Envelope Design

The envelope is the continuous insulated barrier between inside and outside, consisting of walls, floor, ceiling, windows and doors.  Increasing insulation reduces the need for heating/cooling but obviously there is some point where it makes neither economic or environmental sense to increase the level of insulation.  In passive solar design, we can easily make that tradeoff because we know how much energy is likely to come into the building, so we can design for typical conditions. For everyone else, you will need to decide if your primary concern is economic, environmental or both.  If they are economic, you will end up having to make assumptions about future energy costs, and if your priorities are environmental, you'll have to make impact decisions with very incomplete knowledge, and if they're both you'll have an even more complex choice.

The envelope of the house is also the wind barrier, the rain barrier and a vapor barrier and in most cases the physical structure that holds up the house is buried in the envelope.  For more on these issues and how to build the components of the envelope, see the construction section.

How Much Insulation?

Actually we really want to know how well the envelope blocks heat flow, which means not just insulation, but the effects of thermal bridging, windows, doors and infiltration, that is everything that affects heat flow thru the envelope.

In absolute terms it depends on your climate, or rather your heating/cooling load.  It also depends on the type of building and how much surface area it has (see shape discussion below).  In relative terms, here are a few ways you could look at it:

  • Put in enough, so that the mean radiant surface temperature of the envelope is reasonably close to room temperature 99% of the time, ie to maximize comfort.
  • Put in enough so that the predicted energy use is 80% less than what houses currently use (see the 2030 challenge). This is the predicted amount to curb global warming.
  • Put in enough so that all the energy of the house can be supplied on site (ie zero energy).
  • Put in as much as you can afford.
  • Put in how much makes sense based on your estimate of future energy prices.
  • Put in enough so that you can heat the house with a ventilation system.  This is essentially the Passive House standard, where the maximum heat loss can't be more than 3.16BTU/SF.  This approach generally requires more insulation than the others and is somewhat controversial.

Since each approach is dependent on your assumptions and priorities there is really no way to say one is better than another.  Its also the case that both the economic and environmental tradeoffs are changing rapidly as new products and technologies come on the market.  In spite of that, its still the case that how much insulation you need is likely more than building code requires and more than virtually all houses have in them.

Balancing Envelope Components

Whether you've got a specific heat loss target, or are just gong by the "do the best you can" plan, you need to balance the insulation values between walls, ceiling, floor, windows, doors and infiltration.  If you have one weak link in your envelope, then your heat loss becomes dominated by it, and improving other areas won't make much difference.  Its like a dam with multiple leaks: there isn't much point stopping the small leaks if there is a big one.

The best way to balance the components is to build a heat loss spreadsheet (or use software1) and plug in the appropriate areas and U values and compare what percentage of the total loss each component is.  If they're not balanced, then the obvious place for improvement is to start with whatever component has the largest loss.

Consider a simple 2000ft2 two story house measuring 25x40x17.  Assume it has about 12% glazing (which in this case is 240ft2).  This is a typical older city-lot house.  Now imagine doing a energy retrofit: we'll leave the walls at R11, fill the floor with R40, fill the ceiling with R50 and then tighten so that the blower door gets 2ACH50, and assume ACHnat is 1/20 of that, which is about 28CFM, and we'll make up the rest with mechanical ventilation so that we get a total of 75CFM (which is the code requirement), but at this point we'll assume no HRV.  In this plan the windows are the most expensive part of the retrofit.

U-wall U-Floor U-ceiling U Windows Vent  
1/11 1/40 1/50 .3 75CFM  
Wall loss Floor loss Ceiling loss Window loss Infiltration Total
200.9 10 20 72 81 383.9
% total
52.3% 2.6% 5.2% 18.7% 21.1%  

As you can see from the table, if we want to improve this house, the problem is all in the walls, so now lets try bumping the walls up to R22, which is the equivalent of adding 2" of polyiso, as shown in the following table:

U-wall U-Floor U-ceiling U Windows Vent  
1/22 1/40 1/50 .3 75CFM  
Wall loss Floor loss Ceiling loss Window loss Infiltration Total
100.4 10 20 72 81 283.4
% total
35.4% 3.5% 7.1% 25.4% 28.6%  

