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 GBA, 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.”
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…
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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.
That's an attic truss, not really a raised heel truss - and looks like it would be a nightmare to insulate.
Response to Malcolm Tayolor
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.
Blower Door Test
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
Response to Lawrence Martin
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.
raised heel truss
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 redux
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.
ROOF Insulation Question
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!!
Response to Rick Milne
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.
Roof Insulation Follow Up
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!
Response to Rick Milne
EPS can be used under metal roofing. For more information, see these documents:
EPS Roofing Solutions
Engineering Concerns in Metal Roofing
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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