Double Walled Observatory

[Adam J]

I have noticed that a large number of people are using ship lap on the outside of their observatory and then using ply or hardboard on the inside...not sure why a double wall is required?

 

 

[Bizibilder]

It gives a little insulation as well as a clean smooth surface on the inside - its amazing how tangled up you can get inside an observatory with an internal frame when it is pitch dark!! (Won't say how I know  ).

[Dinglem]

I believe it also gives a bit more rigidity to the structure.

[Astrokev]

...and gives creepy crawlies less opportunity to find nooks and crannies to hide in, waiting for darkness to fall when they come out to frighten you to death in the middle of the night.

[Adam J]

2 hours ago, Dinglem said:

I believe it also gives a bit more rigidity to the structure.

I am good for rigidity. I could maybe accept that it will give a nice surface onto which you can hang equipment etc.

[m.tweedy]

8 minutes ago, Adam J said:

I am good for rigidity. I could maybe accept that it will give a nice surface onto which you can hang equipment etc.

All of the above. Vapour barrier and insulation in the cavity. Making use of the spare ply was also a factor

[Adam J]

4 minutes ago, m.tweedy said:

All of the above. Vapour barrier and insulation in the cavity. Making use of the spare ply was also a factor

I would have thought the aim would have been to circulate air thought the obsy to keep it dry not trap it inside with a barrier. You also want to try and keep the equipment at ambient temperature? For a warm room i would agree, but it seems counter productive for the obsy itself.

[m.tweedy]

6 minutes ago, Adam J said:

I would have thought the aim would have been to circulate air thought the obsy to keep it dry not trap it inside with a barrier. You also want to try and keep the equipment at ambient temperature? For a warm room i would agree, but it seems counter productive for the obsy itself.

The insulation is keeping the temperature down during the day. On a night the dome will be open along with the 2 doors. It rapidly cools down. There is also a gap all of the way round between the top of the wall and the dome. It was left unopened for 2 months from January when I was in New Zealand with no condensation issues. The barrier was put in place to stop any water getting inside. It is the stuff that goes under the roof tiles and is working well as there was no water penetration either. I live in the rainy, windy North so needed that little bit extra "insurance".

my observatory build is earlier on in this section.

[Adam J]

17 hours ago, m.tweedy said:

The insulation is keeping the temperature down during the day. On a night the dome will be open along with the 2 doors. It rapidly cools down. There is also a gap all of the way round between the top of the wall and the dome. It was left unopened for 2 months from January when I was in New Zealand with no condensation issues. The barrier was put in place to stop any water getting inside. It is the stuff that goes under the roof tiles and is working well as there was no water penetration either. I live in the rainy, windy North so needed that little bit extra "insurance".

my observatory build is earlier on in this section.

Hi well I was planning on fitting a solar powered fan to prevent the internal temperature from rising too high. Ship lap is very good at keeping rain out of a shed, never had any issues and it rains lots in the uk. 

[RayD]

Thing is, there is no right or wrong answer here, just varying ways for you to achieve what you are after.

If you just want a water tight(ish) room, then a framed shed with shiplap will work.  It isn't that water tight, and that is why higher end sheds and most high spec observatories will have a breathable membrane such as Tyvek or similar. 

As noted above, the OSB does add a considerable amount of stability and weight, and it also gives you more fixing points for the cladding.  Don't forget, cladding is just that, and isn't really designed to offer structural stability, albeit it will stiffen a structure up initially.  However, over time as it gets wet and dry etc. it will begin to swell and contract, and split etc. and this is when the leaks start happening.  This isn't an issue if it is on top of a backing board with membrane.

In addition to the above, the external board also provides a suitable and water tight backing for the internal insulation, which you can't get with cladding and no VP, so you will probably end up with damp issues if you then insulate.

Of course all this comes at a cost, which can be significant, but it does also lend itself to a very controllable environment for both you and your kit when the roof is closed.  Certainly for me this was a big factor when deciding how to go about my build.

Finally, if you only have shiplap cladding with nothing under, consider the security aspect, as it is pretty easy to pull off.

It seems like you have a pretty good idea where you want to go with your build, and as I said at the beginning, there is no right or wrong way, just the way which suits your requirements, available space and budget 

[RayD]

Just to add to that.  Mine is actually triple walled, with an OSB backing board under the cladding and Tyvek, and then with a ply face board on the inside, which not only covers over the insulation to hold it place and protect if from damage, but also gives you plenty of smooth(ish) walls for wiring, pictures, shelves, additional screen in my case and whatever else you want to put on a wall, which may be difficult with it being just a frame work.

