Heat Load Calculator

A while back we posted a series on the mechanics of calculating the heat load of your home.  At the end we promised to offer up an Excel file that is set up for you to do your own calculations without getting a headache or hand cramp

The calculator comes pre-filled with info from our Houston 2448.  Everything in light yellow can be modified.  The file or the rest of the cells are not locked.  This should be considered open-source, AKA modify at your own risk.  If you enter any values into white cells, you may destroy formulas.  There are also no fail safes or error checking in here.  Double check your work.

There are 7 components listed: slab, floor, walls above and below grade, windows, doors, and roof.  Each of these has inputs for area and R-value.  Note that windows should be input as U-value.  When inputting wall area, don't take windows or doors into account.  They are automatically deducted from the wall area in the calculations.  Outside design temperature can be modified for the first four items; remaining values are derived from those.

All the work is shown on the following columns.  The UA value, Δt and Btu/hr values are shown.  Indoor temp can be changed to your desired setpoint.  To the right is a little table with all sorts of nerdy calculations in it.  Percent of load tells you which component is losing the most heat.  Cost/hr tells you how much it costs.  In the example you can see that more than half the heat loss in this house is through the walls.  Of course!  There is only R-15 in the walls!  You can also see that increasing to R-21 doesn't do much for that factor.  Increase the walls to R-30 and you can get that component down to about 1/3 of the heat load.  Still high.  Note that the rest of the load percentages change as you change the area or R-value of an item.

Lower down on the page is a place to take leakiness of the house into account.  Input your target or measured ACH50 as well as volume of the home.  You should only change the HC if you know what you're doing.

The final input is for number of bedrooms or potential bedrooms.  This little calc will determine internal gains from humans.  It takes the number of bedrooms and adds 1 person per ASHRAE standards.  If there will only be two people living in your 3000sf house, enter one bedroom for kicks.

Total peak heating load is given near the bottom of the sheet.  The final table gives an idea of how much of what types of heat is needed to keep the house comfortable.  A forced air unit size and efficiency can be entered.  As you can see, even this is WAY too big for the house.  Even 2 1kW cadet heaters will do fine.  In this case we would recommend a 500W heater in each of the bedrooms and bathrooms with a 1kW in the great room.  Still a bit much but at least reasonable.  Perhaps a mini-split heat pump would do for efficiency as well as adding some cooling if you are in the South.

This calculator should be used for entertainment purposes only.  No guarantees about the results or performance of this tool are made or implied.  If you break it, you bought it.  If you find errors, please feel free to let us know.  If somebody who knows javascript is bored, we would be thrilled to turn this into an online tool.

Massive Influx of Plans

In what might be described as an heroic maneuver, we at Istockhouseplans just cleared out a bunch of backlog and loaded more than a handful of plans.  The final damage?  Seven in one blow!  Listed below is the latest additions to the lineup:

The Wilsada A 1416A joins her brother the Wilsada 1416.  This tiny house duo are each a whopping 200sf.  The bed is in its own nook, the rest of the space being open save for a bathroom.  Therein is the difference.  Wilsada has a narrow 4x6 bath while the A is a slightly more spacious 5x5.  Otherwise the plans are very similar with their multitude of windows.

The Cottage in the Grove C2042 was a joint project with our builder friend last year.  We finally got around to creating the artwork and writing the text for this one.  This was a narrow 1356sf house that has the most charm of any of our offerings yet.  Two suites each with bathrooms and a large open downstairs space.

The Houston A 2448A steps in with a slightly larger footprint than his predecessor.  The numbering belies his true width of 25'.  Other than an even 1200sf, not much changed from the prior version.  The roof line was modified to maintain the 16' roof plane.

Three Edgewoods were finally put on display.  A few years ago we spent alot of time with the original version creating several spin offs for our builder friend.  The Edgewood C C2552C and The Edgewood D C2552D offer variations on the 25'-28' wide 3 bed 2 bath story-and-a-half theme.  Rounding out the triad is the The Edgewood D2 C2552D-2 mashup.  Not only is it a lot of width, it's also a lot of characters in the numerical coding!

Finally is a brand new plan inspired from a century ago.  The Arleta 2850 adds a third true single story full size plan to our lineup (right behind the Houstons).  At only 1353sf, this little charmer is guaranteed to be a winner in the new downsized home movement.  Three beds, two baths, lots of closets, and both material and energy efficiency just enforce her future position in the marketplace.  We're very excited about this one and hope to see it built soon.

Check out our full catalog for all plans.

Energy Efficient Wall Systems

You may remember our post a couple of years ago promoting Fat Walls.  In their 11/11 monthly newsletter, Energy Design Update recently reported on 15 different wall assemblies modeled through TRNSYS software.  The walls were simulated in the climates typical to Atlanta, Pittsburgh, and Phoenix.  Of the 15 walls, three tied in first place for an overall value of R-43.  One of these walls fills a 2x6 cavity with closed cell polyurethane spray foam for a rather high price tag.  The second wall involves 10" thick SIPS.  The third wall is our option number four from the previously mentioned post with 2" more of foam.  That is, a 2x6 wall with blow-in and 4" of outboard XPS foam.  As we mentioned back when we wrote the initial post, this makes window detailing a bit of a bear.  Attachment issues come into play as well.  The advantage of this system is the standard wall framing and no loss of floor space inside the house.

A reasonable compromise might be 3" of foam.  This allows the use of true 2x4 for bucking out windows while allowing 1/2" air space.  Half inch furring strips can then be used over the foam for attachment as well as a rainscreen.

They also modeled a similar wall as our top choice, double 2x4, total 8" thick with 2" of outboard foam.  Our results?  R-40 with U-0.20 windows.  Their results were R-38 with U-0.25 windows.  As you should know, U-0.20 windows are slightly better than U-0.25 resulting in a slightly higher total wall R-value.

So apparently we know what we're doing!