guest-blogsheader image
0 Helpful?

Choosing and Installing Windows in the ProHOME

Windows and doors have both aesthetic and functional roles in an energy-efficient building envelope

Posted on Mar 23 2017 by Mike Guertin

Editor's note: This post originally was published as part of the ProHOME series at Fine Homebuilding magazine. Sean Groom, a former editor at FHB, leads it off with a discussion of how these windows were chosen, and FHB editorial advisor and builder Mike Guertin picks it up with instructions on how to install the windows correctly.

Windows and doors are unique in that they are an important aesthetic component of the house and also a critical part of the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials.. And, as part of the envelope, they uniquely affect energy consumption through their insulating properties and their ability to add heat to a home through solar heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss..

Let’s tackle the performance issues first. Designer Michael Maines started with a combination of fixed, casement, and awning windows to maximize energy performance. For operating windows, the hinged action of casement and awning windows closing onto the weatherstripping seals better than double-hungs, where the weatherstripping between the upper and lower sashes slides past each other at the check rail. Preventing small amounts of air leaking through the window assembly pays dividends when we get to the mechanical systems and for the comfort of anyone sitting near a window.

In a high-performance house, the choice between double-pane and triple-pane windows is fraught. Builders and designers typically have strong opinions on the subject. They fall either into the camp of using the highest-performing windows they can find, or the camp of using high-quality double-pane windows and directing the saved money elsewhere. Maines leans heavily toward the first camp. Mike Guertin, the builder, is firmly in the second camp.

The price differential between double-pane and triple-pane windows for the ProHOME project was roughly $7,500. Using BEopt (the National Renewable Energy Laboratory’s free Building Energy Optimization modeling software package), Maines found the triple-pane windows saved about $32 a year at current electric rates. (It’s an all-electric house.)

Whether you evaluate increased spending on performance from a break-even or return-on-investment standpoint, the numbers aren’t going to work out at any point during the life cycle of the windows.

Paying for a solar array

The ProHOME is not a Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates., so it wasn’t necessary to keep the heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. below a particular threshold. Instead, the goal is to create a pretty good thermal envelope and limit air infiltration to between 1.0 and 1.5 ach50. The significant savings from foregoing triple-pane windows can be used on a photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. array. Accordingly, the windows are double-pane Marvin Integrity units, which have a fiberglass frame and will stand up to the coastal weather in Rhode Island.

Maines still contends, however, there is a comfort argument for triple-pane windows that may sway the opinions of some clients, as the surface temperature of the interior glass can be significantly warmer than that in double-pane windows in the winter. A cold window surface chills occupants sitting near it even if the ambient air temperature at the thermostat is in the 70s.

Careful installation is key

The Warwick CTC students spent about a week installing windows for a couple hours a day. The process started with Bruce flashing the window bucks back to the Zip wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. and then the students installing the sill pan flashing and rough-opening jamb flashing. The window installation is just as meticulous.

We worked in three-person teams, with one inside person who passed the unit out, centered the window and then shimmed it level under the direction of the two outside workers. The outside guys checked the windows for plumb, applied sealant along the sides and top, bedded the flanges into the sealant, and then screwed the unit to the bucks. It took 30 to 45 minutes to install and flash each Integrity window properly. This is in addition to the rough opening preparation we did earlier.

After the window is centered in the rough opening, the outside crew checks for plumb and square.

First the window is centered in the rough opening by the inside guy. Then the outside guys check for plumb and square. They instruct the inside guy to lift and shim if necessary. Very few windows needed any shimming, as it turned out.

With the window plumb, square, and centered, the outside guy draws lines along the side flanges so the window can be set correctly when the sealant is applied.

With the window plumb, square, and centered, the outside guys draw lines along the two side flanges so the window can be quickly reoriented after the sealant is applied. The inside guy tilts the window out about 8 inches at the top — enough for one of the outside guys to run a 3/8-inch bead of TiteBond Ultimate MP sealant along the top and sides of the window buck.

The other outside guy keeps a hand on the window as a backup to the inside guy. The bottom flange is not sealed so any water collected in the sloped sill pan can drain outward.

When the window is tilted back into the opening, flanges are lined up with the pencil marks.

When the window is tilted back into the rough opening, the flanges are realigned to the lines and then screwed off. We used 1 1/4-inch galvanized pan head screws instead of the 2-inch roofing nails that the instructions call for. Screws go in faster than the nails, and there’s less risk a poorly aimed hammer whack will damage a window frame. Since we are fastening directly to the 2x3 window bucks, I’m not concerned that the screws don’t penetrate as deeply as the nails would.

The four corners of the window flanges on the Integrity windows require a sealing patch. This is in addition to the backflashing step we took when preparing the windows for installation.

The adhesive foam pieces have to seal to the window frame and over the open corner of the flanges. We pressed the patch pieces in place with a mini flat bar to ensure good adhesion in the cold temperatures — low 40s / upper 30s.

Each corner of the window flange needs a foam sealing patch.

In addition, a strip of housewrap is taped over the face of the flange. It helps deflect any wind-driven rain that enters in the joint between the trim.

ProtectoWrap’s SuperStick window flashing tape seals the window to the flashing tape applied between the Zip wall sheathing and onto the face of the window bucks. Integrity flanges are mechanically attached to the frame of the window, so the flashing tape must be bonded to the side of the window jamb by 1/4 inch to 3/8 inch.

The head flashing tape Bruce installed weeks earlier when he applied the Zip tape around the window bucks still had a strip of release paper along the bottom edge. We removed the release paper and folded the head flap down over the top flange and window frame.

For more, use this link to see a short video with Mike Guertin on how windows should be installed in a high-performance building.


Tags: ,

Image Credits:

  1. Fine Homebuilding

1.
Mar 23, 2017 7:22 AM ET

10 cents
by Robert Swinburne

I have found that if windows are individual and not huge, that there is not much difference in comfort between double and triple glazed. It is when you get into large areas of glass, whether a large single window or grouped windows that you can really feel the comfort difference.


2.
Mar 23, 2017 9:54 AM ET

This puts some numbers on the
by Jon R

This puts some numbers on the comfort issues.

http://www.payette.com/building-science/glazing-and-winter-comfort-tool

Lots of tall glass may require heat sources under the window to address the downdraft issue.

http://nesea.org/sites/default/files/session-docs/room_side_low_e_coatin...


3.
Mar 23, 2017 4:16 PM ET

Winter Comfort Tool
by Jamie Wolf

Thanks for sharing the glazing and winter comfort tool Jon - really impressive and useful. Choosing glazing is about far more than the ROI equation.


Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!