Remote, and ceiling insulation

Where the devil is...

Remote, and ceiling insulation

Postby PaulL » Wed Aug 04, 2010 10:27 pm

We're thinking about building in Wanaka, and I've been looking at various building styles. I like the look of the Remote style: http://sustainable.cchrc.org/docs/REMOTE_Manual.pdf

In summary, you leave the stud bays empty, run plywood sheathing over the house, then a full vapour/air barrier, then polystyrene insulation on the outside. The viewpoint is that it's hard to make a house airtight when sealing the inside - too many penetrations through the wall for plumbing, power etc. But if you do the sealing outside, you don't have this issue- there are few penetrations. But you want your air barrier inside the insulation, so as to prevent condensation, so the insulation needs to go outside that. Then the cladding. The other benefit is that using polystyrene avoids many thermal bridges and gaps in the insulation - particularly the gaps you get with batts that sag in the stud bays. And of course all your walls are now empty, so easy to run wiring etc.

The remote manual suggests still using normal insulation in the ceiling, but I'm thinking why not go the whole hog and do the roof the same way. Putting the insulation on the outside means I can go in my ceiling cavity without dying from glass fibres, and makes that whole space part of the conditioned envelope - so usable for storage etc.

Anyway, on to the question. If I build the roof with normal trusses (but with no overhangs, to allow easy sealing). I then cover with plywood, and seal all that to be airtight. I now need a layer of polystyrene, and additional rafters so as to create eaves and hold the roofing. There are a few ways to do this:
1. 8 inches of polystyrene (about R5 I think), then attach rafters to the polystyrene (screwing through into trusses with long bolts). This feels funny - the polystyrene is now supporting my roof, and the bolts are surely a thermal bridge.

2. Use 4x2 rafters on top of the plywood, with 4 inches of polystyrene in between. Then 4x2 battens, again with 4 inches of polystyrene. Then tin on top. This feels better to me, as I get less thermal bridging. But I do still get some bridging from the battens and rafters. Most of it is covered by at least one 4 inch layer of polystyrene (R2.5) plus 4 inches of timber (R1). But where the rafters and battens cross, I get a 45mmx45mm square that is 8 inches of timber - so R2. I'm trying to work out what my resulting insulation value is, and whether this is good, bad or indifferent.

My gut feel is to go with option 2. Using tight fitting polystyrene will give me better installed insulation than most people have, and although in theory I have some thermal bridges, at least they're timber not metal, and they aren't too large. I could spend lots of time worrying about it, but in reality it will be fine. Does anyone have thoughts on this?
PaulL
 
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Re: Remote, and ceiling insulation

Postby PaulL » Wed Aug 04, 2010 10:44 pm

Actually, I decided to do the maths. Ugly, but possible. Interested in comments on my logic here.

1. Assume 600mm centres, and treating them as continuous (ignoring the edges of the roof for simplicity). So I consider a square consisting of half a 4x2 on each edge, 600mm wide.

2. The central bit is two layers of polystyrene, so I have 555x555 = 308,025 sq mm of double layered polystyrene, and R5

3. The edges are a single layer of polystyrene + one layer of 4x2, there are 4 of them, but I only count half of each edge (they all border another equivalent square). So I have 45x555x4x0.5 = 49,950 sq mm of R3.5

4. The corners are a double layer of 4x2, there are 4 of them, but I count only 1/4 of each as they all border 4 other squares. So I have 45x45x4x0.25 = 2,025 sq mm of R2

5. So I'm 85.56% R5, 13.88% R3.5, and 0.56% R2. Using the formula from the FAQ, I end up with R4.68

6. I probably get a bit more from the plywood and other bits and bobs, I'll ignore that for simplicity - best case it's probably only R0.1 anyway.

So, my questions.
a. Does this maths look vaguely right? The answer feels about right.

b. Given that this is a real world, full roof insulation value, is that enough? I think a lot of people are theoretically installing R4 or R5, but my gut feel is that many of them aren't actually getting that - poor installation, air drafts, thermal bridges etc are reducing it significantly. This building style should avoid all that - so I should actually get the R4.68, and I should get a very air tight house

I guess I'm looking here for someone to say "yep, I did something like that, and the house was warmer than you could imagine". :-)
PaulL
 
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Re: Remote, and ceiling insulation

Postby pgarch » Wed Aug 04, 2010 11:54 pm

It would pay to run the system past the council. I am not sure on the maths but I would be cautious of using EPS. XPS is a better product with better insulation per mm thickness and will not take on water like EPS. I am not sure your proposed method would be that much better than say 140 or 190 wall framing which would simplify the detailing.

I am currently building a Thermomass (insulated concrete) house in Wanaka. My thoughts (and this is more a gut feel) is that a high mass house has more advantages than theoretical calculations give it. Yesterday just after the sun went down as I walked past a wall I was surprised at the heat that was radiating off it.

Regards Philip
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Re: Remote, and ceiling insulation

Postby PaulL » Thu Aug 05, 2010 10:02 am

One of the key features of the REMOTE system is the ability to put your air barrier outside the framing. Using deeper framing doesn't deal with this issue - you're still left using your gib board as the air barrier. We've recently renovated our current house, and getting an airtight seal on the gib board has been difficult (which is to say, we haven't succeeded). I was also very attracted to not having insulation in the stud bays - being able to add services at a later date would be nice. The detailing on this system didn't look over difficult, the only real issue is that your walls get thicker.
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Re: Remote, and ceiling insulation

Postby Albrecht Stoecklein » Sat Aug 07, 2010 7:54 pm

Hi Paul,

I think your calculations look about right. That's the sort of R-values I would expect.

But - and that's a big BUT!!! - I would feel quite uncomfortable to have rigid insulation hard up against framing or sheathing. The red weathertightness fags go up in front of my eyes. If any water would get behind the EPS or XPS and membrane that water would be trapped between the membrane and the sheathing and can over time do lots of harm.

Just be prepared that the council will ask lots of questions, too.

Two alternative options:

1. Add a cavity between the sheathing and the EPS. If you detail the sheathing well it would also form a very air-tight barrier but would still allow moisture vapour through. Just be extremely careful around flashings, etc and use large overhangs/eaves.

2. As Philip suggests, I thing using concrete instead of the timber framing would give you the additiona benefit of thermal mass, and there are of course not issues with concrete rotting. There are a few systems on the market combining EPS and concrete. A concrete house would also be very airtight and has natural capability to absorb and release moisture.

And lastly, there is a new company in NZ selling an "intelligent" membrane(http://www.proclima.co.nz/). That membrane is placed on the inside of the framing and depending on the microclimate on the two sides of the membrane lets moisture through or not. They also sell flushbox covers etc. These wall penetrations are one of the main infiltration paths.

I personally still would think that the concrete option is best.

And lastly: Research by the Building Research Association has actually shown that most modern New Zealand houses are already very airtight. In particular if they have any sheet cladding material. If you make your house even more air tight you should certainly consider a proper heat recovery ventilation system (Moisture Master HX, Lossney, Intellivent are a few of the products on the market).

Hope that helps

Sunny regards

Albrecht
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Re: Remote, and ceiling insulation

Postby Bahamus » Tue Feb 04, 2020 8:14 pm

I will follow up with very good information.
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