# Question re: thermal mass vs. basement (long)



## cc-rider (Jul 29, 2003)

I had originally designed a house on a slab, with all the plumbing running down one 8â thick wall through the house. All fixtures are either against this wall, or on an outside wall. Now, though, I like the idea of a basement better. But Iâll lose the thermal storage of the slab. 

If I build on a basement instead of a slab, how do I get enough thermal mass inside my building? I had initially thought of a slab over a foot or so of stone, but now Iâm thinking a basement. I donât really want concrete block walls in the house, and I donât have attic storage for water bottles. How much thermal mass would I need for the following situation?

The currently plan is for the main house is basically a 24âx 24â square. There is a 12â x 24â bedroom/bath/laundry wing attached to the east side. This gives me a total square footage of 864 (of course, that includes closets, etc.)

There is an attached, well-insulated sunspace on the south side of the main house, probably on a slab or piers, measuring 12â x 24â. An enclosed stairwell runs along the west side, going to the basement. I donât think I want thermal storage in this room because I am using it to collect heat for the rest of the house. I have 80 square foot of windows on the south, plus a patio door on the east. 

On the wall between the sunroom and the main house, there is 79.5 square foot of windows/sliding doors on the south. (How much, if any, direct gain will I get in these windows?? The sunroom is 12â deep, with about an 8â flat ceiling).

In addition to the 80 square foot of windows on the south wall, there is another 90 square foot of windows on the south as clerestory windows (that open into the main house), plus another 24 square foot of windows on the clerestory into the bedroom addition. (So 90â goes into the main houseâ¦. which is 576 square foot, and 24â goes into the bedroom addition, which is 288 square foot). 

In addition to these windows, there are NO windows on the west, possibly one on the east (in the bedroom addition), and 1 window in the bathroom, 1 in the kitchen, and 1 in the living room on the north. There is an attached air-locked entry on the north side, too, which will have one window on the west, door on the north.

There is a 6â wide âcatwalkâ under the clerestory windows, accessed by a spiral staircase.

My questionsâ¦.

Do I have too many windows on the south? Will I overheat? How much thermal mass would I need to compensate for this? I will be able to cover the clerestory windows with thermal panels at night and in the summer as needed.

Where can I build in thermal mass? Iâd like to have a fireplace in the sunroom (but I donât need thermal mass in there). I will have a corn stove in the main house, and can put a stone wall behind that. Will using tiles over the subflooring create any thermal mass? How do I gain more? I hope to have cathedral ceilings, so will not have an attic.

The catwalk will be under the clerestory windows and about 6â wide. The windows in the clerestory are 3â high (plus a patio door going to a deck outside of that area). The window-wall height in the clerestory is about 8â. Iâll have bookcases under the windows. Will the sunlight coming in these windows âclearâ the catwalk and shine into the house? (There will be a rail, but no other walls) Iâm in northwest Ohio, if that matters. I canât figure out how to predict where the sunlight will fall.

Thanks!
Chris


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## rambler (Jan 20, 2004)

Will help to know where you live - how much heat you need, and your sun angle.

Up here a basement is a normal thing, a slab is a poor house material.

Build overhangs over the window areas, angled sun will come in in winter, will be shaded out when it is directly overhead in summer.

We need too many btu's up here to really store too much. Move a little air so the back side gets some heat too, and not worry about it - night time _will_ need that corn burner running. 

Your situation, in a milder climate maybe, can be very different.

--->Paul


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## suburbanite (Jul 27, 2006)

If you have your lattitude then it should be a simple matter to find the data for the sun's highest and lowest excursions from the equator and use that to figure the angles of the sun at its lowest and highest for the year (at the solstices). Then at least you will know where the light will fall in your rooms based on some simple trigonometry.

It is hard to beat water for heat capacity. It can double as an emergency drinking supply.

A friend of mine has a custom fishtank that takes the upper half of one wall of the living room. Decoration, and thermal mass by default.


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## cc-rider (Jul 29, 2003)

rambler said:


> Will help to know where you live - how much heat you need, and your sun angle.
> 
> Up here a basement is a normal thing, a slab is a poor house material.
> 
> ...


