thermal mass v. insulation

[Sherry]

Hi everyone,

Would it be true to say that it is better to try to place your insulation on the outside of your walls, while the thermal mass is ideally on the interior of the walls? 

If this is indeed the case, would it also be true in a hot and humid (tropical) climate?

Thanks for any help any of you (very smart, by the way) people on this site can offer me.

Sherry

[John_M]

I'm a little confused by the question, but I would want my insulation "in the walls".  Not sure what type of thermal mass you are talking about?

[MountainDon]

Yes, what sort of thermal mass?  When I hear thermal mass I think of concrete floors, concrete or brick walls and the like. Could you expand on your idea? Thanks

[firefox]

If you mean tropical as in it never drops below 70 F, then you prbably don't want thermal mass, but concentrate on insulation
and shading techniques to shield yourself from the sun. Under certain circumstances you can use a thermal mass to cool your house, but
that takes a little doing. IE if you have an area that can be kept cool at night, you can then suck the heat out of it and use it during the day
to keep the house cool.

I was raised in Puerto Rico and the subject of heat never came up other than how to avoid it.
Bruce

[Sherry]

As it happens, Bruce, it is Puerto Rico I'm talking about.  I lived there for a couple years, and I've always had a dream of retiring there and building a home for myself.

My main concern is keeping it as cool as possible, hopefully not needing a/c.  The Puerto Ricans have a real disdain for wooden houses, claiming that termites will destroy them.  It always seemed to me when I lived there that the commonly-found cement block houses didn't have enough windows for good ventilation and light/views.  But maybe they know something I don't, like maybe more windows would allow more heat to enter during the day?

I have been mulling over earthbag construction. Which makes me wonder if it would be better to fill the earthbags with an insulative material like crushed volcanic rock (if it is readily available in PR, that is), or if it would be better to fill them with a thermal mass like earth?  Also, would a thicker wall help keep out the heat?  Or, if I went with the conventional concrete block, would it be a good idea to put some kind of insulation on the outside of the block like a rigid foam?  These are the questions that I'm struggling with, and that I am hoping all of you can help me with.

Thanks, Sherry

[dug]

What are the temperature swings there? When I lived in Tucson it was routinely over 100 during the day but would go down into the 60's or low 70's during the night. Homes with lot's of thermal mass (usually adobe) were fantastic and required little cooling. If the temp does not go down at night then the thermal mass can work against you.

Insulating outside heavy walls is ideal, though not commonly done.

[Sherry]

The temperature swings in the tropics are not as great as you would have experienced when you livied in the desert, I believe.  Usually, up to high 80's maybe low 90's during the day, cooler at night but never what you'd call cold.  The complicating factor is really the humidity, I think.  Makes it feel hotter and less comfortable, even when you're not outside in the sun.  That's why "swamp coolers" really don't work in humid climates like that of PR, as I understand it.  Even in southern PR, which is kind of where I'd be and where it's somewhat drier, it's still humid.

I'm thinking of building in the mountains, where it's a bit cooler at night than lower elevations, maybe 60's or 70's at night?  I'm not quite sure because I never lived in the mountains of PR, only visited there during the daytime.

[John Raabe]

Thermal mass helps even out temperature swings on a daily basis (or longer if insulated) but does nothing for humidity. It is ideal for the high desert as dug mentioned.

When I was designing houses in Hawaii we tried to keep the structure light, the roof overhangs wide, and encourage airflow and ventilation. You can do this inexpensively with cool air intakes from the shaded underfloor area and a high exhaust on the leeward side of the building or (better) the roof.

However, when the temperature and humidity gets uncomfortably high, you have few options as good as electric refrigeration - the machine that built the south!

[dug]

humidity is a big factor for sure. Tucson, for all practical purposes had none, and 90 degrees in the shade would feel quite pleasant in the summer.

Local knowledge can be very beneficial, generally the natives will have it figured out.

