Can a Trombe wall be used in reverse?
lirus
| Posted in Energy Efficiency and Durability on
New construction, Panama City, Fl, 5,000-square-foot container (18) home facing south on piers. Will have 8′ x 40′ porch on west and east sides connecting to 8′ x 80′ porch on south side. Think U shape.
Due to high thermal conductivity of the metal, I was wondering if I could use a Trombe wall in reverse, pulling air from under home and venting it out at top of wall exteriorly. My thought is to remove convected heat transfer between the container walls so less insulation would be needed.
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Replies
Les,
We need more information.
Am I correct in concluding that you are building a house from two steel shipping containers?
If that's what you are doing, do you plan to include any floor insulation, wall insulation, or roof insulation?
If the answer is yes, will the insulation be installed on the interior or the exterior of the containers?
Finally, describe the plan to "pull air from under the home" in more detail. Where will the fan be? Will the fan be pulling air through ducts or channels? What building components will enclose these channels?
Total of 18 containers, see drawing attached. As you put the containers together you create a double wall. The house will be on piers approx 3'6" high with a cinder block skirting. The idea was to use natural convection to cool outside wall (between interior wall and porch wall) to decrease temperature differential between the walls.
I do intend to insulate (mineral wool) all of the interior along with exterior sheeting as I intend to brick exterior. I also plan to use insulation between the corner blocks, where conx attach, to try and prevent thermal bridging.
This is a more "what if" question, than this is what I am going to do as far as the question. I was just reading about thermal conductivity and wondered if I could use the high conductivity of the metal to draw the heat away from the outer wall and reduce the temperature differential. As large as the house is (72' x 80'), it creates a huge earth cooled area underneath. I wondered if I could take advantage of it. I have been in old plantation homes (26 rooms) that are cool on the hottest days of summer due to self shading.
Les,
I still can't visualize your plan.
If you expect to use flowing air to lower the interior temperature of your containers, your plan will only work if temperature of the flowing air is significantly lower than the temperature of the steel walls of your containers. There is unlikely to be a significant volume of air in your crawl space for this purpose, so your scheme won't work unless outdoor temperatures are cool -- and under those circumstances, you probably don't need cooling.
Moreover, using flowing air to lower the temperature of these steel panels will only be effective if your containers are uninsulated. It sounds like you want to insulate the containers. If you do a good job of insulating the containers, then the temperature of the steel walls won't matter much -- especially if the steel walls are on the exterior side of the insulation.
Thank you for your answers. I see what you mean.
The intent of the question was to see if I would be able to use less insulation. Imagine this, 2 plates 9' 6" x 40' separated by 1.5" of void space. Interior space, drywall, insulation, plate, void space (assumed sealed at the moment), plate, insulation, brick, exterior space.
If I understand conductivity from what I read, the sum of the conductances (conductivity x depth of material) equals the conductance of the assemble. It then follows that the temperature difference between the interior and exterior gives the actual heat flow (x area) through the wall assembly. Hope that is correct.
So back to the assembly. The exterior walls are shaded by the porch, so I should not have to worry about solar gain to much. So I am assuming that my temperature differential should just be exterior ambient temperature minus interior temperature. Say 75*F interior and 95*F exterior. I thought that if I did not seal the void space between the plates and allowed the cooler air (usually at least 10*F cooler) from the crawl space to convect up through the void space and out, that this would reduce the temperature differential. That in turn would reduce the need for the exterior wall insulation.
I also though that the convected air might also help with the thermal bridging by pulling some of the porch mass heat away by giving it another path to follow.
Les, Doesn't leaving the void between all the containers mean you will have to insulate every wall of every container, whereas if you butted them together you could just insulate the floor and ceiling of all the interior ones?
Malcolm, I would only leave the voids open in the exterior walls. Most of the insulation would be used inside since thermal bridge will be a big issue. The containers will be butted together and I intend to do as you suggested, floor and ceiling insulation. I thought I might be able to limit my exterior wall insulation with this idea.