Thanks Paul,

I'm in northwest Ohio. Flat, windy, cold.  I looked up the latitude once....but couldn't figure out the charts. 

I'm hoping to figure out how to make natural thermal movement (I can't think of the term) by using the clerestory part, and vents in the floors, etc. YEAH, I keep that corn burner burning NOW in my house in town! Love it! My only source of heat.


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## cc-rider (Jul 29, 2003)

suburbanite said:


> If you have your lattitude then it should be a simple matter to find the data for the sun's highest and lowest excursions from the equator and use that to figure the angles of the sun at its lowest and highest for the year (at the solstices). Then at least you will know where the light will fall in your rooms based on some simple trigonometry.
> 
> It is hard to beat water for heat capacity. It can double as an emergency drinking supply.
> 
> A friend of mine has a custom fishtank that takes the upper half of one wall of the living room. Decoration, and thermal mass by default.


Trigonometry. You say that like it's easy?!  

I just don't like water. I don't like ponds, creeks, nice clean swimming pools, fish aquariums. I just have this "thing" about water, I guess! A nice hot shower is different, however! I don't know if it is just that I'm afraid of water, or I really don't like the 70's waterbarrels-under-every-window type of thing. And I really, truly know that water is the best heat storage....but just can't go there. What is the NEXT best???


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## suburbanite (Jul 27, 2006)

Well, you need to know the magic old ***** chant

soh
cah
toa

sine = opposite/hypotenuse
cosine = adjacent/hypotenuse
tangent = opposite/adjacent

...........|\
opposite|_\hypotenuse
...........adjacent

where the angle in question is the one on the bottom right of the 'mysterious ***** glyph'.

(you gotta remember my undergraduate degree was chemical engineering, which means for me 'hard' is something that requires a laplace transform before you can solve it.)


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## Rick Allen (Jun 5, 2006)

A few thoughts to your questions:

1) A basement is expensive when compared to additional above ground living space.
2) A basement won't add the same resale value as the same living space above ground.
3) Exterrier concrete walls insulated on the outside with the concrete left exposed on the inside adds mass. The winter sun should directly hit the walls for best heat transfer.
4) Plywood subfloors covered with cement wonderboard and finished with ceramic tile add mass to conventional floor structures. Floor joists should be beefed up.
5) Don't worry about too much south window area. We just close out mini blinds in the summer and eliminate the vast majority of heat.

Lots of luck./RA


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## SolarGary (Sep 8, 2005)

Hi,
The rough rule of thumb on south glazing is that you can have up to about 8% of the floor space in south glazing without adding any extra thermal mass.
If you want a more exact reading on this, you might try downloading and running the free HEED software (link below). This software works with a weather file for your area, and does an hour by hour year long simulation -- it has one graph that shows overheating problems. Its easy to use -- it was designed for use by the general public (no trig  ). You can use the "Advanced" menu to add more thermal mass, and see what the effect is.

Thermal mass that the sun shines on directly is worth about 4 times thermal mass that it does not shine on. So, try to put your thermal mass where the sun shines directly on it part of the day. Thermal mass thicker than 4 inches is ineffective on a daily cycle -- that is, no need to go thicker than 4 inches.

Adding thermal mass:
- double up on sheet rock
- stone or tile floors.
- water containers of any sort -- see the water wall link below
- masonary walls

Are you sure you want to cut back on windows so much in the rest on the non-south facing walls? Windows are nice for light ...
You can use thermal shades or plugs to control heat loss at night and keep the size down.

For solar homes the idea you were first thinking about with all the square footage upstairs on a slab works out well. You get good thermal mass from the slab essentially for free -- it also lets you stretch the house out along the east/west axis, which makes for more rooms with a good south solar exposure. A lot of people like a plan with the kitchen in the SE corner for early sunlight, and the main living rooms along the south wall for good solar heat and light. But, I'm sure you can make either way work.

Links ----------
HEED solar home design software:
http://www.builditsolar.com/References/energysimsrs.htm
(its the first one on this page).

For figuring out shading for overhangs, this is a nice tool:
http://susdesign.com/overhang/index.php
Very easy to use -- just move sliders.

