Most building owners won’t consider their roof hatch until someone needs to duck into the space to service HVAC in the middle of July. At that point, they’ve compounded two concerns, the broken unit and the ill-advised choice of having the roof hatch open for whoever needs to climb a ladder into the penthouse space because there are limited access measures to get the newly busted part back up through the roof.
Eventually, the owner realizes that every single time that hatch is opened, conditioned air is practically blowing out the front door. It’s not dramatic, but it costs thousands before the season’s over.
Yet the truth is that commercial buildings require roof access. Mechanical units fail. Membranes require inspections. Sometimes snow needs to be cleared off an edge. Yet every penetration made on top of a building envelope becomes a thermal weak point, and the roof hatch is one of the most considerable openings a building has. Thus, getting it right involves understanding what occurs when access is valued over insulation.
The Nature of Roof Penetrations
The first thing about roofs is that they comprise continuous insulation. The idea behind modern commercial roofing is that it’s sufficiently insulated to maintain energy within in winter and keep it from entering in summer. Then someone cuts a hole in it to access what’s inside?
A standard roof hatch creates thermal bridging, metal transfer, which conducts heat through the hatch frame and immediately through the layer of insulation, connecting the building’s interior temperature with what’s outside. With R-30 or R-40 atop a finished assembly, where a hatch is situated could bring R up to R-5 or lower with no type of assembly or frame connection, R values aren’t theoretical; they’re shown through ice dams and heat influx.
The problem increases because these are not static openings. A roof access hatch door hinges open and shut (one would hope), meaning moving pieces, hinges, and openings that make spray foam insulation ineffective. Where gaskets come into play is more relevant than the minimum spec for this subcomponent. Gaskets that are subpar have low compression set, meaning, eventually, they provide an air leakage path through which conditioned air might escape even when the hatch is fully closed.
Roof Hatch U-Values
We can’t forget about energy code requirements for roof access hatches’ U-values, and there’s no arbitrary number here. A U-value measures how much heat loss occurs from through material; lower values are deemed better insulating capacities. For example, most national energy codes suggest that access hatches achieve U-0.50 or greater, which is higher than expected of something designed to be lightweight for human operation.
Gaining access to such performance requires significant insulation in a hatch curb and door or a thermal break in the frame construction; some manufacturers utilize polyurethane foam insulation in panel hatches and curbs while others rely on thermal break technology to lessen metal-on-metal interaction where interior and exterior meet.
It’s not about how it gets here; it gets here regardless, but using uninsulated hatches will save money, uninsulated hatches that come at an energy penalty, which many buildings succumb to. The math works sometimes on roofs that rarely require access (twice a year for an inspection), but annual operations, and increasingly so in extreme climates, makes energy loss real hard currency.
Building code also presents an interesting conflict here with roof access; fire codes may dictate how many access points are on top for firefighters; energy codes like we’ve just mentioned demand certain thermal performance; safety codes require minimum opening dimensions, life safety determines hatch size, too, but one hatch that satisfies all of these efforts simultaneously will cost more than one that merely cuts a hole in the roof for feasible access.
Sizing is a clear issue for energy performance; bigger means access for more thermal transfer; smaller means restriction on interior movement for equipment. A 30×36 opening is sufficient for minimum code requirements, but if HVAC contractors can’t fit their equipment through, they’re going to want a crane pick from the parking lot – and that’s expensive enough where some owners choose the larger opening and pay for it in energy instead.
Cost Calculations
The math isn’t complicated, but people rarely do it until they’ve gotten past design. Picture a building with 5,000 heating degree days and 1,000 cooling degree days, an uninsulated 36×48 roof hatch will lose approximately 2 million BTUs per year over insulated options based on typical commercial rate costs of $0.03-.04/CF:
Per hatch, removing $75 worth of emissions annually (without considering reduced load demand) from a building doesn’t seem significant. However, multiple hats exist on multiple buildings PLUS this termination costs over its 20-30 year lifetimes, which come to $1,500-2,000 pounds against $500 premium per insulated hatch, when it’s scaled over time, it’s likely to find payback between year 7-10, which works for most commercial building economics; albeit sometimes it’s worth it regardless if annual maintenance doesn’t occur as roof hatches lose their effectiveness after years of wear and tear.
There’s a difference between loss due to pure conduction loss versus gasket failure vs air sealing vs insulation value. A hatch could have plenty of insulating value in panels but still fail at the point at which allowance occurs around gaskets going through periodic thermodynamic temperature cycles allowing even more air loss to occur around them.
It’s recognized that air leakage loses more energy than any pure conduction, which holds true whether LEED certified or not, and fails with gasket compression levels come into play over time, EPDM maintains better sealing quality than others; silicone holds better with extreme temperatures; minor detail specification gets lost but determines better performance down the line, or subpar performance after molecular breakdown over thermodynamics and UV exposure over time.
Manufacturer best practices promote compression gaskets with multilayered seals and continuous gaskets without joints/seams versus cheap gaskets that can fracture, the visual difference isn’t apparent until they’ve been in use for five years plus, which is exactly why those assessments go ignored during initial assessments.
Integrated Design Considerations
Integrative designs mean building designers are putting greater thought into roof access placement by placing them in vestibules/mechanicals rather than conditioned spaces to reduce impact in addition to designs that include small enclosed stairs instead of hatches where additional insulated edges can better join up with penetrative roofing.
Curb height plays into this as well, higher curbs provide more workable insulation around framed edges as well as integrated fields, while 8-inch curbs are easier for operatives 12-18 inch taller curbs perform better against snow drift/rainfall, which means maintenance personnel need more distance to access so this means energy positioning comes across more effectively than safety/accommodation accessibility over time.
But let’s be realistic about what’s likely to happen in commercial buildings, the roof hatch gets specified during construction, installed during construction, and then gets forgotten about until something major fails rendering questionable performance degrading reliability, and there have been some state changes along the way with gaskets compressing/hardware corroding/hatches overcoming play, this makes security and thermal integrity questionable, but they’re so minuscule over time that it’ll never occur until it really compounds monthly costs yet no one makes those assumptions.
Building managers who perform regular annual maintenance on their hatches (gaskets/lubricant on hardware/replaced hold-open arm) maintain their energy performance closer to original intentions; operating managers who’ve forgotten failed expectations see dips in performance quantifiably attributed although energies aren’t connected at all times like they should be, the right mechanism would be maintainability better suited in design.
Access hatches with accessible hardware/replaceable gaskets/materials highly correlated prevent energy losses better than designs driven by initial costs offering no expected long-term payback.
Trade Offs
There are no perfect tradeoffs between efficiency/access, their compromises should be made depending upon climate realities and intended use for maintenance and cost thresholds, but a basic storage facility in New England might be fine with basic materials, however, a hospital in Minnesota needing consistent maintenance won’t.
It’s important to make those determinations, not just assume some lowest-common-denominator product whatever regular roofer makes available, they don’t care about energy impact, they do care about life safety standards; performing load analyses integrated with lifecycle cost projections citing intersections of energy-efficient skylights vs what makes the most sense for subcomponent performance based on expected use practical versus negligent reality.
Therefore, good buildings understand the value of all parts. Most building owners will never realize their roof hatch works perfectly, they will notice it’s either not accessible or it’s becoming a big hole that’s costing thousands energetically every year it’s left improperly secured/opened, but worth it every time it becomes an integrated design system instead of just another construction detail.
