Browse Forums Eco Living 1 Feb 19, 2010 8:52 am I have been reading this forum for a while but this is my first post so I hope I get it right. My issue is this: we are in the process of designing what we hope will be a reasonably energy efficient home in Melbourne and we have been toying with the idea of including at least one RBV wall to add thermal mass. The designer has suggested that this be the wall to the living area on the north side of the house. However I'm having difficulty grasping how the sun hits the interior of a north-facing wall if that makes any sense! Shouldn't the mass be placed in the sun's pathway, which in our living area is going to be impossible as we have designed it to be 7 metres wide for optimum light distribution. Does anyone have any advice to offer? Re: Thermal mass walls 2Feb 19, 2010 10:08 am Knowing the height of your windows and the angle of the sun, you can determine with a bit of simple trig how far the sun will enter the room.
Anyway I did one calculation * for you. The sun needs to be lower than 20 degrees in the sky to project more than 7 metres in the room onto the wall. At noon time (in the winter solstice of Melbourne) it will project about 4.6 m into the room. Have a play with this Sunposition calculator to find out what I mean. So in the dead of winter from around sunrise to 10am the sun might hit the back wall as long as the room isn't too narrow. Nowadays there are software programs for architects and building designers that can model sunpath on a 3D representation of your house and you can where the sun strikes within the rooms. So does it matter if the sun hits the mass directly? The literature is a bit incomplete on the matter. Apparently it is less effective for the sun to light the mass directly but not completely ineffective. I feel that as long as you are in a sunlit room, radiant energy is going to just bounce around inside until it strikes the shaded mass anyway. I think it is a different matter if the room is ONLY getting indirect light. In that case the mass is going to be far less effective at storing heat. I presume you are also using a slab for mass which already gives you a head start on thermal mass. Walls are just an icing on the cake. There is also nothing wrong on using the mass on the side walls of the room. At least it will get some direct sunlight (though weaker) at some point during the day. If making use of stored heat is important then putting it on the south side is the worse thing you can do. * Assuming height of window height 2700 and room 7000mm depth Angle of sun = Inverse Tan (2700/7000) = 20 degrees ** Assuming winter solstice noon day sun 30 deg altitude and window height 2700 Depth of projection = 2700 / Tan 30 = 4.6 m Re: Thermal mass walls 3Feb 19, 2010 10:45 am ^^ Then it is the thickness of the mass to determine the desired thermal cycling of the substrate in your area. IE Northern aussie needs a very thick or dense mass so that it's temperature never gets to ambient ...and a southerner would be looking for a mass that would completely warm up and release the energy over only a few hours say till 10-11 pm (bed time ) ...and be ready to warm up first thing in the morning... Lots of houses in the south use a solarium to generate the glass house effect... Hot air created is drawn 'thermally' upward continually replacing previously warm air.. Eventually the room selected warms up. In the summer time you grow a deciduous vine for shade but also use the heat/roof vent to draw in air from the south side as it is always in shade(air locking north)(unless you don't have a veranda) in which case you would have vegetation or shading. This is in MHO and some conclusions that I have drawn from the industry. Where you are coming from is where you are going to... Re: Thermal mass walls 4Feb 19, 2010 11:15 am Regarding mass for the temperature climate of Melbourne: Location of mass = internal walls (situated within the insulated envelope) on northern side Thickness = 10-15 cm (any more than this has not been to improve performance) Area = approx 6m2 of mass for 1m2 of north window. Colour = recommended to be light coloured but not shown to make much of a difference. Note that the mass needs to be shaded from summer sun and only exposed in winter. This is a what we call a conventional direct gain system and is the most cost-effective solution to achieve the desired outcome (NB mass materials are expensive unless you plan to make your own mudbricks) There are alternatives using Trombe walls (indirect gain) and Sunspaces (isolated gain). They are more complex to build and ideally you will get a