Ten Common Mistakes Made By New Home Builders

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Ten Common Mistakes Made By New Home Builders

These bad ideas should have been buried long ago; the problem is, they’re not dead yet

Posted on Aug 4 2017 by Martin Holladay

Designers and builders who do their homework before construction begins have few problems. Unfortunately, some projects happen backwards: the design and construction are well under way before the homework begins. That type of project can be problematic.

At GBAGreenBuildingAdvisor.com, we see examples of the latter group all the time. Designers, builders, or homeowners who are in the middle of a construction project will post basic questions on our Q&A page. “I’m looking at the rafters and trying to decide how we should insulate the roof,” they write, or “We’re trying to figure out the best place to put the HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. .”

Answering these questions is part of my job; however, I don’t look forward to another ten years of similar questions. I’d rather be unemployed.

To help reach that goal — putting me out of the Q&A business — I’m providing a list of ten common mistakes. Let’s banish these blunders.

Mistake #1: Forgot to install raised-heel trusses

Raised-heel trusses should provide enough vertical space above the top plates of the exterior walls to allow for the installation of a generous depth of cellulose or fiberglass insulation, plus about 2 1/2 inches for a ventilation baffle and an air space.

For a code-minimum home in Alabama, raised-heel trusses might need only 13 inches of vertical clearance at the top plates, while a pretty good house in northern Maine might need 19 inches of vertical clearance. First, figure out how much insulation you want to install, and then let the truss company know your needs when you place your truss order.

Mistake #2: The insulation contractor did a bad job of installing fiberglass batts

This age-old problem is still with us, as Carl Seville’s many blogs on the topic regularly remind us.

If you are the owner, there are three possible solutions to this problem:

  • Write specifications that insist on adherence to the Grade I installation standard, and make sure that insulation contractors understand your expectations before they bid. And supervise, supervise, supervise, with the aim of ensuring that the standard is met.
  • Do the work yourself.
  • Specify an insulation material other than fiberglass batts.

Mistake #3: Forgot to install any basement wall insulation

In you are building in Climate Zone 3 or anywhere colder, your building code probably requires basement walls to be insulated.

Just because it’s required, though, doesn’t mean it’s always installed. If your builder says, “No one does that around here,” push back or find another builder. Insist on basement wall insulation. Here is a link to an article that describes the right way to do the work: How to Insulate a Basement Wall.

When it comes to slab-on-grade foundations, the comparable sin is forgetting to install vertical rigid foam at the slab perimeter. When it comes to improving the thermal performance of the foundation, this type of insulation is always a good idea, although it’s often omitted — sometimes for a valid reason (worries about termites), but usually due to simple ignorance.

Mistake #4: Designed a mechanical room that is too small

I’ve written about this issue before, so I’ll be brief. Make a list of all the different appliances and pieces of equipment that belong in this room — perhaps a furnace, water heater, well pump, pressure tank, water softener, and HRV — and make sure everything will fit. Remember to include room for ducts and pipes, as well as room to access the various pieces of equipment for maintenance.

Then make the room a little bigger.

Mistake #5: Forgot to perform a blower-door test

If you schedule your first blower-door test at the right time — usually after windows and doors are installed, but before the drywall is hung — you’ll be able to identify leaks in your thermal envelope. It’s much easier to locate these leaks and seal them before the drywall is installed, so don’t wait until it’s too late to schedule your first blower-door test.

And if you are still one of those builders who thinks that blower-door tests are a waste of time, you need to wake up.

Mistake #6: Installed an oversized furnace and an oversized air conditioner

Almost every new home in the U.S. has an oversized furnace and an oversized air conditioner. This problem persists because (a) equipment manufacturers don’t offer as many low-load options as they should, (b) HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractors have a financial incentive to sell oversized equipment, and (c) most HVAC contractors lack the skills to perform an accurate load calculation. (If you doubt these statements, you should read these two articles: Saving Energy With Manual J and Manual D and Manual J Load Calculations vs. Rules of Thumb.)

Finding the right professionals to perform your load calculations and to design your HVAC system can be an uphill battle. For some hints about how to proceed, see Who Can Perform My Load Calculations?

Mistake #7. Located ducts in unconditioned spaces

In some regions of the country, HVAC contractors routinely locate ducts in unconditioned attics or unconditioned crawl spaces. These ducts are basically outdoors. When ducts are located in unconditioned spaces, the duct systems are responsible for tremendous levels of energy waste.

