How the two differ in kind
Insulate a loft generously and the weakest line in the whole ceiling is the strip directly above the exterior wall. Three things compete for the same space there: the insulation, which wants its full depth; the ventilation path from the eave vents, which needs a clear channel; and the truss or rafter itself, whose depth at the bearing is whatever the geometry gives.
With a STANDARD truss heel, the top chord meets the bottom chord almost at the wall plate, so the available depth over the plate is small. The insulation is squeezed down to a fraction of what it is elsewhere, and the vent path either takes what is left or is blocked entirely. The consequence is a continuous cold line around the building at exactly the point where the wall's own junction is already a thermal bridge — which shows up as ice dams at the eave, condensation and mould on the ceiling at the wall junction, and a measured performance well below the insulation's nominal value.
A RAISED-HEEL truss lifts the top chord at the bearing, creating a taller heel so there is room for the full insulation depth over the wall plate AND a clear ventilation channel above it. At manufacture it costs very little — a slightly taller truss and a corresponding change to the fascia and the cladding at the eave — and it resolves the problem geometrically rather than by compromise. It cannot be retrofitted.
BAFFLES are the other half and are required in any vented roof regardless of heel height. They hold an open air channel from the eave vent up over the insulation, which loose-fill would otherwise drift into and batts would otherwise fill, and they stop wind-washing — outdoor air blowing into and through the edge of the insulation, which degrades it. What a baffle cannot do is create depth: on a standard heel it preserves the vent path by taking space the insulation needed.
The factors that actually differ
Show
| Raised-heel truss | Baffles and compressed insulation | |
|---|---|---|
| What it provides | Depth over the wall plate for full insulation AND a vent path. | A clear air channel, whatever depth exists. |
| When it can be decided | At design and manufacture. It cannot be added later. | At any time, including as a retrofit. |
| Cost | Very little at manufacture — a taller truss, and eave details to suit. | Low per baffle, and required regardless. |
| What it solves | The compressed-insulation cold line over the wall, and the vent path, together. | The vent path only — and wind-washing at the insulation edge. |
| Effect on ice dams | Substantial, since the eave cold line is where the heat escapes that drives them. | Helpful, by maintaining the ventilation that keeps the deck cold. |
| Effect on the eave appearance | Changes it — a taller heel means a deeper fascia and a different soffit line, which is an architectural decision. | None. |
| Is it required | No, but it is the only way to get full depth at the plate in a vented roof. | Yes, in any vented roof with insulation near the eaves. |
| Wind-washing | Reduced, since the insulation is deeper and better protected at its edge. | This is one of its jobs — a baffle shields the insulation edge from incoming air. |
| Retrofit options | None short of re-roofing with a new structure. | Straightforward, and usually combined with air sealing the ceiling beneath. |
| Used together | Yes. A raised heel still needs baffles to keep the channel open. | Yes, and on a raised heel they have room to work properly. |
Which one, and when
Choose raised-heel truss when…
- New construction with a vented roof, where the cost at manufacture is small and the benefit permanent.
- A high insulation target, where compressing the insulation at the plate would undermine the whole specification.
- A cold climate with ice dams, where the eave cold line is the mechanism.
- Anywhere the wall-to-roof junction's thermal performance is being calculated rather than assumed.
Choose baffles and compressed insulation when…
- Any vented roof — baffles are required regardless of heel height.
- A retrofit, where the truss geometry is fixed and cannot be changed.
- Loose-fill insulation being added, which will drift into the eave without them.
- Wherever the eave vents are being obstructed by insulation, which is the commonest attic ventilation fault.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why is the eave the coldest part of the ceiling?
- Because three requirements compete for a space that is running out. The insulation wants its full depth; the ventilation path from the eave vents needs a clear channel above it; and the structure's depth at the bearing is whatever the truss or rafter geometry provides, which with a standard heel is very little. The insulation is therefore squeezed to a fraction of its depth over the wall plate, at exactly the point where the wall-to-roof junction is already a thermal bridge — so the two weaknesses coincide in a continuous line around the building. The visible consequences are familiar: ice dams forming at the eave in a cold climate, condensation and mould appearing on the ceiling at the wall junction, and a roof that measures well below the insulation value it was specified to.
- What exactly is a raised heel?
