Shed Roof Insulation

[Nick C]

As I mentioned on my layout thread, I'm renovating my shed/summerhouse/workshop/railway room in order to waterproof and insulate it - and I've been wondering about the best method to insulate the roof.

 

It's a "log cabin" style building, which makes it difficult to attach anything to the walls - apparently the beams need to be able to expand and contract independently. 

 

Looking online, it seems that there are two methods:

1: Waterproofing - OSB - Insulation - Vapour barrier - Existing roof timbers - Existing joists

2: Waterproofing - Existing Roof Timbers - Existing josts - Insulation between joists - Ceiling

 

Can anyone explain the pros and cons of each?

 

To me option 2 looks cheaper, as I don't need a new OSB covering, and less insulation (don't need to cover the overlaps) - but could encapsulating the existing timbers cause problems? Option 2 also means I can split the job - get it waterproof first, the insulate when I'm happy there's no more leaks.

 

It's a slightly pitched roof, maybe 10 degrees, and the existing tongue-and-groove timbers are getting damp at the lowest ends in a couple of places - I'm assuming the felt has failed under the fascia and is allowing water in there - I'll remove it as soon as the weather allows, but can anyone suggest how best to dry the timbers? I don't want to trap moisture between them and the new covering.

 

I'm intending to use EPDM for the new waterproofing layer - unless there's a good reason not to? I'm also intending to use 50mm PIR insulation for both the roof and the floor in a floating-floor arrangement, as there's no room to add anything under the existing floorboards.

 

It's under 15m^2, so as I understand it I don't need to worry about building regs, but I'd still like to do it properly so it lasts...

 

Edited March 17, 2021 by Nick C

[Ron Ron Ron]

The same choices I was looking at at the end of last summer Nick, so I'm interested in what others have to say on the matter.

 

In my case, I couldn't decide either way and decided the existing roof covering would do just one more winter, before it became a bit of a risk.

 

To a degree it might depend on the internal truss structure as some sheds don't have much getting in the way of applying internal insulation, where as other sheds would involve lots of cutting to fit etc.

 

With my shed, with its apex roof, the easiest option would be external (warm roof) insulation, with large 50mm insulation panels below something like OSB.

There's a wide choice of waterproof roofing materials and I'm inclined towards some of the metallic products.

 

Don't forget ventilation, either.

 

This is one of my bigger jobs for the coming spring.

 

 

.

[Nick C]

Thanks Ron

 

Unfortunately it definitely won't last one more winter - I'm worried it won't even last until it's actually dry enough to do something about it, after checking in the heavy rain last night and seeing more areas along the back that are showing signs of dampness.

 

These photos were taken a couple of weeks ago:

 

 

 

 

 

 

[Damo666]

On 27/01/2021 at 17:00, Nick C said:

It's a "log cabin" style building, which makes it difficult to attach anything to the walls - apparently the beams need to be able to expand and contract independently. Looking online, it seems that there are two methods:

 

1: Waterproofing - OSB - Insulation - Vapour barrier - Existing roof timbers - Existing joists

2: Waterproofing - Existing Roof Timbers - Existing josts - Insulation between joists - Ceiling

 

Can anyone explain the pros and cons of each?

I would say it's more important to allow the walls to move according to the season (expand / contract due to the moisture in the air). The roof will move less so, as you may be aware as it's got a waterproof layer on it already (Felt?) and if this moved too much then it would have ripped.

 

Option 1 is the preferred solution, as this will allow any internal vapour (from you and any other water sources) to escape through the ceiling T&G boards (I note that you have removed the existing waterproofing, this needs to be done, don't cut corners and leave it there, it will act as a vapour barrier in the wrong place).

 

Option 2 will (a) lower your ceiling head-height and (b) more importantly, trap water vapour between the insulation and the waterproofing. The vapour in the air will condense once it gets to the cold side of the insulation and then cannot escape because of the waterproofing. It will then saturate into the timbers and then you've got big problems. For this to work you would need to introduce ventilation above the insulation and below the roof T&G. It might be possible to introduce cross ventilation in the side walls of the cabin.

 

There is an argument that these log-cabins are quite air leaky anyway, so the issue of moisture being trapped within the structure is reduced, but I wouldn't risk it on my cabin.

 

@AY Mod I would like to propose there be a sticky for the basic principles of insulating sheds (or lofts for that matter). I see this question come up frequently and I know @chris p bacon has offered his advice many times, others with a construction professional background have provided advice too. There are really only a few options when it comes to shed / loft insulation solutions, anything else risks damaging your investment (shed or the structure of your home), so the answers are essentially consistent: Warm Roof v Cold Roof and how you manage moisture in the air.

[LimboBrit]

On 27/01/2021 at 18:00, Nick C said:

1: Waterproofing - OSB - Insulation - Vapour barrier - Existing roof timbers - Existing joists

I would definitely recommend method #1. Condensation is much less likely to be a problem as I learnt from my own experience. The vapour barrier will prevent warm humid air from reaching the cold surface of the OSB and condensing. With method #2 there is nothing from preventing condensation from forming on the existing roof timbers and soaking into the insulation with the inevitable problems of mould.