If you are involved in a residential construction project, you should not compromise on this important principle: all ducts must be located inside the home’s thermal envelope. For more information, see Keeping Ducts Indoors.

Mistake #8. Forgot to seal duct seams

To me, duct seams should be like plumbing connections: they shouldn’t leak. If you want your ducts to convey warm air in winter and cool air in summer to the rooms where the air needs to be delivered, then you don’t want leaky ducts.

Not every HVAC installer understands this basic principle, however. Look for evidence of mastic or high quality tape on all duct seams. If you don’t see signs of duct sealing work, insist on a duct leakage test.

For more information, see these two articles:

Mistake #9. Installed hot water pipes that are too long

If a bathroom or kitchen is 30 or 40 feet away from the water heater, you’re going to wait a long time for the hot water to arrive. Long hot water piping runs waste water and waste energy.

This problem is best addressed at the design stage. Ideally, the kitchen and bathrooms will be located close to each other and close to the water heater. If that’s impossible, your house may need two water heaters or a demand-controlled hot water circulation loop. For more information, see:

Mistake #10. Installed recessed can lights in insulated ceilings

There are at least three reasons why you don’t want any recessed can lights in an insulated ceiling:

  • Almost all recessed can lights — including so-called “airtight” fixtures — leak air. Because the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. causes air near can lights to be pressurized with respect to the outdoors, air leaks in an insulated ceiling cause more problems than air leaks in walls.
  • Recessed can lights take up space that should be filled with insulation, thereby lowering the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the ceiling insulation. This fact explains why recessed cans create “hot spots” in your ceiling. In snowy climates, these hot spots often cause ice dams.
  • When the lamp in a recessed can fixture is turned on, it gives off heat. The hot bulb accelerates the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season., pulling more air through cracks near the fixture. Each fixture becomes a heat-loss chimney equipped with its own engine.

There are lots of alternatives to recessed can lights: track lighting, wall sconces, and pendants, for example. Ideally, you won’t need any electrical boxes in your ceiling. If you end up with a few electrical boxes in your ceiling, specify airtight electrical boxes or spend some time air sealing the holes in the back of each box (where the cable enters the box) as well as the crack between the electrical box and the drywall.

If you insist on the recessed can look, one alternative is to install the new LED pancake fixtures that fit into shallow electrical boxes. (Shallow electrical boxes are easier to air seal than recessed can fixtures.) For more information on this option, see:

Correct your mistakes before construction begins

If you do your homework before construction begins, you'll have plenty of time to make all your mistakes on paper. Those mistakes are much easier to fix than mistakes made with concrete, 2x6s, and plywood.

Martin Holladay’s previous blog: “PV Systems That Divert Surplus Power to a Water Heater.”

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Image Credits:

  1. Image #1: Fine Homebuilding

Aug 4, 2017 10:07 AM ET

Pre-Mistake #1
by Armando Cobo

1. Lack of building knowledge and lack of code knowledge, relying on subs to know code.
2. Not paying for a complete set of drawings and specifications.
3. Consistently looking for the cheapest bid, usually getting the worst subs.

Aug 4, 2017 11:02 AM ET

Hey now
by Malcolm Taylor

That's an attic truss, not really a raised heel truss - and looks like it would be a nightmare to insulate.

Aug 8, 2017 11:01 AM ET

Response to Malcolm Tayolor
by Martin Holladay

Good catch. Thanks for your comment.

You're right -- the original photo that appeared with this article was a poor choice. I've replaced the photo with a more appropriate one.

Aug 9, 2017 11:53 PM ET

Blower Door Test
by Lawrence Martin

If you do the blower door test before you install the drywall, then how can you use airtight drywall on the attic floor / ceiling? It seems that it is necessary to install at least ceiling drywall before the blower door test in that situation

Aug 10, 2017 4:45 AM ET

Response to Lawrence Martin
by Martin Holladay

You're right, of course, that the scheduling of the blower-door test depends on the air barrier details. At a minimum, you'll need to have your primary air barrier installed (including your ceiling air barrier or roof air barrier) and your windows and doors in place before you can perform the blower-door test.

In a house where the primary air barrier is at the sheathing layer (including the roof sheathing), the blower-door test can usually be performed before any drywalling.

