Image Credit: All photos: Golcar Passivhaus The house as a structural ridge beam made of steel. The blue Solitex membrane is the exterior air barrier. Above the Solitex are battens and counter-battens. This detail drawing shows the different layers of the roof assembly. Short 2x4 rafter tails were scabbed onto the eaves. The scabbed-on rafters are on the exterior side of the building's thermal envelope. The building has two roof layers. Although skylights provide welcome natural light, they are weak links in the home's thermal envelope.
Editor’s note: The author, Bill Butcher, is a Certified Passive House Consultant who was project leader on the Denby Dale Passivhaus and is currently leading the Golcar Passivhaus project, a 2,700-square foot three-bedroom house in Golcar, West Yorkshire, England. Some of the technical and construction-related terms used in Bill’s essay have been translated from British English to American English.
At the Golcar Passivhaus, we’ve gone for an insulated roof assembly to create extra space in the attic area, as opposed to the uninsulated roof we created for the Denby Dale project. You get the advantage of having the extra space and room, but inevitably an insulated roof assembly is the more expensive option, as it is a more complicated build.
The roof detailing at the Golcar Passivhaus is in fact similar to that at our Stirley Farm EnerPHit project.
We have used steel structural ridge beams to carry the main load of building (see Image #2, below). These steel beams are inside the thermal envelope (so they do not create a “thermal bridge”).
I-joists for rafters
For rafters, we used a slightly smaller I-joists than we used at Stirley. In this case the I-joists are 302 mm (11.9 inches) deep. We have improved our I-joist detailing from the recent Stirley Farm project by placing 25 mm (1 inch) of rigid foam insulation in both sides of the web of the I-joist so that it forms a neat unit with flush sides (see Image #1 at the top of the page).
This approach has three main advantages:
- It reduces the thermal bridging through the I-joists.
- It ensures that our mineral wool or fiberglass infill in the rafter bays is consistent and flush as opposed to squashed in the I-joist.
- It reduces air movement around the insulation and thermal bypass within the web of the I-joist.
The roof needs to be windtight and vapor-open on the outside; on the inside, it needs to be vapor-closed and airtight.
In Stirley we used a wood-fiber vapor-open sarking board for the roof sheathing. In contrast, at Golcar we’ve used Solitex windtightness membrane (an air barrier membrane) because of the need for traditional detailing at the eaves and rakes (see Image #3, below).
To further reduce the thermal bridging of the I-joists we’ve used a continuous 50 mm (2-inch) rigid polyurethane insulation board on the interior of the roof assmbly, sandwiching the Intello membrane (a smart vapor retarder).
Build-up of the roof
The layers of the roof assembly are shown in Image #4, below. From the outside in, here are the layers:
- Natural slates
- Battens and counter-battens
- Solitex membrane, taped
- Full fill of 325 mm (12.8 inches) fiberglass batts in between the 302 mm (11.9 inch) I-joists, so that the batts compress, reducing the potential for sagging and minimizing thermal bypass problems
- 25 mm (1 inch) of rigid polyurethane insulation attached to the sides of the I-joist webs
- Intello smart vapor retarder as an air barrier (taped inside)
- 50 mm (2 inches) of rigid polyurethane insulation with laminated plasterboard on it
Note: We’re not actually using the Intello to its fullest potential here; in theory it could have been replaced by plastic sheeting. We are not going to be able to utilize its ‘intelligence’ and vapor-release properties because we’ve blocked it with the rigid polyurethane insulation. But we’ve used it here for buildability and robustness reasons and cost.
This is not going to create problems with moisture / condensation as we have a vapor-open membrane on the outside, so moisture won’t be trapped in the roof. As a rule of thumb, if the vapor control layer is within the first third of the insulation there is no danger of interstitial condensation.
Aesthetics of the roof/wall junction
The junction between the roof and the wall outside needed to fit in with the West Yorkshire vernacular. At Denby Dale we had a big overhang for shading which disguised the build-up due to the thickness of the insulation within the roof, which gives the building a different look than the normal West Yorkshire vernacular.
At Golcar we haven’t needed an overhang but we have had to create an extra “false” roof with smaller rafters (see Image #5, below) to cover the thickness of the walls; otherwise it would have created an aesthetic problem.
Two roof levels
We have had to ensure continuity of insulation and airtightness between the two roof heights. To achieve this we’ve had to build timber “framing” so that we have continuity of insulation, as well as airtightness and windtightness (see Image #6, below).
We’ve done a bit of cost/benefit analysis on the roof using PHPP, and we concluded that we did not need such a depth of insulation as we used in the Stirley Farm warm roof. This time, we’re going with 302 mm (11.9 inch) deep rafters as we didn’t need 406 mm (16 inch) I-joists. We didn’t need as good a U-factor as we needed at Stirley.
That is the beauty of using PHPP to plan your project; you can take your time and see what you can get away with. So we’ve got a lesser U-factor in the roof at the Golcar Passivhaus, but it still delivers the Passivhaus requirement of 15 kWh/m2*year.
We are using 3 Fakro rooflights (skylights) which are Passivhaus-certified and quadruple-glazed, with an excellent whole-window U-factor of 0.58 W/m2K (equivalent to U-0.10 in the U.S.). They also came with airtightness and windtightness skirts and insulated flashings, so they’ve really thought about how it would work within a Passivhaus / low energy building (see Image #7, below).
The problem with rooflights in a Passivhaus is that – due to the need for water and wind protection for the roof – you can’t wrap them within the insulation in the same way as windows and doors in a wall. You have to put the frame right into the cold of the outside of the roof. The thermal edge losses (Psi values) for rooflights are around 10 times worse than the wall / window installation detailing at the house.
Rooflights are always going to be a weak point thermally. But since the Golcar house is relatively large, we can get away (thanks to PHPP modelling) with using three rooflights.
Bill Butcher is the director of the Green Building Store in the U.K. Bill is a Certified Passive House Consultant who was project leader on the Denby Dale Passivhaus and is currently leading the Golcar Passivhaus project in West Yorkshire, England.