The Solar House, Daniel Chiras nice book on building and living in a solar house. More good ones here:
http://www.builditsolar.com/Projects/SolarHomes/guidesps.htm

Water walls for thermal mass:
http://www.builditsolar.com/Projects/SpaceHeating/AWaterWallIntro.pdf

Lots of info on designing and building solar homes on my site:
http://www.builditsolar.com/Projects/SolarHomes/solarhomes.htm



Gary


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## artificer (Feb 26, 2007)

If you want thermal mass, look into Insulated Concrete Forms. You can build the walls from them as well. There is also nothing to prevent you from laying down insulation in the basement, and putting in gravel a couple of feet thick. Make sure its compacted, then cast your slap on top of it. I've seen both air being blown through the gravel and hoses with water used to transfer the heat in/out of the mass.

If you want the ultimate in thermal mass... look at http://www.monolithic.com/
Very strong construction method, with massive amounts of concrete inside of the highly insulated shell.

Michael


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## sammyd (Mar 11, 2007)

A guy at work poured his whole house except the roof.
He uses practically no fuel for heat. He's got it tucked in some trees as well with a south facing deck and windows.


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## Cabin Fever (May 10, 2002)

I canât really answer your questions without a lot of research. With that said, let me give you a few perspectives on a basement. A basement is a cheap way of doubling your homeâs floor space. Our basement cost $20K which amounts to about $15/square foot. We do not heat our basement. I suppose the basement does get some heat that radiates from the sheet metal forced-air runs and some heat leakage from the furnace and hot water heater....but thatâs it. All of the forced air vents are closed in our basement. We live in Northern Minnesota and the basement gets no colder than 50ÂºF during the coldest part of winter. Most of the winter it is 55Âº down there and during the summer the basement is about 65Âº. With just a tiny bit of heat, the basement could be very livable on a 24/7 basis. 

An idea for you might be to consider an air circulation system that would take the solar heat gain upstairs and circulate it downstairs into the basement. The basement walls and slab would have more thermal mass than the just the slab you are thinking of. During the night, the basement walls and slab would give this heat up and your air circulation system would move this warm air upstairs where you are living.


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## djberg (Mar 14, 2007)

Cabin Fever has it right. We have very similar results with our passive solar home in Vermont. We carved the basement into a little hill and used the excavation dirt to shelter the North side of the house - like an old-time bank barn. The "basement" is just half into the ground on the South side and completely underground on the North. We have two bedrooms down there and they are the nicest rooms in the house - warm in winter and cool in summer.

Don't think of your basement as an extra expense. Think of it as an inexpensive way of getting extra rooms that need very little energy.


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## cc-rider (Jul 29, 2003)

My problem is that I'm in the FLATLANDS of Ohio. I don't really have any hills.... I have about a 2' drop from the front of my property to the back. I'm a bit concerned that if I excavate in order to make a basement, and use the dirt as fill to berm a side..... it will look strange. A bump in the middle of a flat field. 

I don't even have any trees to soften the picture..... just a blank corn field.


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## Cabin Fever (May 10, 2002)

You don't need a hill to have a basement. Many (Most?) basements are almost totally underground....maybe just a foot or two of the basement is above the ground level all the way around.


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## cc-rider (Jul 29, 2003)

Cabin Fever said:


> You don't need a hill to have a basement. Many (Most?) basements are almost totally underground....maybe just a foot or two of the basement is above the ground level all the way around.


I think I was trying to envision a way to have it NOT SO FAR underground (because of the ground water situation), but berming the sides would look funny -- like a bump in the middle of the field. 

I've also thought about making the basement only 3-4' inground, and then putting a wide porch around the entire building...making the basement LOOK underground (under the porch, at least!). 

So many options....


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## djberg (Mar 14, 2007)

Even a two or three foot slope from North to South will allow above ground windows on the South. You might consider planting evergreens on the North of the house and deciduous trees at the Southwest and Southeast corners. White pines and most maples grow fast. In time, those will do a lot to keep your home warmer in winter and cooler in summer. 

There are many guide books with info on planting for energy conservation. I think that evergreens should be planted near the house ( or on the berm ) on the North side and then about 20' away from them, in a semi-circular wind wall from Northwest to Northeast. 

That planting pattern was very common on old midwest farmhouses.


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