Windows T/F

Which of the following are true concerning window installation detailing?

Psi-glazing edge is reduced with increased insulation wrapping of the frame

False

the value of the item is that it separates two Ψ-values people routinely merge. A window carries both. Ψ-install belongs to the joint between frame and wall, and lapping insulation over the frame is exactly what improves it — that is the move that can drive it to zero or below. Ψ-glazing-edge belongs to the spacer bar at the perimeter of the sealed unit, buried inside the frame, and it is set by the conductivity of that spacer: aluminum is poor, stainless or a plastic composite is what a Passive House uses. Insulation on the outside of the frame does not reach it. Keep them apart in the model as well as in the head, because PHPP takes them as separate entries and the installed U-value build-up needs both.

Ensuring an airtight connection between window and frame will improve the energy balance of the window

False

read the stem precisely, because the trap is in the words “of the window”. Airtightness at the frame-to-wall joint matters enormously to the building: leakage there drives infiltration losses, carries moisture into the assembly, and shows up in the blower-door result. But the window’s own energy balance as PHPP models it is solar gains minus transmission losses, Qs − Qt, and neither term contains an air-leakage component. The leakage is accounted for elsewhere, in the ventilation and infiltration figures derived from n₅₀. So the sealing is essential and it improves the building, while leaving the window’s modeled balance unchanged. Two different accounts, and the question asks about only one of them.

The position of the window in the wall build up has little influence on space heating demand

False

position is one of the most consequential choices in the whole detail, and it acts in two directions at once. Thermally, a frame set into the plane of the insulation and lapped by it runs a near-zero or negative installation Ψ, while the same frame stranded in the masonry can run a severe positive one; PHI’s best practice reaches about −0.12 W/(m·K). Optically, the further back the window sits, the deeper the reveal, and a deep reveal shades the glass — which cuts winter solar gain and therefore raises the heating demand, all else being equal. The two effects can pull against each other, which is why the position is decided deliberately rather than left to the wall section, and why PHPP asks for reveal depths on the Shading worksheet.

In externally insulated masonry wall constructions, it is critically important to position the window in the insulation layer

True

On an externally insulated masonry wall the insulation is the continuous layer and the masonry is not, so a frame fixed to the masonry sits outside the thermal envelope’s continuous line and bridges straight through it. Setting the frame in the plane of the insulation, and lapping the insulation onto its outer face, is what keeps the line unbroken — the single most consequential decision in the installation detail. Think of a window as an insulation element rather than a structural one and the position follows. One caveat from the chapter is worth carrying: lapping insulation over an aluminum facing part is close to ineffective, because the aluminum draws cold in beneath the insulation faster than the insulation can hold it out.

The same Psi-install value can be expected for all four sides of the window

False

the sill is almost always the worst of the four. The head and the two jambs can usually be lapped generously with insulation, but the sill has to shed water: there is a drainage detail, a sill pan or a flashing, and often a support that has to carry load. All of that competes for the space the insulation would otherwise occupy, so the sill typically ends up with less cover and a higher Ψ-install than the other three sides. Two consequences. When you model the installation you may need different Ψ values on different sides rather than one figure times the whole perimeter. And when you detail, the sill is where the effort pays — it is the side most likely to be both the largest thermal bridge and the place water gets in.

In retrofit projects, wrapping existing (in-place) of frames on the inside has the same benefit as wrapping it on the outside

False

internal wrapping can actively make things worse. Insulating on the outside puts the frame and the surrounding wall inside the thermal envelope and keeps both warm. Insulating on the inside does the reverse: it leaves the frame and the masonry around it on the cold side, so the junction runs colder and the surface temperature at the reveal drops toward the mold threshold. On a masonry wall with an already low U-value the effect on the junction Ψ can be adverse rather than merely smaller. This is the window-scale version of the general retrofit rule — external insulation wherever it is possible — and where interior insulation is unavoidable the junction needs a hygrothermal check and usually a wedge of flanking insulation to lift its temperature.

The drainage detailing at the window base has a minor influence on the Psi-install value

False

the drainage detail at the base is one of the largest single influences on Ψ-install, not a minor one. The reason is that drainage tends to require continuous material running from inside to outside at exactly the point where the insulation should be unbroken. A metal drainage pan carried through from the interior to the exterior is the worst case: a continuous, highly conductive path across the full thickness of the detail, running the entire width of the window, and it should never be used. The fix is to break the metal, use a non-metallic pan or a membrane, and keep whatever must cross the line as small and as low-conductivity as the water management allows. This is why the sill is usually the worst of the four sides.

A Psi-install value of  0.003 Btu/hr.ft.F can be regarded as thermal bridge free

True

The thermal-bridge-free threshold is Ψ ≤ 0.006 Btu/(hr·ft·°F), which is 0.01 W/(m·K), and 0.003 is comfortably inside it. Two things worth attaching to the number. It applies to the installation Ψ the same way it applies to any other linear bridge — there is not a separate, looser limit for windows, even though the window perimeter is usually the longest thermal-bridge line in the building. And meeting it is a modeling permission rather than a pass mark: a junction that qualifies may be entered as zero or omitted, and if every junction in the building qualifies the thermal-bridge surcharge disappears from the calculation. A window installation at 0.003 is doing what a well-lapped frame in the insulation plane should do.

Negative Psi-install values are not entered into PHPP due to the fact that they are ‘thermal bridge free’

False

You may enter a negative Ψ-install, and there is every reason to — it reduces the modeled heat loss and so lowers the heating demand. Two clarifications keep this straight. Omitting a negative value is permitted, because leaving out something that would have helped you is conservative; what you may not do is omit a positive one. And the rule that governs claiming them is the important half: to take credit for negative thermal bridges you must have calculated all of the thermal bridges in the building, favorable and unfavorable alike. That is what stops a model from collecting the corners and quietly forgetting the balconies. Note also that “negative” and “thermal-bridge-free” are not the same idea — the threshold is a limit, the sign is a direction.

Fixed windows in Passive House projects may have the appearance of frameless glass with good detailing

True

it follows directly from the best installation detail rather than being a separate styling choice. The ideal is a frame set in the plane of the insulation with the insulation lapped across its outer face; on a fixed unit, where there is no sash to operate, the lap can cover almost the whole frame. What remains visible from outside is glass and a thin edge, so the window reads as frameless. The performance and the appearance improve together, which is unusual and worth knowing: covering the frame is what drives the installation Ψ toward zero or below, and the frame is the weakest part of the window thermally, so hiding it also raises the glazing fraction. It is a good argument for large fixed units paired with a smaller operable one.