[Shibby]

Oh, no. Cost going up again... For my obs, I was only planning on adding extra stud+ply and insulation to the warm room, but maybe I should do the scope area too.

So, the vapour barrier: is this to prevent moisture from inside getting in to the walls? Where would this moisture come from, just the air? And the insulation would be to stop it getting too warm in there? Perhaps keeping out the creepy-crawlies is reason enough.

[RayD]

27 minutes ago, Shibby said:

Oh, no. Cost going up again... For my obs, I was only planning on adding extra stud+ply and insulation to the warm room, but maybe I should do the scope area too.

So, the vapour barrier: is this to prevent moisture from inside getting in to the walls? Where would this moisture come from, just the air? And the insulation would be to stop it getting too warm in there? Perhaps keeping out the creepy-crawlies is reason enough.

No it's really not essential Lewis.  The warm room would be fine as that just makes a nice space to be in, but the scope room, as with mine, I guess you could say is very much a nice to have.  Thing is my kit is very expensive, so a bit more on insulation at this stage to keep it as stable as possible in all weathers just made sense to me.  Once the roof rolls back it cools down quickly, but with it closed it stays cool in the heat (proven to work over recent weeks), and will stay warmer in the cold weather.

As I said it's about what suits your needs, and many people get on fine without insulation at all.  However, the vapour barrier I would say, in my opinion, is pretty much an essential, particularly if only cladding is being used.  It basically lets the inside breathe, so moist air building up inside can get out, but is water resistant, so it reduces the likelihood of damp and mould, but also keeps things nice and dry.  I used Tyvek. which is a paper like material from Dupont, but Screwfix do a cheaper one which would work fine. 

Of course I'm sure there will now be several people reminding me that they have had their kit in a shed in a septic tank at bottom of t' garden without a VP for 50 years with no hint of a problem.  I would say good for them, but that doesn't mean that best practice doesn't include using one.

[tooth_dr]

My own personal experience - I cladded my warm room with OSB and it suffers from damp, sometimes the top of the work top is wet to touch.  The main obs part, is simply tongue and groove boards, with no liner, and in fact a  couple of the boards have shrunk so there is a visible gap ie you can see in and out This allows plenty of air to circulate, it's never damp, and I guess it's not far of the ambient temp, or at least it cools down very quickly.

[ollypenrice]

I've decided that I prefer marine ply on the outside. It's quicker and easier to make, provides more rigidity by triangulating the frame to which it is fitted, is easier to sand and re-finish and can be 'cladded over' by a second layer when, evenually, it becomes disreputable-looking! I've found that the thin stuff lasts just as long as the thick.

I have only lined and insulated my warm room and the inside of the corrugated steel roofs over the scopes (to stop condensation drips and reduce daytime temperatures.)

So my favourite materials for the outside are marine ply and corrugated galvanized steel. (The steel has proved so much better over the years than the alternatives I've tried that I kick myself for having tried them.)

Olly

[Adam J]

5 minutes ago, ollypenrice said:

I've decided that I prefer marine ply on the outside. It's quicker and easier to make, provides more rigidity by triangulating the frame to which it is fitted, is easier to sand and re-finish and can be 'cladded over' by a second layer when, evenually, it becomes disreputable-looking! I've found that the thin stuff lasts just as long as the thick.

I have only lined and insulated my warm room and the inside of the corrugated steel roofs over the scopes (to stop condensation drips and reduce daytime temperatures.)

So my favourite materials for the outside are marine ply and corrugated galvanized steel. (The steel has proved so much better over the years than the alternatives I've tried that I kick myself for having tried them.)

Olly

Alas from an aesthetic perspective the wife would not go for ply and corrugated steel structure in her garden or I would have done so from a functional / cost perspective. Do you have any pictures? At any rate the obsy is completed not just trying to decide if it needed an internal wall in addition to the ship lap. 

[RayD]

2 minutes ago, Adam J said:

At any rate the obsy is completed not just trying to decide if it needed an internal wall in addition to the ship lap. 

It wouldn't achieve much now.  All it would do in reality is give you somewhere to hang star chats and alike, and would add a bit of long-term rigidity for when your cladding starts to move about.

