want DBMS (Dynamic Benefit of Mass Systems) then you need the mass to touch the interior. With ICFs you are benefiting with the mass as (1) concrete strength and (2) energy savings only from the mass, which is concrete, being a continuous barrier that is eliminating the air flow. No DBMS with ICFs. For DBMS benefits you need to build what can be referred to as: C-I-C or CFI (brand: All Wall System, formed and cast in place, some precast products)
H. John Griffin II, PE<br>[email protected]<br>(954) 325-7578
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07 Mar 2009 11:01 AM
Don't know about dynamic thermal mass, but in case you decide for any type of heat pumps, you will benefit from floor heating due to its lower temperature.
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07 Mar 2009 12:27 PM
John; not all ICF systems have 2" of foam on the inside. A combination 2" EPS - 6" concrete with 20mm PP or PEX Tubing with-in the concrete core (fluid filled) - 3/8" MgO2 board on the interior (or Heraglit, Tectum) would certainly result DMBS as well as far greater benefits. (Lop-sided ICF)
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07 Mar 2009 01:18 PM
XPS under the basement slab is a good idea, as it is almost always a good idea to have insulation between the climate controlled space and an area that varies in temperature from that climate controlled space.

You will get some thermal mass benefit, to help moderate temperature variations, from the concrete in the ICF wall. The more insulation you have between the living space and the thermal mass, the more it diminishes the effect, so the earth berm will have less effect, but your home is better insulated from the ground temperature.
Brad Kvanbek - ICFconstruction.net
jmagillUser is Offline
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07 Mar 2009 03:16 PM
There is a diminishing return on thermal mass just as there is with insulation. In other words too much mass is not good as well. Too much mass will not balance the thermal flywheel.

I agree that you would be best off with a lopsided ICF wall. Concrete to the interior of the home, insulation to the exterior and berm. The berm will act as added insulation.( though of lower value)

If you design the home properly for solar you should be able to get away lots less heating and cooling. Incorporate a way to vent hot air during the night an you may get away without air conditioning.( depending on your location.) Being able to cool the interior mass during the night makes a huge difference in a thermal mass home during the day.

allwallcoUser is Offline
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07 Mar 2009 10:12 PM
jmagill; What are you talking about when you wrote:
There is a diminishing return on thermal mass just as there is with insulation. In other words too much mass is not good as well. Too much mass will not balance the thermal flywheel.??

This makes no sense to me. Maybe you can explain the thermal flywheel concept so that it will make sense, please.

What I do know is that when you properly design a home with C-I-C you keep the doors and windows shut and you bring in fresh filtered air (many easy cost effective methods. about $150.00) You don't open the doors and windows and allow the mass to get to temperatures that aren't comfortable. Hence, you don't have to use energy to get your home back to the right temperature. You open doors when the temperature outside is comfortable.

With the proper envelope design and thermal mass walls "C-I-C" you will have a home that will cost you $0.027per SF. (National avg is $0.10/SF) When 2,400 SF homes operate complete energy at a cost of $65.00 per month and 3,500 SF homes operate at a complete energy cost of $185.00 per month this creates a LONGER time to recover the upfront costs of Solar Energy. This said, when solar energy is applied to a properly designed new home, you have to extend your break even years times four(4) unless sized properly. 5 years to break even becomes 20 years, etc. Solar is great when you can (1) get local power to buy the extra energy and (2) when you size the Solar Energy it to provide ONLY the energy I just described to you, which greatly reduces the size and cost of your Solar Energy application.

The FIRST step to conserving energy and minimizing a homes carbon footprint is to Build the home so that it uses the least energy possible. C-I-C walls (AllWallSystem.com), spray foam under the roof plywood tight to the top of the walls (use closed cell for additional strength- freebie benefit), shaded windows in cooling areas, thermal windows in heating areas. Remember more glass = more money.(period) for heating or cooling areas. For entry home buyers I suggest tankless water heaters, for people that can 'afford' to be more conscious I point them to solar hot water, then to solar energy. I'm hopeful there are more Energy Incentives in the forefront.

How much concrete is optimal for thermal mass on the interior walls in the conditioned space? The amount that is used on the houses above that are out performing any other methods of building homes, would be my answer.

I find many people keep trying to apply theories, thoughts and possible engineering opinions to how to build a home. I base Energy Savings on properly built homes that have been tested, but more importantly homes that are "operating" at the costs referenced above. I'm always willing to learn and look forward to further discussions. Educating home builders in regards to the Ease of construction of Energy Efficient homes is my real niche.
H. John Griffin II, PE<br>[email protected]<br>(954) 325-7578
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07 Mar 2009 11:01 PM
The all wall system claims to the "best of 16 walls tested" Oak Ridge National Laboratory. Do you know what they are best at? And/or where we can find the test report?

I believe ICFs are a better system, but if I am wrong I need to know so I can change systems.
Brad Kvanbek - ICFconstruction.net
allwallcoUser is Offline
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07 Mar 2009 11:17 PM
I've attached links to ORNL, within the below link:
http://allwallsystem.com/design/allwallOverICFs.html


H. John Griffin II, PE<br>[email protected]<br>(954) 325-7578
ICFconstructionUser is Offline
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07 Mar 2009 11:43 PM
Instead of robbing this thread we should start a new one on the ICF forum.
Brad Kvanbek - ICFconstruction.net
Bruce FreyUser is Offline
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08 Mar 2009 05:49 AM

The amount of "thermal mass" you need or want depends a lot on where you live and what you want to accomplish.

It seem to be an accepted fact that for a continuously conditioned space, themal mass that touches the interior space is the most beneficial.

For a typical diurnal cycle, having more than 2" of thermal mass material does not seem to have any added value.  If you are trying to capture more than one day, then obviously more thermal mass is better as may be the case for passive solar or other "thermal storage" systems.

The thermal mass of a typical ICF wall is of little benefit in the first case because it is insulated.  In cold climates where the exterior temps are always below the interior, there seems to be no energy benefit from the thermal mass of a typical ICF wall.  The thermal mass of an ICF wall DOES have an energy benefit where the outdoor temps fluctuate above and below the interior temp, although I do not believe it is significant.  There are many good reasons to use ICF but my view is that thermal mass is not one of them.

Bruce

jmagillUser is Offline
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08 Mar 2009 06:41 AM
Posted By allwallco on 03/07/2009 10:12 PM
jmagill; What are you talking about when you wrote:
There is a diminishing return on thermal mass just as there is with insulation. In other words too much mass is not good as well. Too much mass will not balance the thermal flywheel.??

This makes no sense to me. Maybe you can explain the thermal flywheel concept so that it will make sense, please.


Thermal flywheel, you need that explained? Okay.

Heat from the sun in a solar home is absorbed during the day and released at night. You only need enough mass to contain the heat you can store. Too much creates a longer lag. This is due to the rate the heat travels through concrete in a single day.

The goal is too balance your windows and mass so that you can store and then use the amount of heat from sunlight for a single  day. Proper use of windows and air flow in the summer months will reverse this thermal flywheel by cooling the mass during the night so it can absorb extra heat during the day keeping the home comfortable.

Proper sizing of mass can and does mean no energy use for heating on sunny days.

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