properly engineered design if you decide to pursue this course. Mass is used differently for hot,arid (desert) or hot,humid (tropic) climates and not pertinent to this thread. Re: Thermal mass walls 5Feb 19, 2010 11:48 am Many thanks dymonite69 and onc_artisan for your prompt replies. Yes, we will be having a concrete slab floor and hopefully terracotta coloured tiles. I'm pretty weak on maths but I'll definitely have a play around with that web calculator. The height of the ceiling is only 2700m so it looks like getting sunlight in to a depth of 4.6m in winter would mean having a window flush with the ceiling. You have both allayed my fears a bit though that this would be a next to useless expense. What is MHO? Re: Thermal mass walls 6Feb 19, 2010 12:11 pm The additional RBV won't be useless it is just that the benefit can't be quantified that well. It will help but hard to know by how much. IMHO = In my humble opinion. But remember what ever we suggest that YMMV = Your mileage may vary. Re: Thermal mass walls 7Feb 19, 2010 3:59 pm Floor coverings tend to reduce solar gain, the softer they are the more insulating they are. ... meaning not helpful for solar passivity. Where you are coming from is where you are going to... Re: Thermal mass walls 8Feb 19, 2010 4:51 pm onc_artisan Floor coverings tend to reduce solar gain, the softer they are the more insulating they are. ... meaning not helpful for solar passivity. What onc is saying is that certain materials like carpet can't absorb much heat but will just get very hot and make the room uncomfortable during the day. Thermal mass absorbs lots of heat and its temperature doesn't rise too sharply. When the sun goes down it releases this heat at a more steady and comfortable rate. Re: Thermal mass walls 9Feb 19, 2010 4:53 pm Thanks Dymo Where you are coming from is where you are going to... Re: Thermal mass walls 10Feb 19, 2010 5:11 pm Liliana, It doens't matter which wall your thermal mass in on. Thermal mass absorbs heat during the day and releases it at night. The wall location doesn not matter. What does matter is that the wall is located in the area you want to moderate the temperature in. If you want a passively heated/cooled solar house (that is, no air conditioning or heaters) then you need high levels of insulation and thermal mass. You can even trade one off from the other. That is, you can reduce your insualtion to half the value (say from R3 to R1.5) and double your thermal mass (by adding twice the amount of internal masoanry) the end result would be the same. Quadruple your internal masonary and you're ahaed. The effect of the combined insualton and thermal mass is to smooth out the dialy temperature swings between night and day. By ventialting during summer nights and lettign the sun in during winter days you can even shift the smoothed out temperature up or down a few degrees. Cheers, Casa Demolition August 2009, Construction Started September 2009, Completed December 2010 Re: Thermal mass walls 11Feb 19, 2010 5:43 pm I don't think you can approach the two as interchangeable. It all depends on how much 'free heating' during the day and how cold it gets at night. For instance, if you have lots of sun and very cold nights then it is worthwhile to both have lots of mass and insulation. Unless you live in a generally hot climate you can't lose with a decent amount of insulation. Mass is your heat battery and insulation is your method of preventing heat leaks. Too much mass in a generally sunless climate can be a liability. You will need to use extra artificial heating just to get the mass up to temperature. Once it stored it will start to leak immediately unless you have enough insulation. Re: Thermal mass walls 12Feb 19, 2010 6:08 pm Too much thermal mass in the wrong place will make a cold area, great in summer but winter it never heats up. The floor and walls need to get all the direct sunlight possible in winter. Where you are coming from is where you are going to... Re: Thermal mass walls 13Feb 20, 2010 6:48 am dymonite69 I don't think you can approach the two as interchangeable. It all depends on how much 'free heating' during the day and how cold it gets at night. dymonite69 For instance, if you have lots of sun and very cold nights then it is worthwhile to both have lots of mass and insulation. dymonite69 Unless you live in a generally hot climate you can't lose with a decent amount of insulation. Mass is your heat battery and insulation is your method of preventing heat leaks. [quote="dymonite69"Too much mass in a generally sunless climate can be a liability. You will need to use extra artificial heating just to get the mass up to temperature. Once it stored it will start to leak immediately unless you have enough insulation.