If the ceiling drywall will be the primary air barrier, then of course the ceiling drywall needs to be installed and taped before the blower-door test can be performed.

For more information on these issues, see Blower Door Basics.

Aug 10, 2017 6:38 AM ET

raised heel truss
by Charles Bado

It's both an attic truss and has a raised heel, space below the attic floor is 16" deep to allow for blown in cellulose.

[Editor's note: Charles Bado is referring to the photo below, which appeared in an earlier version of this article.]


Raised heel truss - Bado 2.jpg

Aug 10, 2017 11:41 AM ET

Raised heel truss redux
by Malcolm Taylor

It does have a raised heels, but I bet it was incidental to any energy concerns. The height was necessary for structural reasons - as witnessed by the absence of any thought as to how the sloped top chord would be insulated. Unless the idea was to have an unconditioned attic - which seems like a lot of work for little return.

Sep 1, 2017 10:35 PM ET

ROOF Insulation Question
by Rick Milne

I thought I had it all planned and have been reading GBA for years including all your articles on insulating a low slope unvented cathedral roof... but have run into a quandry on insulating my built up residential roof in Southern coastal area of BC.
I was initially favouring the higher density rock wool but the cost for a 2700 sq ft roof at R28 minimum became unreasonable. I was leery of polyiso because of the articles on GBA of the penalty at low temperatures and while getting quotes from local Victoria,BC commercial roofing suppliers for EPS type II began getting cautioned about EPS being less stable at high temperature under a steel roof, messy to cut and place and that I should stick to the higher density commercially proven polyiso given the mild winters. Here in BC the EPS is the most cost effective BUR material available and I am in a more remote area.
I do plan to put batts up under the roof deck later to beef up the R values but appreciate any advice. The manufacturers info on EPS indicates it's good to install up to 165F.
I guess even though I thought I had it all figured it seems sometimes varying opinions can confuse!!

Sep 2, 2017 5:52 AM ET

Response to Rick Milne
by Martin Holladay

Polyiso is a perfectly good choice for your application, even though you live in a (somewhat) cold climate.

Your reference to R-28 insulation had me scratching my head for a while -- that seems low to me -- until I figured out that you are probably planning to install R-28 of exterior rigid foam, supplemented by fluffy insulation on the interior (hopefully, to bring the R-value of the entire assembly to about R-49).

Assuming you are aiming for R-49, and assuming that you live in Climate Zone 5 (use this map to determine your exact climate zone), you probably want to install rigid foam with a minimum R-value of R-29, according to my calculations. (So our assumptions are similar).

For more on the logic behind my recommendation for R-29 exterior foam, see Insulating Low-Slope Residential Roofs.

While polyiso is good insulation, you have to de-rate the R-value of the polyiso in cold weather. For more information on this issue, see Cold-Weather Performance of Polyisocyanurate. My article quotes John Straube as saying that cold-climate builders should assume that polyiso performs at about R-5 per inch. For a thick layer of polyiso, that's conservative (since the upper inches of polyiso help keep the lower inches warmer). If you specify 6 inches of exterior polyiso, you will be fine.

Sep 2, 2017 10:11 AM ET

Roof Insulation Follow Up
by Rick Milne

Martn...thanks for the quick response. I live in zone 4 and R28'is minimum code for a cathedral roof. I will add batts later. I would rather use the more cost effective eps foam than polyiso so my question is will a type II eps foam at 7 inches under plywood sheathing have the longevity and stability and stand up to the heat a metal roof may generate or are my commercial suppliers just a bit biased by pushing the polyiso. They have even suggested putting a layer of polyiso on top of the eps foam to provide protection from heat which is opposite what GBA recommends. I have read no articles on GBA that indicate eps has any of these issues. They also think taping the top seams under plywood decking is a waste of time and money!

Sep 3, 2017 5:33 AM ET

Response to Rick Milne
by Martin Holladay

EPS can be used under metal roofing. For more information, see these documents:

This last document notes, "The insulation materials used with metal roofing are identical to those used with traditional roofs, and the selection process is much the same. The most common insulation materials used today are polyisocyanurate (commonly referred to as polyiso), perlite, expanded polystyrene, and extruded polystyrene..."

If you have any doubts about the suitability of the EPS you have specified for your project, you should talk to the technical help specialist at the headquarters of the EPS manufacturer or distributor.

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