- A truss detailed so that the top chord is lifted above the bottom chord at the BEARING point, creating a taller heel where the truss sits on the wall. That extra height is depth available above the wall plate — enough for the insulation to run at its full thickness right out over the plate, with a clear ventilation channel above it, which a conventional heel cannot provide. The heel height needed follows from the insulation depth plus the vent channel, so it is calculated from the specification rather than chosen from a catalogue. Terminology varies — energy heel, raised heel, oversized heel — and the requirement is the same. It is specified at the time the trusses are ordered, which is why it has to be identified during design rather than discovered on site.
- What do baffles actually do?
- Two things, and the second is less well known. The obvious one is maintaining a clear air channel from the eave vent up over the insulation to the roof space, which loose-fill insulation would otherwise drift into and batts would otherwise fill — a blocked eave vent is the commonest ventilation fault in a loft and it defeats a ventilation strategy entirely. The second is preventing WIND-WASHING: outdoor air entering at the eave blows into and through the exposed edge of the insulation, moving air within it and carrying heat away, which degrades its effective performance in a band near the eave. A baffle forms a barrier between the incoming air and the insulation edge, so the air passes over rather than through. Both jobs are needed regardless of heel height.
- Can a raised heel be added to an existing roof?
- Not in any practical sense, because it is the truss's geometry at the bearing rather than something attached to it. Changing it means changing the roof structure, which is a re-roof with new trusses rather than a retrofit. What CAN be done on an existing roof with a standard heel is to make the best of the depth that exists: baffles to keep the vent path clear, air sealing the ceiling plane beneath — which matters more at the eave than anywhere else, because that is where warm air leaking upward meets a cold surface — and using a higher-performance insulation in the constrained band so the reduced depth achieves more. Spray foam or rigid board at the eave under the baffle is a common approach. None of it fully substitutes for depth.
- How much heel height is needed?
- Enough for the full insulation depth over the wall plate plus the ventilation channel above it, which makes it a calculation from the specification rather than a standard dimension. The insulation depth comes from the thermal target; the channel comes from what the vent path requires, which is typically a modest but non-negotiable clearance beneath the sheathing; and the total is the heel height, measured at the bearing. Because insulation depths have risen substantially as thermal standards have tightened, the heel heights now required are considerably greater than traditional construction provided — which is why raised heels have gone from unusual to routine in cold-climate construction. The other input is the fascia and eave detail, since a taller heel shows on the elevation and has to be designed rather than discovered.
- How does this relate to ice dams?
- Directly, because the eave cold line is where the heat that drives them escapes. An ice dam forms when heat from the building warms the roof deck enough to melt snow, and the meltwater refreezes over the cold eave beyond the heated envelope. The heat gets there two ways: conduction through the ceiling, which is worst where the insulation is thinnest — the compressed strip over the wall plate — and air leakage through the ceiling plane, which is also concentrated at the wall junction where the top plate meets the ceiling. A raised heel addresses the first by restoring the depth, and air sealing addresses the second. Together with adequate ventilation, they are the cure; the ice-and-water membrane at the eave is the insurance against the events they do not prevent.
- Do unvented roofs change this?
- They change it substantially, because the competition for space at the eave largely disappears. An unvented assembly insulates at the roof line rather than at the ceiling, with sufficient insulation in contact with the deck to keep it warm enough to avoid condensation, and there is no ventilation channel to accommodate — so the eave detail is about continuity of insulation around the wall-to-roof junction rather than about fitting three things into one space. It brings its own requirements: the proportion of insulation that must be above or against the deck varies by climate and by code, air sealing is critical, and the assembly has to be designed as a system. Where it is used, the heel height question becomes an insulation-continuity question instead, which is a different problem with the same underlying aim.
- What should be done first on an existing loft?
- Air seal the ceiling plane, before adding any insulation — and pay particular attention to the perimeter, because the junction where the partition top plates and the exterior wall plate meet the ceiling is both a major leakage path and the coldest line in the roof. Then install baffles at every rafter bay at the eaves, so the vent path stays clear once insulation is added and so the insulation edge is shielded from incoming air. Then insulate, running the insulation out as far over the plate as the depth allows. Doing it in the other order buries the leaks under the insulation, makes them unreachable, and lets warm moist air pass straight through the new insulation into the roof space — which is the mechanism behind frost on the underside of the sheathing.