This Thermal House - the Basics of EcoRenovation
 

In the time that I have been active at EcoRenovator, I have noticed that knowledge of the basic physics involved in heat loss and heat gain is not universally understood here. For instance, the notion that 'heat rises' when in fact it is hot air that rises, while heat travels in all directions. This and other examples of partial understanding can lead not only to renovations that are not as effective as we expected, but also to expensive errors and oversights.

I came across this very useful, short and succinct article called, This Thermal House which explains the physics involved in heat loss & gain.

The article begins:

I think , This Thermal House can make a valuable contribution to our basic understanding, and hopefully, the effectiveness of our action.

Best,

-AC

Hi AC,
I glanced at the article to get summary points because I could easily see that I'll need to put a lot of energy into reading it to get all the implications. I'll do that later because I enjoy physics anyway.

One thing I immediately got out of it though was that insulating walls came to only 10% of the improvement of fully insulating a house. The other thing that jumped out at me was that insulating floors contributes 38% of the improvement. And I've noted elsewhere that insulating floors can create problems of their own when moisture laden air enters a crawl space in warm weather, condenses in the cooler crawl space and then causes mold and wood rot problems down there. The real problem is that by insulating between floor joists, which most people do, you keep the conditioned air out of the crawlspace and make the condensation problem worse! And because the joist themselves are not warm, even though the floor now is, you invite a perfect storm of wood rot through the floor joists that will eventually lead to moisture driving through to the floor itself. So if you have wood floors and you decide to insulate beneath the floor you will warm up the floor. But you will also cool off any crawlspace you might have. That's really bad and will likely cause problems eventually, even including cupping of the floors.

I've learned all this just a bit late because a few years ago I did exactly the wrong thing and put insulation between the floor joists. I since taken most of it back out. Luckily I haven't yet put in new wood flooring that I eventually plan on putting in. Next project (after current construction on other things) is to seal, insulate, and put in conditioned air into the crawlspace and treat it like living space (except for not living down there). An added benefit is that if I ever go ahead with my ten year plan of putting in solar hot air panels that hot air will go right into the crawlspace and act as a heat reservoir while simultaneously making the floor warm.

The main thing I'm hoping to convey from this message is that one shouldn't automatically read your article and start insulating the floor. Depending on the circumstances it could create other problems along with warming your house. Overall the article you reference is really good and useful and i'm already learning from it. I just don't want people to jump to destructive conclusions on their own. That already happens too much, as we both know.

This is very interesting information you have here. It is also in opposition to [my understanding of] the recommendations from my state energy office and also various non-profit energy groups in my area.

So, this is something I really want to get straight, because my floor insulation is pretty much non-existant. I have insulated the walls very well but I have not gotten really serious about the floor part of the house project yet.

So, I'd be interested to know your source of information on this issue.

And also, have you considered that for the moisture in the air to do its wickedness, it needs to actually get to the floor/joist area. What I mean is that if you were to, for instance, spray foam up onto the bottom of the floor, it would make an air-tight barrier.

And also, as I understand it, the moisture problem occurs when warm, moisture-bearing air is chilled below its dew point and the moisture precipitates out. This is the kind of thing that happened with a lot of fiberglass filled wall cavities... the warm indoor air leaked through the walls and dropped its moisture in the fiberglass, causing problems.

The fix for that seems to be creating a sprayed-on foam seal against the inner surface of the outside wall, about an inch thick minimum (the exact minimum thickness depends on the local temps & humidity). Then the wall is filled with an insulation of your choice. The idea is that the temperature gradient is such that the dew point is never reached in the fiberglass (or cellulose) portion of the wall, but the 'dew point' is reached in the air-tight foam layer, where the humidity-bearing air is prevented from entering.

I think a floor version of this idea would do it.

I have done a bit of my floor insulation, and I have used 2" rigid foam boards that have been cut to a bit less than the joist spaces, and I push it up against the floor and fasten it there and use one-part foam around the edges and seams. My idea is to immobilize the air inmigration.