My thought was to "reverse" a Trombe wall. If I understand a Trombe wall correctly, it uses solar gain to heat a air space/heat-sink and then convects the air using vents at bottom and top into the interior. This only works (efficiently) if you have a high differential temperatures. In Florida our differential temperatures are smaller and the overall temperature is higher than the interior temperature (excluding winter ). I thought that the crawl space air (earth as heat-sink, removing solar gain) could be allowed to flow though the exterior wall void spaces, carrying away some heat from exterior wall and the interior wall would see a lower temperature air. I think that if it would work I could reduce my exterior wall insulation. I think that in winter it would work on reverse.
Les,
Your idea won't work, for a variety of reasons. The main reason is that you want insulation. Insulation will give you better performance than your air-flow scheme.
The second reason that the scheme won't work is that there is a limited volume of crawl space air. The third reason that the scheme won't work is that crawl space air in Florida isn't very cool during the summer.
If you are insulating the ceiling and floor of your building on the exterior of the containers, you will also be insulating your perimeter walls on the exterior, not the interior. So there is no way to get any air flow where you want it -- because the steel walls of your containers will be on the interior side of your insulation.
If you think that you can insulate your roof on the exterior, and your walls on the interior, you are sadly mistaken. The steel will be a thermal bridge.
Martin, Thanks again for your answers.
In my answer to your first response to my post, I stated "I do intend to insulate (mineral wool) all of the interior along with exterior sheeting as I intend to brick exterior. I also plan to use insulation between the corner blocks, where conx attach, to try and prevent thermal bridging." I could have been more clear with the use of "sheeting", this was meant as rigid foam insulation.
The crawl space volume is 72' x 80' x 3.5'=20,160 ft^3 minus some amount for piers. The volume of the void space I was discussing is 1843.3ft^3. I do not know what the cf/m would be, but lets say its 100%. 20,160ft^3/1843.3ft^3/m=10.9 min to remove all the crawl space air once. Obviously that cfm is greatly over stated for a convective type flow, but I think the volume for a 10%/20% cfm flow is there. I stated about 10*F cooler than ambient air. The intent of the idea was that if I could expose the interior void space wall to 80*F crawl space air versus an exterior wall exposed to 90*F air it might help.
I would point out that I at no time in my posts have I said I would insulate only on one side of the walls. I said that my idea "could reduce my exterior wall insulation". Nothing was said about reducing roof or floor insulation (interiorly or exteriorly) at all. If I could reduce the cost of the exterior wall insulation by any % it would help. The way containers fit together creates a natural space for air to flow between the walls if left unattended, which is something I will attend to on interior walls.
Green builders scratch and claw for every way possible to save just a few kW a month. No magic bullet, no easy solution, but by constantly trying new ideas and variations of old ones. I only intended to present the idea that a Trombe wall uses solar gain to release it as needed up North. Why couldn't we in the South use the Earth to cool the ambient air temperature that our exterior walls are exposed too.
Thanks again for your time.
Les,
If you disagree with my opinions, go ahead and build the wall the way you want. It doesn't sound as if you expect the air flow to have any contact with the steel containers -- only with the brick siding. Is that right?
If it makes you feel any better, Trombe walls don't make any sense, either. It's always better to install good insulation than a Trombe wall.
Martin,
I plan to build a wall as follows, drywall, mineral wool insulation, steel wall, sealed void space, steel wall, rigid foam insulation, brick. Insulation above in attic and below in crawl space. Conventional. Spending to much money to make a mistake.
This was just out of curiosity to explore an idea about a different way to use what was available and test the knowledge that I had gained from the site and other resources. From the knowledge I have gained about thermal conductivity and conductance and their role in heat transfer, is the key is the temperature difference. I mean aside from all the calculations, if the temp inside is equal to the temp outside you have no heat flow. So I was proposing a thought exercise in using the earth as a heat sink to cool ambient air and expose the outside of an exterior wall to that temperature vs the ambient. Just a thought. A thought like taking the sealed (welded) void space and pull a vacuum on it.
Thanks again
Les,
I don't want to discourage you but if you haven't built with shipping containers before you might want to read this:
http://www.tincancabin.com/2013/12/the-shipping-container-cabin-in-perspective/