[/quote]"If you want a passively heated/cooled solar house (that is, no air conditioning or heaters) then you need high levels of insulation and thermal mass..." Once you start artificially heating or cooling thermal mass can work against you. Demolition August 2009, Construction Started September 2009, Completed December 2010 Re: Thermal mass walls 14Feb 20, 2010 7:04 am I should add that a passive thermal house only works if the average of the day and night temperature are within (or close) to the thermal comfort range. For Sydney, say in winter days are 25 degrees and nights are 5 degrees, then are you happy with 15 degrees (plus a little during the day and minus a little at night). You can then shift this up a few degrees by allowing sunlight in during the day. Of course the temperature will be somewhat above the average during the day and somewhat less during the night, but the more insulation x thermal mass you have the less variation. The end result should be a house that doesn't go below 18 degrees in winter. In summer, say days are 35 degrees and nights are 20 degrees, then are you happy with with 28 degrees (plus a little during the day and minus a little at night). You can then shift this down a few degrees by allowing night time ventilation. The end result should be a house that does not go above 28 degrees during summer days. If you had very large thermal mass and very large insulation then the temperature inside would hardly fluctuate and would be more like the average of several days temperature. Passive thermal would not work in far north Queensland where the average temperature is well above the comfort range. Here you need air conditioning to obtain thermal comfort (and lots of insulation). It would also not work in Tasmania where you need supplementary heating. Again lots of insulation would help. Demolition August 2009, Construction Started September 2009, Completed December 2010 Re: Thermal mass walls 15Feb 20, 2010 1:50 pm We are probably getting a little to technical for our original poster. Thermal mass for your purposes is predominantly to reduce your heating requirements. It will have some benefit for moderating summer temperatures as well. Passive solar design is predominantly about radiant heat control using appropriately placed windows and fixed shading devices. All other energy saving components such as insulation, special glazing or foil barriers can be part of passive solar design but are not unique to it. What you are trying to achieve is controlled entry of 'free' radiant heat into the house in winter to offset any artificial heating requirement. Don't underestimate the power of this. The interior house can absorb many times more heat from direct sunlight then is received from the heat contained in the air. Uncontrolled entry of radiant heat is the predominant cause for overheating in Summer but is an asset in winter. This energy is so significant that a poor design will allow excessive radiant entry in winter resulting in overheating! One way to moderate this effect is with appropriate amounts of thermal mass to absorb the 'excess' energy and release it at night time when the sun sets and the temperatures begin to fall. So good passive design includes: 1) Good window placement - north is best because it can be easily shaded in summer 2) Correct window aperture - to admit just the right amount of radiant heat (too much = overheat in day and excessive heat loss at night, too little = underheating) 3) Correct shading - to allow low winter sun in but block out high summer sun 4) Adequate thermal mass - to absorb some of the radiant energy (too little = overheating, too much = won't get heated and you will need to heat it artificially) NB windows are both an asset and liability - they easily let in radiant heat during the day but also let out heat at night) We live in a slightly cooler climate then Melbourne: http://www.bom.gov.au/climate/averages/ ... 