Although the walls are still the largest loss, the losses are more balances, so now lets look at going up to R30:

U-wall U-Floor U-ceiling U Windows Vent  
1/30 1/40 1/50 .3 75CFM  
Wall loss Floor loss Ceiling loss Window loss Infiltration Total
73.7 10 20 72 81 256.7
% total
28.7% 3.9% 7.8% 28% 31.6%  

This time we don't get nearly as dramatic a payback.  The first change reduced the heat transfer by 100BTU/hr/°F, and second change only by 26.7BTU/hr/°F.  In the second change, even if we added another R11 as we did in the first change, instead of R8, the change is still only 34.4BTU/hr/°F because the leak we're plugging is not so big.  While this change, may not seem worth it, it really depends on the marginal cost: depending on how you're adding the insulation, adding 4" or even 8" of additional space might not cost much more than adding only 2".

At this point, ventilation has become our largest loss, so replace our simple ventilation system with a 70% efficient HRV.  Keep in mind that our infiltration losses are still 30.6BTU/hr/°F, so we're only saving on the ventilation side but we reduce that do about 15BTU/hr/°F, leaving a total ventilation energy loss of 45.6BTU/hr/°F as shown in the following table:

U-wall U-Floor U-ceiling U Windows Vent  
1/30 1/40 1/50 .3 46.7CFM/HRV  
Wall loss Floor loss Ceiling loss Window loss Infiltration Total
73.7 10 20 72 45.6 214.7
% total
31.2% 4.7% 9.3% 33.5% 21.3%  

With this improvement in ventilation energy, we've removed 35.4BTU/hr/°F, so depending on the marginal cost of adding the HRV, on pure energy terms this was a bigger improvement than the previous one where we added R8 to the walls.  If we wanted to continue to improve, we'd look at windows and the wall again.

Effect of Size, Shape and Shared Walls

The shape of a building can have a large effect its surface area, and hence its heat transfer amounts, for example a 20x100 one story ranch house has a surface area of 5920ft2 while a 25x40 two story house has a surface area of 4210ft2 but since most of that additional surface area is in the floor and ceiling where its usually easy to add a lot of insulation, and since shape has no effect on windows and infiltration, which are often large losses, the actual heat loss transfer difference between them is minimal.

What does have a large effect is shared walls and large buildings.  As the building gets larger the ratio of floor are to surface area starts shrinking: another way tot look at it is that the amount of surface area per square foot of floor space shrinks.  The buildings in the examples below have ratios in the range from 2.96 to 1.39.  As the examples show, as the building gets larger, the relative losses thru the envelope go down, and the effect of ventilation air goes up.

The following examples compare different configuration of a 2000SF living unit.  The first two are single family, and the last two are multi-family, ie a bigger building.  In each case, the heat transfer amounts are per unit, meaning that in the multi-family case, the result is an average across all the units--needless to say the units with more exterior surface area will transfer more heat than the others, but the question here is not about individual units, but the average effect on the entire unit.

Since this is a comparison of the effect of size and shape, everything else is kept constant. Below is a summary of each component:

U-wall U-Floor U-ceiling U Windows Vent  
1/30 1/40 1/50 .3 75CFM  

These values are chosen to represent typical green building insulation levels in a moderate to cold climate.  The floor R-value assumes a 12" joist floor and some kind of fluffy insulation in it and that its over some kind of unheated crawl space which is likely to be warmer than outside temperature, so the loss is reduce to 60% of what it otherwise would be.  Given how little heat is transferred thru the floor anyhow, removing this fudge factor really wouldn't change the picture much.  Its assumed that the attic has more space for insulation, and since more is often installed there, this is probably a reasonable assumption.

The wall value is smaller due to assumed less space available for insulation. The windows are low to medium cost double glaze unit and that the house has a 12% glazing ratio, which for these 2000SF units means 240SF of glass.  The ventilation is assumed to be composed of infiltration of 28CFM due to a 2ACH50 tightness and a non HRV ventilation systems that delivers another 47CFM of fresh air.  Since we're comparing shape here, changing to an HRV lowers all the numbers equally, but doesn't change the comparison.

