Reciprocal Roof insulation in cold climate

Hi there! I am really interested with the reciprocal roof structure with roundwood timber framing and I am wondering how to insulate it efficiently and solid enough to be able to stand on it. In Tony Wrench's book, he builds a green roof using straw bales directly on a canvas over the structure with a rubber membrane on the bales, old carpets to protect the membrane and turf on top.

I think it's awesome and very inspiring! Cheap, good enough and fast!
But in my opinion it's not enough insulation, may be interesting for the mouses or squirrels to take shelter there in the winter (maybe I don't know).
I live in Québec, Canada and the building code ask for R-40 minimum of insulation on the roof and we have heavy rain and snow loads to consider.
I am really motivated to experiment with natural material and try to change the way people build their house around me.

The problem with that type of roof and the roundwood is that it creates odd triangles and bumps and curved because of the uneven nature of the roundwood. It is not easy to work with conventional building techniques around a roof like that.

I show you here a wall and roof system that is used in our region with timber frame and hemp wool with wood fiber pannels to avoid thermal bridge. It's expensive but it's efficient and a pain in the ass to work on a round roof (my guess). I have also access to hempcrete, strawbales, large quantities of small diameter poles, surely cob but I didn't look for clay around yet.

Please help me.

Hello Gabriel;
https://www.greenbuildingadvisor.com/  might be useful.
I am not an expert, but I live in a tough climate and have seen many problems.
I think achieving a vapour barrier on the underside of that roof will be difficult;  possibly closed cell spray foam insulation on the lower surface would do it.
It would ruin/change the look of the underside.
A friend recently tried hempcrete in coastal Nova Scotia and it got mouldy;  you might be lower humidity.
If this is aimed for part time occupancy I would be tempted to avoid trying to air seal it; heat the space when occupied; let the air move through.
Many older houses have been ruined by partial vapour barriers trapping moisture between layers.
If you put a rubber membrane in the midst of insulating layers, you will get condensation on the membrane wet warm side (probably the inside, most of the time, in Quebec), unless there is enough insulation on the cold side to stop condensation on the wet warm side.
Also, depending upon your location in Quebec, we will see more temperature and humidity fluctuations.
good luck Doug

As Douglas said I'd be concerned with condensation if the waterproof barrier is on the outside of the roof - and if on the inside, you'd need some sort of water shedding on the outside - maybe shingles would work though?

Do you need to stand on the roof? What's the maximum slope you can have with a reciprocal roof? I've been looking at thatch and it seems a great material (assuming you can a) get hold of it and b) have the labour to construct it.

1919 mushroom house of Charlevoix, Michigan -  thatched in 2013

As a rule of thumb 10” of thatch has the same R-value as 7” of fiberglass batt, or an R-value of R26.6

(From Hendricks Architects on thatch )
So you would need about 15 inches of thatch to get an R value of 40. Since the normal thatch thickness is about 18 inches it seems that you wouldn't need any extra insulation. To shed the snow and heavy rain you'd need a slope of 45 degrees though.

trying to insulate that safely, effectively and without rot in Quebec would be a nightmare

Gabriel -

Does your structure need to be code compliant?  I don't know how the rules work in Quebec, whether there is any wiggle room for alternative designs and experimentation, or whether the law is the law.

I assume that you are trying to preserve the interior aesthetic of the reciprocal frame, while meeting the code requirement for a highly insulated roof, and that you aren't entertaining constructing a steeper roof structure above the reciprocal ceiling beams.

Even so, Nancy's suggestion of thatching the roof seems a good one - high R value, natural and renewable materials, long lived, and adaptable to not-so-square roof layouts.

The Rural Development Commission in the UK published  "The Thatcher's craft" a long time ago, but it's still a good reference for three styles of thatching - long straw, combed wheat straw, and water reed.  It is available for free download from various sources (e.g. http://thatch.net/thatchers/thatcherscraft.htm ).  However, if you can find a hard copy, the instructional value of the plates is much better than the digital scans, due to the detail which is visible in the printed plates.  None of the scans I've seen (including the ones available from the UK gov't agency which was the successor to the RDC) has the same quality as the print version.  I just acquired a hard copy from a used book store last summer, and had no idea what I had been missing.

Hermann Phleps also has some details on other styles of thatching in his classic book on log construction, "Der Blockbau".  I have the English language translation by Roger MacGregor which was published by Lee Valley in the early 1980s as "The Craft of Log Building".  Among the thatch variations Phleps presents are: tamped thatching, sheaf-thatched roofing, loose laid thatch, and thatch-tiled roofing.  Before seeing Phleps's book, I was unacquainted with several of these styles, even though they apparently had ample, if geographically confined, historical precedent.

Phleps also presents details for flatter-pitched turf roofs of the Scandinavian or Alpine European styles in this same book.  These seem more like what Wrench is advocating in the reference you provided.

I should do a proper review here on the Permies forums of Phleps's book, but I haven't gotten there yet.  It is a treasure trove of information on traditional log and timber construction, written by a man who was born in rural Transylvania 1877 and began teaching architecture in 1907.  He documents traditional procedures and methods, ranging from Scandinavia to the Carpathian Mountains.  He gives many examples of structures which, as of the time of the original publication in the early 1940s, had been standing for several hundred years - from stave churches to hay barns.

I hope this is some help - at least some ideas to think about.

Tony Wrench built his roundhouse up the road from where I live. The climate here is pretty mild, although it does get cold (for us) in the winters. I wouldn't expect to see negative numbers (-1C and below) for many days each winter though, and rarely below -6.

Straw bales are surprisingly good insulation. This thread here on Permies suggests that a single bale is around R28-R35, depending on its size and orientation. You could very easily double up on the straw bales, pegging them together with sharpened wooden stakes if you like, to achieve an R-value of 50 or more.

To add to this, the turf ("sod") layer will provide additional insulation. I don't have any figures to hand for that but it will be significant.

In terms of waterproofing, the EPDM/pond liner will want to be placed between the turf and the straw. Hopefully the reasons for this are obvious but the straw will fail very quickly if it is allowed to get wet.

The condensation issue might not be as severe as some imagine. The roof design would be described as a "warm roof", meaning that the insulation is on the inside and the cold, water impermeable layer beyond it. This means that there isn't a cold surface for the water vapour to condense on. Placing a vapour barrier between the straw and the rafters would remove any risk of water ingress but I would worry about the straw being unable to breath.

A final consideration is that Tony didn't have to consider large snow loads for his design. This will, of course, only add to the roof insulation but you should consider using larger timbers that he does to cope with the increased weight.