what is the downside of interior thermal mass?

You have to have the right climate (swings above and below comfortable). Interior temperature has to vary (the more the better). Cost. Slow response of your heating system. Interferes with the effectiveness of thermostat setback (how much is open to debate, but we see people with high mass interiors that decided they couldn't use setback at all). Overshoot and undershoot (as radiators).

Is it worth a typical ~11% building savings? There are other ways to achieve that too.

Thanks for the replies

The U factor I want to achieve is 0,15 W/m²K for walls and for windows max 0,8 W/m²K.

Further I like to have:
- Fast building time 4-5 months in the winter season
- Low temperature hydronic heat system.
- wall heating, because radiation is very comfortable heat transfer.
-passive cooling by preventing solar heat and the use of thermal mass and night ventilation on hot days.
- High ceilings in the living room and kitchen. About 3,5 meter.

I thought that SIP with on the inside a plaster or concrete layer with tubes was a good solution. But maybe SCIPs are better.

You might find that if you have radiant floors (more common) and a well insulated house, there isn't much more to gain by having additional radiator area. Ie, it is a case of diminishing returns. Also note that heating the exterior walls will be comfortable but will increase energy usage. It might be hard to find people with SCIP experience.

Rpatterson, you'd need a special reason to incorporate mass for mass' sake outside of the desert Southwest. I built a concrete house (AAC) in Pa for two reasons: as a buffer for aggressive passive solar and as a stable base for stucco. Sound deadening and fire resistance are other good reasons. That said, mass does add something even here. With no hvac for three weeks, internal temps have ranged from 66 to 72, with exterior temps hitting the 40s. I'm in the deep woods so passive solar is little help until the leaves fall in Nov. Most of my neighbors are doing without hvac as well, but they have to move quicker in the morning than I do. Fielder's choice as to who's better served.

[quote] Also note that heating the exterior walls will be comfortable but will increase energy usage. [/quote]

How much it will increase and why? With wall heating you can lower the thermostat 1-2 degrees Celsius, because of the bigger radiation part.


[quote]You might find that if you have radiant floors (more common) and a well insulated house, there isn't much more to gain by having additional radiator area[/quote]

If only the floor is heated, the walls remain cold, thus uneven temperature distribution, which leads to less comfortable climate. (see attachtments) and your not so flexible with your floor finishing. I like a wooden floor by example and with that you will isolatie your floor heating.

When you heat the exterior walls, you have a greater temperature differential across the insulation. This causes more heat loss through it. For example, if the wall is 21C instead of 19C and it is 0C outside, you get 10% more heat loss (21-19)/(19-0). In a very well insulated house, all the temperatures (walls, floors, ceilings (but not windows)) will be very close to the same, so you won't be uncomfortable like you would be in an old house with cold walls and floors.

I have not seen any data that indicates that once you have a radiant floor, you can add wall radiators to get an additional 1-2C decrease in interior temperature with no decrease in comfort.

Also note that once you start using your thermal mass as a radiator, it no longer works well as passive thermal mass.

Maybe using just the interior walls as radiators is reasonable. Tubes and fins can be put into wallboard and plaster. Even a wood floor works well as a radiator in a well insulated house.

[quote]I have not seen any data that indicates that once you have a radiant floor, you can add wall radiators to get an additional 1-2C decrease in interior temperature with no decrease in comfort.[/quote]


I could find some Dutch data graph but without reference:

y-axis -> wall temperature
x-axis -> air temperature
blue -> too cold (te koud)
yellow -> comfortable (behaaglijk)
red -> too hot (te warm)

Maybe using just the interior walls as radiators is reasonable. Tubes and fins can be put into wallboard and plaster. Even a wood floor works well in a well insulated house.

I now have renovate two old houses. With wall heating on interior walls and isolate the existing exterior wall as good as possible.
But I thought when building a new SIP home, the heat loses trough an exterior heating wall are negligible, but i have to calculate that.

Also note that once you start using your thermal mass as a radiator, it no longer works well as passive thermal mass.

In the summer your heating is off. On very hot days when you have a heatpump, you can reverse it and cool the thermal mass at night.
In winter times you can use the mass for a solar energy buffer. If you have solar collectors on your roof.

For offices with lot of glass surface, people and computers, then it's better not use the thermal mass for heating I think.

I understand your point about using mass to store cool/heat from a heat pump or solar collectors, but active storage (like a water or ice tank) will work much better.

An ice tank, jonr? Running the AC at night in a high mass house is set-ahead with more buffering and less temperature variation. I thought you liked programmable tstats.

Perhaps it wasn't clear - active, as in insulated water or ice tank. A programmable thermostat still works well as do increased COP on a heat pump and night sky radiative cooling (using water).