[firefox]

We lived right on the beach in a 4 story concrete block apartment house with nice tile floors and lots of glass louver windows that you hand cranked open and shut. I am not sure, but I think there was some overhang over the windows. At the very most you slept in your birthday suit and a sheet over you at night. We diden't have A/C or even a fan, but I don't remember heat or humidity ever being a problem.
Ofcourse you have to understand that that sort of thing doesn't mean much to a preteenage boy. And besides, all we had to do was run downstairs, cross over the driveway and your toes were in the sand of a 3 mile long beach protected by a barrier reef 5 miles out that kept the sharks out. In fact, we were in the water so much that our parents would check us for gills on a regular basis.

If you can get land up in the mountains in the rain forrest, you shouldn't have a problem with heat. When we first got there we lived out near San Justo which is a smjall inland town, but not in the mountains, and it didn't get as hot as on the beach.

I think that if you stick to cement block construction with tile floors,
and shade protection, you will be quite comfortable. Maybe a window a/c unit for the bedroom for occasional use. Lots of cross ventilation. Having a shade roof over the regular roof is a big
advantage if you have the typical concrete roof. But I am sure there are lots of different Ways to set this up.

BTW I am envious of you. I would love to retire down there, but
it would cost too much for us to afford it.
I know you will love it down there.
The best of luck to you.
Bruce

[firefox]

One more Very Important Thing. You do not want to make anything
out of wood. If I remember rightly, the termite is the Puerto Rican National animal. You have been warned.
Bruce

[Sherry]

It seems that sometimes the humidity makes using A/C unavoidable if you want to be comfortable, but I just wanted to try to design a house in order to avoid using it as much as possible.  I know part of the equation involves acclimating to the climate again and avoiding doing certain things during the hottest part of the day.

John, I hear what you're saying about electric refrig. having built the south, but A/C sure does use a lot of energy and takes $$$.

As I read all about all the innovations that help people in various climate conditions save energy and money (swamp coolers, rocket stoves (is that the right term for them?), passive solar heating, heat pumps, geothermal,  etc., it just makes me wonder why nobody has ever invented something other than standard A/C to deal with a humid-type heat.

Thanks for recounting your experience in PR, Bruce.  Was one of your parents in the military?  Makes me nostalgic to get back there someday soon.  And thanks to everyone else who took the time to answer me.  If anyone happens to hear of some new innovation in this area that I haven't heard about, please tell us about it.

[firefox]

My Father went down to PR and started a textile business after he
was discharged from the Marine Corps. He also had a turist shop in Old San Juan called Dolphin Court. It was right accross the street from the presidents palace on one side and overlooked La Princessa
(State prison) on the other.

Putting a shade roof above the regular roof will make a big difference.
Just do whatever it takes to keep the sun off of the main structure,
Not just the roof.
Make sure concrete walkways are decoupled from the house with insulation.
Maybe mount PV arrays on top of the shade roof.

Try to design so that breezes will flow through the house unobstructed by orienting windows tat line up with each other,
as well as openings in the interior walls. Things like transoms
high on the walls.

good luck!
Bruce

[Sherry]

I'm not really familiar with a "shade roof".  Can't remember seeing any in the tropics, either. (I've lived in both Puerto Rico and Panama.)  Maybe it's something I've seen but I didn't recognize as being a shade roof?  Or is it a fairly new concept?  (It's been about 30-some years that I haven't been back to the tropics.)  Can you tell me what it is?  I mean, I sort of envision it as a roof on top of another roof, maybe with some separation between the two, but I'm just making a guess here from the name of it.  If that is what it is, then what holds up the shade roof?

[MushCreek]

We inadvertently put a 'shade roof' on a house here in FL. Our old roof was so flat you couldn't do shingles on it. They wanted a small fortune to do a built-up roof, plus I'm a hard-core DIY'er. I built a second roof over the original one, and it was vented at the eaves and at the ridge. The original attic was about 40 degrees cooler on a summer day! Most houses here in FL are concrete block with poor insulation (none in the walls). If you could keep the sun off of it, the concrete changes temperature slowly, and would stay about the same day and night. If you could build partly underground on the slope of a mountain, the ground would probably help keep you cool.