Your thoughts?

Best,

-AC

Yeah I'd have to agree with AC_Hacker here. It sounds like you have a problem that needs to be delt with by use of some form of vapor barrier, not removing the insulation.

Back to the article though, it seems pretty well written by someone just getting into the swing of ecorenovating.

Not so fast. This was written by Tom Murphy of the "Do the math" blog. A fantastic blog on energy conservation.

Going to the source, his blog, there are a few problems pointed out in the comment section.

My biggest problem is with the percentages based on the unusual shape of the house. This theoretical house is square, one story, and large, at 2422SF. This makes the floor and ceiling very large compared to the walls. My rectangle ranch style house has much more SF of walls than floor or ceiling.

I guess I breezed over that because it was just an example. I didn't take it that every house has the same areas. Good point though. Readers should be aware of things like this!

Yeah, the geometry of the building shape is very important. That is one of the things that really fascinates me about the spreadsheet tool, that is the gold-standard tool for Passive House design. The tool is called the Passive House Planning Package (PHPP for short) and it is a very comprehensive model which includes the effects that house geometry will have on the total heat loss, and consequently the required insulation to meet Passive House standards.

Description of the latest PHPP is available HERE

A 'limited functionality' version is available for free HERE.

The full-boat version is 160 EUR, available HERE.

Other relevant links of interest available HERE

But, getting back to the essay, and the different geometry of your house, compared to the model in the essay, are you able to adapt his rationale to your house and come up with working percentages that apply to your particular house geometry?

Best,

-AC

Here's the article that I based most of my comments on. It rings true to me.

http://www.buildingscience.com/docum...ace_2010r2.pdf

My understanding, such as it is, is that the moisture condensation problem common to walls and floors are very severe with floors with vented crawl spaces that have insulated but partially exposed floor joists. It's actually a lot worse than walls because warm air travels up and also there is little sun or air circulation to dry it out that walls might have.

Here is another link with a broader overview about conditioned crawlspaces and the inadequacy of current code. I think it's pretty good information.

http://www.buildingscience.com/docum..._download/file

Best,

Edit:
Hey AC, I was starting to feel bad that this damp wood phenomena might affect you directly. But the more I think about it seems like you might be safe. I'm basing that on the fact that your heat source is directly coupled to the floor. So there is a good chance that the closeness of that heat source to the unprotected edge of the joists will overwhelm the normal cooling effects. I hope so.

Oops, Sorry. I had mixed up this blog with another I had read about the same time. The other one was more pushing insulating just the ceiling and floor because there was so much more area that it mattered much more. But my walls are much bigger because of the shape. After rereading this one again, I see Tom was not making that suggestion.

I could model my house with Tom's figures. It will be much more complex and take some time with the "in progress" state of my house. (another project goes on my list.)

Free MIT course materials: Introduction to Building Technology
 

I was looking for something else, and came across this course fro MIT, apparently free, called, "Introduction to Building Technology".

I came upon it while searching for some graphics, and was struck by the quality of the images, which led me back to HERE.

It looks like on-line materials about the course, but could be useful

But free info from MIT, such a deal!

Looking through it, there don't seem to be any lecture files, but there is enough other material that it could still be useful.

-AC

Hi AC,

I just glanced through the course notes, specifically chapter 7, moisture control. And they have a whopper there. It shows that a vapor retarder should be on the warm side of the walls. That's just wrong.

I agree with SF on this. Who wants to be wrapped in plastic. All moisture is retained within the enclosure then, kind of like being in a wetsuit. Yuck. Bad info is just being handed down from generation to generation on this subject and when someone points it out all you hear is crickets chirping. Why doesn't anyone question these unexamined assumptions?

EDIT:

Here's a good article that talks about all the myths about the required use of vapor retarder and also the very few times and places they are needed:

http://www.greenbuildingadvisor.com/...vapor-retarder