Annette Jebb

I think the easiest way to make your idea work is to design each room as a separate structural module, but have the rooms share a wall.

For the second room, I would not try to make its centre tree-trunk beam depend on the first room's beam. Instead, both ridge beams could meet at the shared wall, with that wall designed from the beginning to support the beam ends.

That support could be a properly designed timber post, reinforced pier or another structural detail incorporated into the wall. The important part is that the roof load has a clear path down to the foundation rather than relying on cob alone around a beam pocket.

If you know where the next room will eventually go, I would also plan the doorway in that shared wall from the beginning. That way you are not later cutting a large opening into a wall that may be supporting part of the roof.

This could reduce the second room to three new walls instead of four, while still keeping the dividing wall between the rooms. I think keeping those cross walls could also be useful because a very long chain of rooms will need good lateral stability.

The oval shape may make this more difficult. One compromise might be to keep the outside ends and corners rounded, but use straight walls where one room joins the next. You would still get much of the character of a cob building without making each future addition complicated.

For the roofs, I would keep the same ridge height, pitch and orientation for each room. When another room is added, its roof can simply continue the same general roof line rather than creating valleys or complicated intersections.

I would also make the roof overhang on the future-addition end of each room easy to remove. While the room is standing alone it protects the wall, but when the next room is built that section can be altered and the new roofing joined properly.

Your plan for a long southern wall also makes sense for passive solar heating, especially with your strong winter sun. I would still avoid making the overhang unnecessarily deep on that side. A combination of correctly sized overhangs and adjustable window shading may give you more control. This guide on plantation shutters vs roller blinds (https://iseekblinds.com.au/learn/plantation-shutters-vs-roller-blinds) explains some of the differences in controlling sunlight, privacy and heat, which may give you some ideas even though your building style is quite different.

I would also be careful about choosing the room width only according to the length of the available trees. Timber species, diameter, taper, knots, roof weight and actual span all affect how much a beam can safely carry.

If it were my project, I would spend some money at the beginning getting one repeating structural detail checked locally: foundation, wall, bond beam, ridge-beam support and roof connection.

Once that detail is worked out, you could repeat essentially the same small room each dry season.

That seems to fit your original plan quite well: one manageable room at a time, simple roofs, locally available timber, shared walls between rooms and no complicated roof junctions as the house grows.

For this particular situation, I would lean towards a properly fitted blackout curtain and pelmet system, mainly because your windows tilt inwards and because near-total darkness appears to be the first priority.

Solid internal shutters can provide excellent thermal performance when they fit closely against the frame. Traditional shutters worked particularly well because the shutters, folding pockets and window reveals were designed as one system. Retrofitting them to modern tilt-in windows is more difficult. The panels need somewhere to fold, they may interfere with handles or the inward swing of the window, and you would need to open the shutters before opening the window.

A curtain system would be more forgiving and easier to alter later. However, the details matter more than the thickness of the fabric alone. I would use:

a wall-mounted box pelmet that closes the gap above the curtain;
a track wider than the window opening;
curtains that return towards the wall at both ends;
floor-length or sill-sealing curtains;
generous overlap where two curtain panels meet; and
a separate blackout lining or tightly woven blockout fabric

.

The aim is to create a reasonably still pocket of air between the glass and the curtain. A pelmet helps prevent warm room air from dropping behind the curtain, cooling against the glass and returning to the room. Government passive-design guidance similarly recommends snug curtains, sealed edges and pelmets rather than relying on heavy fabric alone.

For severe light pollution, the weak points will probably be the perimeter rather than the curtain fabric. Side returns, overlapping centre panels and a close fit at the bottom may be enough. Small magnetic tabs, removable side channels or hook-and-loop strips could also control the remaining edge leakage without creating a permanently sealed installation.

Solid shutters may still outperform curtains thermally when they are well fitted. Historic Environment Scotland recommends using existing timber shutters at night and identifies draught-proofing as the first priority for reducing heat loss through traditional windows. However, shutters with adjustable louvres are different from the traditional solid-panel shutters in your museum photos. They provide excellent privacy and adjustable light control, but small gaps around the panels and louvres mean they should not be treated as true blackout coverings. This guide to PVC plantation shutters https://iseekblinds.com.au/blog/complete-guide-to-pvc-plantation-shutters explains that distinction and the normal sources of light gaps. For transparency, I am associated with iSeekBlinds, which published that guide.

Condensation is the point where I would be cautious with either option. Once a curtain or shutter insulates the room from the window, less indoor heat reaches the glass. The glass can therefore become colder, which may increase condensation behind the covering if indoor humidity is high. Neither system removes moisture from the room. Keep trickle vents and required ventilation paths open, open the curtains or shutters during the day, and check the glass and frame regularly during the first winter.

Before committing to joinery, I would test one room with a temporary system. Hang an inexpensive blackout curtain wider than the opening, add a temporary cardboard or plywood pelmet, and block the side gaps temporarily. That should reveal how much darkness and comfort you can achieve. You could also compare the window and room-side temperatures with an infrared thermometer on similar cold nights.

My overall comparison would be:

Solid shutters: potentially better insulation, durable, easy to wipe clean and architecturally attractive, but more expensive and complicated around inward-opening windows.

Blackout curtains with a sealed pelmet: easier to make, easier to modify, likely better for near-total darkness and more compatible with seasonal ventilation, but only thermally effective when the top, sides and bottom fit closely.

For your windows, I would try the curtain and pelmet system first. If it does not provide enough thermal improvement, removable internal secondary glazing behind the curtains may deliver a greater benefit than replacing the curtains with retrofit shutters.