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Good building design and insulation choices for A-frame and cathedral style ceilings.  RSS feed

 
Philip Nafziger
Posts: 65
Location: Columbia, Ky
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Hopefully my subject line is fairly self explanatory. I have a 16x26 cinder block cottage, currently in build phase, that I would like to put an A-frame style roof on with two dormers and maybe a small loft depending on how steep I want to go with the pitch.

I have seen first hand the amount of condensation that can happen in these types of systems when the cool air from outside meets the warm air inside and most definitely will avoid that at all costs. Conventionally the easy fix is bubble wrap, felt etc, but if I can avoid that as well I will want to. Of course the majority of heat is lost through the roof so I want as much insulation as possible.

I have thought about many of these aspects extensively, but I can't seem to come up with a good solution.

Can I avoid using underlayment beneath my metal roofing with a type of rain screen? Has anyone done this and if so, how?

I can structurally use 2x6's to frame my ceiling but something with a little more depth may be better like a 2x10, so that more insulation can be packed in, I am guessing.

I live in timber rich south central Kentucky and can get my hands on unlimited amounts of sawdust. How does one go about dense packing sawdust in a ceiling?

Thanks, Phil

 
Luke Eising
Posts: 31
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I am mostly done building my A-frame house with a cathedral ceiling.

You're right in wanting to use rafters to create an a-frame effect inside and out (instead of those fake scissor truss cathedrals)

The problem is the rafters are too small to get enough insulation inbetween. We don't want to build a false roof or false ceiling, but even with 2x12 and sprayfoam (10.5" x r6) you can barely get to the r60 I need in my state, and that's outrageously expensive and heavy. Then you still need occasional horizontal ties to keep your roof from pushing your walls apart

I hit upon the perfect solution for me...It should work for you too. You need a single purlin at the top of the roof from one end of the house to the other (great place for a decorative log).

-You then rest your rafters on that and on the wall. Voila: no more outward push, roof load is pushed directly down on the walls.
-Instead of dimensional lumber you buy/build parallel cord trusses, to whatever width you need (mine are about 16"). You have built a vaulted parallel cord truss (google the pic).

OSB, wrap, steel roof on the outside. 2x2 stringers and 3/4 rigid insulation on the inside (creating a channel for soffit to ridge venting), plank/drywall the inside and blow it full of nice cheap, eco-friendly celullose insulation.
 
Terry Ruth
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Philip,

Framing - Scissor trusses are by far much stronger and can span much longer distances than stick building rafters. You can see this in any clear span build using metal or wood. Last one I worked in clear spanned steel 400'. Long story as to why I will not get into. The height for some mfgs can be limited by assemble jig tools and drive one into a piggy back truss or lower chord pitches and most will PE stamp the designs as part of the cost. If stick framing you follow IRC 2012 "Ceiling and Roofs" span tables, etc, or whatever your jurisdiction requires, or, you have cost of a PE which can be expensive.....If you do so by the time you do it right and are done adding the perlins and collar ties you have a truss or close to it and probably need an internal load bearing wall if spanning over 20' tributary, 40' max total span between outer walls. What is more cost effective depends on local cost of materials and labor and if you are DIY. Continuous sheathing following the mfg span rating in most cases (simple 1-3 stories) is all you need for the walls to take out shear, racking, bending, see IRC 2012 "Walls"....same for roof.

Insulation - most loose fill will settle and leave insulation gaps in time(not good) requires ~ 3-5 PCF. Not sure how you verify that to get the rated r-value. 7 1/4" thk mineral wool batt can sit in a 5 1/2 lower chord or whatever is needed structurally since it is rigid, will not settle. That is at the upper ceiling plane @ a nominal r-value of 30. Add R-6 @ 1.5" thk from Roxul mineral wool rigid board on the upper chord as a thermal break. Whole r-value = 36. If you need more stack and stagger. It is best if the board is continuous to the walls and foundation as outsulation, as well as having all penetrations air sealed.

Ventilation - On top of the rigid board that comes in 16 or 24" OC add furring strips to hold it down taking care not to damage it as in using a man lift vs walking the roof to install(it can take 750 PSI compression which is high, or, tell the installers to use the wood to walk on since it can take 25% deflection also high but too much for live load), and a ventilation gap to soffits and ridge vent. On top of them sheathing (planks (solid sawn real wood 1/4 gaped butts or T&G panels(unsealed or do not taped), best) or OSB (no taped ZIP panels) if you must and is all you can afford but it is junk...plywood is better but expensive). Now add a water and ice barrier to lower 4' to prevent ice dams (read about them there are other ways but it gets complicated) and use a light colored metal roof to reflect radiant short wave solar heat. That is a vapor breathable envelope since all materials are inert and high perm. The water and ice and sheathing barriers sit outside the air streams, toxins if any will release to it. Warning metal roof corrosion protection will eventually wear off and some only allow certain W&I for warranty.

Here is a 24" OC scissor/stick combi design with reduced end gables and look outs, hip garages, I am working on for a client, trusses clear spanning 50'. Had I sticked framed this I'd need a tributary interior concrete footing and continuous wall which cost more and restricts my design, and my framers be at the job site another 2-4 weeks(perhaps in snow and rain delays) that cost me $$$. I'm 8/12 upper chord 4/12 lower still working on increasing the lower pitch. Cantilevered trusses with a energy heal so my soffit and 24" overhangs work is easy . Once I get cost on it I'll get a Timber bolt cost comparison the best structure out there since it is continuous load bearing.


Lots of edits since I have after thoughts can't remember it all at once and cannot spell or type worth a beans. Good luck to you bro, hope that helps.

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Terry Ruth
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BTW: Here is my conceptual truss detail clear spanning 50+'. The truss designers have FEM (Finite element model) software that designs the spans and lets me know if I need of I am deflecting too much. They do not offer the software on the free market I know of but there are ways to do the math I can do. If you deflect too much things can crack.

Once they give me the design I update my model so I can see what is going on and how it affects my architecture...My walls are ten feet high so I can get that open feeling, if they lower my lower chord pitch I may want to make the walls higher.

Dormers in conjunction with a cooling tower would be a great way to expel stack effect heat at the ceiling. Seasonal cooling towers come in remotely controlled louvers high r-values if you can afford them.
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Terry Ruth
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Here is an example of dormers working in conjunction with a cooling tower for indoor ventialtion. That is a CAD model sample from the pro software I use, I can model natural homes that look like that so you can see the design before you build. Another great choice would be clearstory with remotely or manually controlled windows. I also design with natural self tinting windows where needed to reduce heat and allow it in when needed. Another great option is a outer solar reflector deck, free solar lighting energy and heat when needed.

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