[RayD]

10 minutes ago, ollypenrice said:

corrugated galvanized steel

I love the simplicity of this, but also think my neighbours would be up in arms during heavy rain with the ding ding ding ding ding of the rain on it.  Is it noisy?  I can imagine it getting very warm in your location, so fully understand the need for insulation.

[The Admiral]

2 hours ago, Adam J said:

Hi well I was planning on fitting a solar powered fan to prevent the internal temperature from rising too high. Ship lap is very good at keeping rain out of a shed, never had any issues and it rains lots in the uk. 

I've no experience of observatories but do have a shed . The problem with shiplap I find is that the overlap is not great and it doesn't take much shrinkage to result in gaps through which rain can blow in. Knots are another problem, because if they fall out there's a hole. I guess it all comes down to the quality and type of the wood used for construction.

Ian

[ollypenrice]

5 minutes ago, RayD said:

I love the simplicity of this, but also think my neighbours would be up in arms during heavy rain with the ding ding ding ding ding of the rain on it.  Is it noisy?  I can imagine it getting very warm in your location, so fully understand the need for insulation.

I doubt your neighbours would hear it. It does ring a bit in heavy rain (we do get very, very heavy mountain rain sometimes) but one of the sheds is close to our flat's kitchen and we don't hear it.  It's only like rain hitting cars and caravans, I'd say.

Olly

[Adam J]

1 hour ago, The Admiral said:

I've no experience of observatories but do have a shed . The problem with shiplap I find is that the overlap is not great and it doesn't take much shrinkage to result in gaps through which rain can blow in. Knots are another problem, because if they fall out there's a hole. I guess it all comes down to the quality and type of the wood used for construction.

Ian

Its good stuff it came from a wood merchant not the local B&Q.

[The Admiral]

This thread has piqued my interest in construction method, 'cos it strikes me that if you double skin the walls, then there must be a proper construction method to eliminate potential damp and rot problems. Just randomly surfing t'interweb searching for 'garden rooms', I found this example: https://www.greenretreats.co.uk/construction/. I'm not saying that this is necessarily the right way to do it, but it looks as though a cavity and vapour barrier is a good idea (like house walls really, and ventilation of roof spaces). Just my 2 cents worth!

Ian

[m.tweedy]

If it helps here is a link to my build

https://goo.gl/photos/PVWWztA8CTGjSeJN6

if it is inappropriate please feel free to delete it.

The Carpenter said he was treating it like a timber frame house build (apart from the rotating roof that was a first for him :-) ).

[Rusted]

My own experience is that metal roofs make a racket in heavy rain.

We stayed at a farm with a corrugated metal porch across the yard and it was dreadfully noisy!

I clad my home-made shed in 12mm plywood [with factory machined grooves at 4" intervals] 14 years ago.

I haven't touched it with any sort of paint or protection from new and it is still fine.

It has remained flat, strong and quite attractive if you like the grey/brown [long abandoned building] "knotty pine, weathered" look.

The shed has decent ventilation but wood and plywood stored inside gets spotty and my iron tools rust slowly.

My fuzzy logic suggests that insulating an observatory will delay cooling and retain any daytime heat buildup.

Shiny aluminium remains the coolest useful surface material I have tried so far. 

Shiny stainless steel and galvanized iron temperatures absolutely soar in warm sunshine! 

Only new, white, fiber cement roofing & cladding boards are cooler if you can manage the weight and relative fragility.

Roofing felt is by far the worst material on the planet for obs. roofs. IMHO.

Not only is it very heavy, but temperatures inside soar as you would expect with a dull, black and rough, solar absorbent surface.

I built a fairly large workshop with a felt roof on plywood and recorded 103F with the doors and windows wide open!

When I covered a 4-6" thick, concrete slab, shed roof, with recycled aluminium sheet, it made it instantly habitable as a workshop.

With the original black felt covering it regularly went up to 100F inside, day and night for ages, after hot sunshine.

My only experience with white paint was two thick undercoats on a bare, GRP car. That made it cooler in sunshine.

Though the windows probably dominated in solar gain. Titanium white paint has a good reputation for rejecting solar gain.

Dense shade plants are probably even better provided you can still find the sky.

Big pots of shading/concealing plants can easily be moved/rolled/sack-trucked away when needed.

Louvered/slatted blinds must have some useful purpose for shading/ventilating an obs.

All the sunshine which never reaches a building never needs to be rejected/reflected, AC'd or insulated against.

Not a lot of architects, builders, engineers and/or planners seem to know this.