[MountainDon]

Old Land Rovers, the really old ones, had a "tropical Roof" option. This was simply another metal roof top spaced above the normal metal roof. This was pre A/C and used in places like Africa and Australia; anywhere in the hot topics. They worked quite well at keeping the interior cooler.



So a "shade roof" on a building would be constructed in a similar fashion. The idea being to have a air space. In a slightly different vein it is the same principle I used in making the heat shield to protect the wooden wall in our gazebo from the wood burning chiminea. If we had been more crapped for space in the cabin I would have done the same thing there.

[Jens]

check out papercrete, and living roofs.  A living roof would (I assume) be much cooler under the surface than any other roofing material, as well as have its surface water-from rain- evaporate from it, which may make for a bit of additional cooling.

[MikeOnBike]

Here is a good example of a shade roof and an explanation of how it works.  This one is rather large but the concept is the same.

http://greenbuildingelements.com/2009/06/25/a-roof-designed-for-our-hotter-future/

http://www.dwell.com/articles/operation-desert-shed.html

They basically work like a shade tree to keep the radiant heat from your structure.

[John Raabe]

Very interesting solution for a high desert house. Thanks for the links.

[MountainDon]

Near us there is an apparently abandoned steel frame structure with completed concrete slab and a roof. It was begun by a church that must have run out of money. It's been sitting there for 5 years at least. It might be a perfect starting place for something like this.

[RainDog]

 So... if you built an open sided pole barn, or had one built seein' how cheap companies do that, you could put together a flat topped structure underneath without all the concern of water leakage, etc, plus it would be energy efficient in the summertime... Couldn't you just sheath and paint the roof over the ceiling joists and be done with it?

What's the downside? Open sided pole barns are dirt cheap to have built, but I never see a shade roof over a house, with the exception of mobile homes periodically.

[Sherry]

I have a tendency to spin things out to extremes, to their most logical conclusions if you carry out some line of reasoning to as far as you can take it.  So if you'll bear with me here......

If you build a pole barn structure in a climate like Puerto Rico's---now you've got a foundation and roof, and you put down some finish flooring.  (You might even raise the first level an entire story the way the ones on the Katrina thread on this site are to prevent any possible future flood damage and to have an under-roof spot for storage and parking.)  Now all you really need is something to keep out bugs and critters, like screening, right?  Like a whole house made like a sort screened-in porch.  That's assuming you didn't need insulation because you had no intention of air conditioning the space.  Am I off-track here?

I guess I'm saying, if you went so far as to build the pole barn described in the post, you wouldn't need much more on the structure part to already have a house that was comfortable in that climate, right?

[MountainDon]

... something to keep out the riff-raff?

[RainDog]

 I still don't quite get why you don't see shade roofs more often, especially in the south.

You get some pole barn company to come out there and put up a roof real fast (hard to beat their low prices), then you've got a weather-proof site for the rest of your build and for your equipment, you don't have to be up on a pitched roof takin' chances, you don't even have to worry about cutting and setting rafters and ridge board.

Those right there are significant advantages, is there some great disadvantage that I'm missing? Is it just looks?

They aren't generally built onto slabs, so it wouldn't effect your choice in foundations as far as I can see.

[firefox]

Note: the key between the F and H key is broken so I am usinj J instead.

Actually, when I sujested that she use a shade roof, I wasn't aware that any existed. This was bassed on myinvolvment with Military vehicles and communication shelters. the Com Shelters are essentially aluminum boxes. the sides and tops are two pieces of aluminum sheetin with foam inbetween. these jet very hot when out in the sun, so they all come with provisions for a shade tarp. Believe me, they make an extraudinary difference.

It just makes sense to extend them to houses in hot climates.

You do want to have a bij jap between the shade roof and the house
so you jet a larje volume of air movinj accross.

besides the termites in PR, you have the two lejjed kind that remove more than wood. so you want to have a solid structure underneath.

Althoujh the pole barn is a jreat idea, you could probably jet by with a frame  above the house say 3 feet above caryinj a lijht weijht
metal roof.
bruce