This idea is in it's infancy, so feel free to be blunt with input. It'll need tweaking and could easily end up scrapped as just being bad, infeasible or expensive.

I've been reading on Vacuum Insulated Panels and the technology that is our trusty Thermos. I'm finding air is a poor conductor, but since it convects, it acts as poor insulation. The thermos works by combining a low emissivity surface to prevent heat transfer through radiation and an air chamber under vacuum to minimize both conduction and convection. I'm seeing some numbers that claim R-30/inch for these conditions, but the tradeoffs for how much vacuum is required is not super easy to find.

So, one of my ideas is this: What if you made a roof with an internal layer of insulation, a low emissivity layer, a vacuum layer of some sort, possibly another low emissivity layer, more insulation, and finally the water proofing that is a roof?

An initial design might be this: You roof with metal SIPs. Apply aluminum foil or something. Then you lay out PVC pipes, either with or without "framing" depending on the structural needs. You then complete a sandwich with another layer of metal SIPs with or without an aluminum foil layer.

There's a lot of ways to tweak or alter this design. I don't know how strong the structure would be and if you'd want to add structural framing to support it, or possibly make the framing internal to the SIP sandwich.

As far as performance, I'm thinking the PVC pipe could be plumbed to alternately be placed under vacuum for minimal heat transfer, or plumbed to passively source cool humid air for evaporative cooling in the summer.

The viability of the idea might just be how air tight you can make the system or how much energy (either manual or automatic) it takes to maintain the vacuum.
Any thoughts?

Hmm? Where did all my line breaks go? I'll edit the post for readability if someone can help me.

If you want to create your own vacuum panels, make them flat (to minimize thermal bridging) and fill them with fumed silica (to prevent them from collapsing). Expect them to slowly lose vacuum, so install a fitting/valve so they can occasionally be re-evacuated.

You can edit your post and add < br >.

You have to reach almost absolute vacuum for it to work, otherwise the air bridges the gap....and over the surface area of a house that is almost an impossible task. The pump to reach that level of vacuum is super expensive, so you couldn't have one at your house to keep up with leaks. I had the same thoughts as you after reading about using vacuum and spent a week researching it. I thought windows would be the best fit for vacuum.

I see many used vacuum pumps that are more than adequate (~.01 Torr) for vacuum insulating panels for a few hundred dollars. But I do expect you will run into other practicality problems.

Posted By kited on 04 Aug 2015 12:43 AM
Hmm? Where did all my line breaks go? I'll edit the post for readability if someone can help me.

Use a different web browser than whatever you're using for composing the posts. (FireFox seems to work fine.)

Posted By jonr on 04 Aug 2015 12:49 PM
I see many used vacuum pumps that are more than adequate for vacuum insulating panels for a few hundred dollars. But I do expect you will run into other practicality problems.

One reason for the PVC pipes was they could be individually evacuated. I suppose that would just make more work switching. The same vacuum would just take longer to do them all at once, but the end result would be the same pressure.

The other reason I liked the PVC pipes is plumbing in humid air in the summer for evaporative cooling.

So, would the PVC pipe itself conduct enough to act as a thermal bridge?

What other problems do you anticipate? Plumbing issues? My first guess on structural concerns was the pipes would add strength. I guess they are flexible, though, so while the project might hold a hefty load, it would deflect considerably. Still, the reason for span table sag calculations is to prevent drywall cracks and "bouncy floors." Can't a metal SIP be a finished interior for fire concerns? I'd think a deflection would be tolerable.

Ok. Is anyone else seeing < br /> ? If it's my browser, I'll stop worrying about it. Can't change browsers just right now.

I should have said <br> (no spaces).

It seems like reducing air pressure should reduce thermal conductivity, since there are fewer molecules to transfer heat. However, the mean free path (between molecular collisions) increases inversely with pressure, so the remaining molecules can transfer heat more quickly. Thus, the thermal conductivity is almost independent of pressure until the pressure is reduced by about a factor of between 100,000 to 1,000,000 for dimensions of interest in this application. It is feasible to produce such a vacuum, but it requires some expertise, including heating the system to drive off moisture and condensates. It is difficult to maintain such a vacuum. Study the literature such as http://www.electronics-cooling.com/2002/11/the-thermal-conductivity-of-air-at-reduced-pressures-and-length-scales/ to understand that reducing the pressure does not reduce thermal conductivity until the mean free path approaches the dimensions of the system.

Posted By Lee Dodge on 06 Aug 2015 11:38 AM
It is difficult to maintain such a vacuum.

Yes i agree Lee....If it were easy and cost wise it would have already been done. You have to maintain under 1 mbar, and need a many thousand dollar scroll pump and not a used one that is already worn out. Remember this vacuum state needs to last the life of the house. Also one nail in the wrong place and you are done....forget using nails to hang things on the wall.

That's a good article, but I'm under the impression the conductivity at 1 atm of air is sufficiently low for insulating purposes. The problem is convection. If one side is warm it will excited the molecules, which will then mosey on over to the cold side to warm it. I haven't found a good reference on what magnitude of vacuum will properly effect convection.

From other sources of input, I think the thing to do might be: step 1, buy one PVC pipe, plumping, vacuum gage and some kind of cheap (probably manual) pump. Test how well it holds. Step two, try it on a small scale.

Who said walls? I'm talking roof here, for this particular idea. A wall application could be for an earth berm house. If your concrete walls had copper tube vertically laced in them, and it was plumbed top and bottom through horizontal PVC through the insulation and into the berm and the loop connected with a vertical run of copper. in the winter you could apply a vacuum to minimize neat transfer into the earth. In the summer you could go positive pressure which would convect your foundation walls heat into the earth.

Posted By newbostonconst on 06 Aug 2015 01:03 PM


If it were easy and cost wise it would have already been done.

Humanity would never advance if everyone assumed nothing can be improved upon. Granted there are a LOT of bad ideas for every good one. Odds are still better than the lottery I'll hit upon something.

Granted there are a LOT of bad ideas for every good one.

Trust me. I have a lot of them ;P