The building envelope of our modular Passive House

One of the biggest challenges in all Passive Houses is meeting the air sealing requirement of .6 ACH @ 50Pa. It is particularly challenging on this project for two reasons:

  • First, the relatively small size of the house means there is more surface area relative to enclosed volume than on a larger house. Since air changes per hour calculations are based on air infiltration in relation to volume, the same size leak in a small house is a “bigger” leak than in a larger house.
  • Second, the house is going to be shipped down the highway and there is the chance of leaks opening up during shipment. The design therefore has to allow for blower door testing and access to every inch of the air seal layer after transport.

In our previous Passive House projects we have set the air seal layer at the interior face of the exterior wall (we were using SIPs, so that meant the interior face of the SIP panel).  After installation of windows and doors and taping of all joints we blower door tested the house and it was quite easy to tape up any leaks. Only after they had been addressed did we apply finishes over the air seal layer.

Because modular houses arrive at the site completely finished on the interior – paint, trim and all— we can neither locate nor easily patch leaks from the interior. So we turned the process around and set the air seal layer of this house at the exterior layer of the sheathing for the walls. The house will come to the site fully finished on the interior but naked on the exterior. Only after addressing all leaks will we apply exterior insulation and siding.

The exterior wall assembly, from inside to outside, is as follows:

  • ½” gypsum wallboard
  • Double stud framing (2 layers of 2×4’s for total framing thicknessof 9-1/4”
  • Densepack fiberglass insulation in stud cavity
  • ½” Huber ZipWall sheathing with Huber tapes at joints
  • 1” glass-faced polyisocyanurate foam board
  • Tyvek “StuccoWrap
  • Hardie beveled siding

Total R value: 41

We used fiberglass insulation rather than cellulose because it is lighter in weight, a big factor in shipping the house to the site. We are using the polyiso boards outside the sheathing to provide a continuous thermal break outside of all framing members. The glass facing keeps the foam relatively vapor permeable, allowing the wall to dry in both directions if water gets in. For budget reasons we substituted StuccoWrap, a kind of Tyvek with a rumpled surface that allows water drainage and air movement, for a layer of furring strips to create a drainage layer behind the Hardie siding.

We considered building the house over a conditioned crawl space, but the costs of insulating the crawlspace walls and floor to meet PH requirements were prohibitive. So we are building the house atop a sealed crawlspace, built in the standard manner, and then going to great lengths to insulate and air seal the floor framing system.
Beracah’s construction system does not allow access to the underside of the floor framing (they are built on the ground from bottom up), so the floor system cannot be insulated in the factory nor sealed from below. We therefore made the subfloor of the first floor our air seal layer. All joints were taped in the factory before any interior wall framing went up. The house will come to the site with no finish flooring installed on the first floor, so that we can air test and patch leaks at the site.

The floor/crawlspace assembly, from inside to outside, is as follows:

  • Vinyl plank finish flooring
  • ¾” Advantek subfloor
  • 2×12 floor framing filled with densepack fiberglass insulation
  • 1-1/2” polyisocyanurate foam insulation applied to the bottom face of joists, fully taped and sealed
  • 3’ of crawlspace area
  • 20 mil Stego Wrap vapor barrier, taped and floated, laid over gravel substrate and taped to dampproofed foundation walls

Total R value: 51

After the crawlspace is complete, and before the house is set in place, the polyiso boards will be stored in the crawlspace, since the only access to that space will be a sealed hatchway from the mechanical room. Once the house is set, the plumber and electrician will make all their connections through the floor and seal them. After blower door testing, the insulation subcontractor will then densepack the joists and apply the polyiso boards, and Beracah’s field crew will install the first floor finish floor.

The crawlspace is essentially sealed. By code we are required to have a small amount of openings in the crawlspace walls or ventilate the cavity from the house. Obviously the latter option is out. So we will provide the required amount of openings with operable louvers. We will then keep them closed and monitor humidity levels, with a small dehumidifier placed in the crawlspace to handle any humidity build-up.

Because the roof will be tilted up into place at the site, having a finished attic with the air seal layer on roof surface would require a lot of onsite finishing work, and significantly add to costs. We therefore set the air seal layer at the top of the attic floor decking. This was taped at the factory before any of the roof framing was applied. It can now be air tested in the field and accessed before attic insulation goes in.

The roof assembly, from inside to outside, is as follows:

  • ½” gypsum wallboard
  • 2×12 second floor ceiling framing filled with densepack fiberglass insulation
  • ¾” Advantek decking, with all joints taped
  • loose-fill fiberglass insulation filling the eave spaces
  • 6-1/2” Nailbase panels (open-faced SIPs) laid over deck in central attic storage area

Total average R value = 105

It would have been much easier to leave the attic completely inaccessible and fill it with loose-fill insulation, but since the house has no basement our client wanted to provide attic storage space. We are using a sealed attic ladder from Conservation Technologies. The Nailbase panels will provide the walking surface for the central storage space.

The R values of all parts of the assembly are higher than in our previous projects, particularly attic and first floor assemblies. This is partly due to the poorer surface/volume ratio of a small vs. large house mentioned above. It is also due to the fact that we are building over a crawl space and not able to take advantage of the earth for summer cooling, as does a house with a basement. We played a lot with the PHPP (Passive House Planning Package), the PH energy modeling software, in order to find where to place most cost-effectively all the insulation we need. The arrangement described proved the most efficient.


Windows are Zola Window Company‘s UPVC triple-glazed tilt/turn units with insulated sash and frames. We are using their high solar gain units (.62 SHGC/5.7 unit R-value)  at south facing windows and their low solar gain units (.25 SHGC/5.7 R-value) at all other windows. The outside face of the frames is placed in the plane of the exterior sheathing, simplifying sealing around the perimeter with the Profil Tescon tapes. Gaps between windows and rough openings are additionally insulated and sealed with Great Stuff foam.

Now some pictures…


The  open web joists laid out prior to installation of second floor decking



Zehnder Comfotubes snaking through the joists


The exterior wall framing prior to insulation, revealing the double 2×4 studs with 2×10 top plate



Densepack insulation installed



2.4 pounds per cubic foot, as required (comparable to 3.5 #/cf of cellulose)



door framing after insulation, showing the thermal break between the two layers of framing



Insulation being blown in at right, finish exterior Zipwall sheathing at left

Insulation being blown into the second floor ceiling rafter assembly.

IMG_1743Taping of the bottom of the sheathing to the framing. Another strip of sheathing will be applied in the field and fastened to the framing and the sill plate, locking the structure to its foundation.

IMG_1734IMG_1736Attic stair hatchway now insulated, taped, and ready for installation of attic stair. Stair could not be installed in factory because its height above the deck would make it too tall to ship down the road.

IMG_1744The insulated basement hatch. Grey line around the edge is the gasket. No access will be necessary after completion of construction, but toggle screws into the mechanical room floor framingmake access possible if ever necessary.


Penetration through building envelope sealed with Profil’s Roflex gasketing system

IMG_1748They look lonely and not very pretty yet, but both modules of the house are now complete and waiting for shipment to the site on August 11.