3801.shtml Using passive solar design with 19% double glazed north windows (by total floor area), 450mm eaves and slab-only as thermal mass and insulated window coverings our heating energy requirements are at least 1/4 for an average house in this district. It rates around 7.5 energy stars. BTW our summer cooling requirement would not exceed one hour of AC per day on average (most summer days zero use, heat waves 2 hours use max). Interior temperatures maintained between 21-25 degrees. With hindsight (since it is impossible to get someone to model this prospectively) we probably could have achieved an even better result if we used less glazing and maybe taking the eave out another 500m for better summer shading since it is still possible to overheat on very cold but full sun days. Re: Thermal mass walls 16Feb 20, 2010 3:16 pm dymonite, With regard to you wanting, in retrospect, deeper eaves, you can add sun shades above the windows. Looks good on a contemporary house. I don't think you can have too much thermal mass (or insulation). If you had infinite thermal mass and insulation you would get the average yearly temperature in your house, which would be perfect! Such a theoretical house would sit at about 20 degrees Celsius all year round. Lovely! On a more practical basis, with less thermal mass and insulation, we shift the temperature up with winter sun entry and shift the temperature down with summer night time ventilation. I think if you do this you're house will sit between 18 to 28 degrees all year round (might tuck down to 16 degrees while you're in bed on a cold winter's night). Add a few tweaks and you should be able to get 20 to 26 degrees Celsius (without any air conditioner or heater). This is what I'm targeting and I'll let you now how it goes once it's built. On a one day basis, yes too much thermal mass would not heat up during a winter's day. However, let's assume we build our house to last for more than one day . Whatever you've stored in your thermal mass you get to keep or use. After a few sunny winter days your house will be up to the right temperature. And you will have more capacity to ride out a few rainy days when there is not sunlight. Think of it as a thermal flywheel. Cheers, Casa Demolition August 2009, Construction Started September 2009, Completed December 2010 Re: Thermal mass walls 17Feb 20, 2010 3:49 pm Casa2, I suspect your calculation are a little oversimplified regarding the equilibrium temperature that the house will reach. The factors that influence mean climate temperature are not the same as that determine mean house temperature. Otherwise you are suggesting that your town's weather is behaving as it is one gigantic building. The other concept that you propose, namely that you can store several days worth of radiant energy is somewhat controversial. Let's assume that 1m2 of window admits 500W of sunshine for 6 hours a day during winter. That is equivalent to 3kWh. However, 1m2 of window may also loses 70W by convection over the 24 hour period. ** Let's say this is about 70W x 24 hrs = 1.68 kWh. This is not even considering losses via the rest of the building envelope. It would be true that you can 'borrow' a little energy for each sunny day but I am doubtful that you get more than an extra day of 'free' heating unless you super-insulate your house. * U value (single glazed window) 7 x 10 degree mean temperature indoor-outdoor gradient ** you can improve this somewhat by drawing window coverings and dropping the U value or adding double glazing. Re: Thermal mass walls 18Feb 20, 2010 8:09 pm 3 kW in, and if you use double glazing and curtains drawn art night, you'll only loose half as much as the 1.68 kW. Double this figure to allow for the rest of the building envelope and we get to keep 1.32 kW. That's all that's needed, save a little each day and then if you have enough thermal mass release it over 3, 4 or 5 days when the sun doesn't shine. Demolition August 2009, Construction Started September 2009, Completed December 2010 Re: Thermal mass walls 19Feb 20, 2010 8:35 pm Casa2 3 kW in, and if you use double glazing and curtains drawn art night, you'll only loose half as much as the 1.68 kW. Double this figure to allow for the rest of the building envelope and we get to keep 1.32 kW. That's all that's needed, save a little each day and then if you have enough thermal mass release it over 3, 4 or 5 days when the sun doesn't shine. Maybe true for a Passivhaus with R-12 insulation and R-1.5 triple glazing (heating requirement of 15kWh/m2/year or 0.041 kWhr/m2/day) In your example 1.32 kWh for every 6m2 of floor space or 0.22 kWhr/m2 collected each day. An R-4 double glazed house will leak heat three times as fast. Re: Thermal mass walls 20Feb 20, 2010 9:31 pm dymonite69 Your sun calculator is average. It doesn't have Perth but it has Geraldton Render your bathroom walls, two opinions versus the one, makes you wonder. 3 6062 The spacing of the studs looks pretty large especially for a load bearing wall. 3 11117 Nope, only on the inside, I've literally has my ears to the brick outside and can't hear anything. I can sort of 'reset' the noise by pressing firmly on the studs. This… 10 11770 |