Currently, all losses are in Btu/Hr/°F, since we're only looking at relative contributions.

Building 1: Single family, 2000SF Ranch style, one floor,  20x100x8

Wall area=1920ft2    Volume=16,000ft3     Surface/Floor=2.96

Wall loss Floor loss Ceiling loss Window loss Infiltration Total
64 30 40 72 81 287
% total
23.3% 10.4% 13.9% 25.1% 28.2%  

Building  2: Single family,  2000SF, two stories, 25x40x17

Wall area=2210ft2    Volume=17,000ft3    Surface/Floor=2.1

Wall loss Floor loss Ceiling loss Window loss Infiltration Total
73.7 15 20 72 81 261.7
% total
28.1% 5.7% 7.6% 27.5% 31%  

Building 3: Eight row houses, each 2000SF, 2 stories of 25x40x17, total size 40x200x17

Wall area=8,160ft2    Volume=136,000ft3       Surface/Floor=1.5

Wall loss Floor loss Ceiling loss Window loss Infiltration Total
34 15 20 72 81 222
% total
15.3% 6.7% 9% 32.4% 36.5%  

Building 4: Sixteen Condos, each 40x50, 4 per floor, 4 floors, total size 80x100x35

Wall area=12,600ft2    Volume=280,000ft3       Surface/Floor=1.39

Wall loss Floor loss Ceiling loss Window loss Infiltration Total
26.2 7.5 10 72 81 196.7
% total
13.3% 3.8% 5.1% 36.6% 41.2%  

What these examples clearly show is that as the units start sharing more internal walls, the heat transfer per unit goes down.  It also shows that although keeping the R values of the components the same makes a good comparison, it doesn't make sense in real life: in building 4, almost 80% of the heat transfer is from ventilation and windows.  Doing that, another flaw in the comparison comes up, which is that given the lower surface/floor area ratio, the infiltration value is likely to be proportionally smaller.  Assuming a linear reduction based on surface/floor ratio, we can say that the infiltration value for building 4 is more likely to be around 19CFM, and if we then use a 70% efficient HRV to supply the remaining 66CFM, the infiltration/ventilation loss goes from 81Btu/Hr/°F to 35.3Btu/Hr/°F, so now reexamine building 4 with these assumptions and only R21 walls:

Wall loss Floor loss Ceiling loss Window loss Infiltration Total
37.5 7.5 10 72 35.7 162.7
% total
23% 4.6% 6.2% 44.3% 21.9%  

The infiltration/ventilation loss was so large that even with reducing the wall R-value from 30 to 21, the building still has a lower heat transfer rate than the previous configuration.  Now the problem is all windows, and once more the comparison is not really accurate for this type of building since shared walls don't have windows in them, the glazing ratio tends to go down, but if we reduce is by half, which would reflect the smaller wall surface area per unit, we'd compromise daylighting, so we might not actually build a building this shape, although plenty of them get built.  Although an attempt is made in these example to use realistic numbers, the comparison is meant to show the concept, not necessarily a real building.  As it turns out, getting good daylight mostly means increasing the exterior surface area, so its at odds with reducing heat transfer and so a tradeoff must be made.

Insulation and Renewable Energy

As the price of renewable energy comes down, the old adage that insulation is all that matters could change.  On site energy generation offsets any imported energy, so rather than increasing insulation, your can also generate energy and have the same net result in terms of annual external energy use.  While economically there might not be much of a difference between the two,  in terms of how they function, the two are often very different, particularly if the on-site energy is generated intermittently as with solar or wind.  However, if you are able to include battery or heat storage, or your on-site power in continuous, for example small hydro, then the tradeoff is straightforward.  A more thorough look this tradeoff is in the zero energy section.


1: however if you're reading this website, you probably don't have energy modeling software.  However, the simple models you can do in excel are good enough for most decisions.