[Damo666]

40 minutes ago, Nick C said:

Thanks Ron

 

Unfortunately it definitely won't last one more winter - I'm worried it won't even last until it's actually dry enough to do something about it, after checking in the heavy rain last night and seeing more areas along the back that are showing signs of dampness.

 

These photos were taken a couple of weeks ago:

 

 

 

 

 

 

I recognise these issues exactly. How old is your cabin?

 

If the timbers are soft then look at a way of stabilising the fibres (wood hardener) before doing anything else.

 

I would then apply a large dose of wood preserver to the end-grain of all the logs. I've used Blackfriar Exterior Wood Preservative Gold Star in the past (Other similar treatments like Sikkens, Sadolin and Cuprinol are available, and other suppliers too). Give them a couple of coats over a few dry days. Do this on the inside corners too, it won't be as effective but some will saturate into the joints. Everywhere you are seeing that discolouration on the logs / planks, liberally apply several additional coats of preservative.

 

Did you apply wood preservative before the cabin was assembled or after it was erected?

 

I'm not sure what your external finish is, but is this still porous?  Can the end-grain absorb water, if so then it'll take the wood preservative I've mentioned above.

 

[Nick C]

13 minutes ago, Damo666 said:

Option 1 is the preferred solution, as this will allow any internal vapour (from you and any other water sources) to escape through the ceiling T&G boards (I note that you have removed the existing waterproofing, this needs to be done, don't cut corners and leave it there, it will act as a vapour barrier in the wrong place).

 

Option 2 will (a) lower your ceiling head-height and (b) more importantly, trap water vapour between the insulation and the waterproofing. The vapour in the air will condense once it gets to the cold side of the insulation and then cannot escape because of the waterproofing. It will then saturate into the timbers and then you've got big problems. For this to work you would need to introduce ventilation above the insulation and below the roof T&G. It might be possible to introduce cross ventilation in the side walls of the cabin.

 

Thanks - that's the best explanation I've read as to the reasoning for that order of things - particularly with regards to the vapour barrier. I'll stick with option 1 then.

 

Just now, Damo666 said:

I recognise these issues exactly. How old is your cabin?

 

If the timbers are soft then look at a way of stabilising the fibres (wood hardener) before doing anything else.

 

I would then apply a large dose of wood preserver to the end-grain of all the logs. I've used Blackfriar Exterior Wood Preservative Gold Star in the past (Other similar treatments like Sikkens, Sadolin and Cuprinol are available, and other suppliers too). Give them a couple of coats over a few dry days. Do this on the inside corners too, it won't be as effective but some will saturate into the joints. Everywhere you are seeing that discolouration on the logs / planks, liberally apply several additional coats of preservative.

 

Did you apply wood preservative before the cabin was assembled or after it was erected?

 

I'm not sure what your external finish is, but is this still porous?  Can the end-grain absorb water, if so then it'll take the wood preservative I've mentioned above.

 

 

I'm not sure how old it is as it was built by the previous owner of the house - I'd estimate 6-7 years old (we've been here four years and I think they were here for three). The preservative was definitely applied after assembly as there was none under the window frame - since those photos I've removed, stripped, repainted and refitted the window. I've also coated the inside of the end wall with Cuprinol wood preservative.

 

The external finish is Cuprinol "Garden Shades" - I've just put an additional two coats on around the window area (to get into the bits that hadn't been done, particularly the ends of the logs), and will re-do the rest as soon as it's dry enough!

 

Presumably I need to dry out the wood somewhat before applying the hardener or preservative (I've read it should be 14%?) - I've got a dehumidifer for the inside, but how is best to dry the outside, other than waiting for better weather and leaving it exposed to the sun for a few days?

 

I'm also worried about the bottom corner (see the last photo) - is that dampness coming up from below, or just from water running down the joints? There's no sign of any damp-proof membrane, nor does there seem to be any mention of it in the instructions on the manufacturer's website - all they say is that the beams on which it sits are treated.

[Damo666]

1 hour ago, Nick C said:

I'm also worried about the bottom corner (see the last photo) - is that dampness coming up from below, or just from water running down the joints? There's no sign of any damp-proof membrane, nor does there seem to be any mention of it in the instructions on the manufacturer's website - all they say is that the beams on which it sits are treated.

 

The company I specified a few years ago states this in their assembly guide:

 

Quote

"We suggest that if a concrete slab is poured, a damp proof membrane is positioned under the concrete in the normal way. This will prevent damp soaking up into the slab and potentially into the bearers of the cabin. Further to this, we strongly suggest a concrete base is formed so that its size extends to the outer face of the walls only – Whilst it may be easier to dispense with careful measurement and simply form an oversize base, this only increases the possibility of rain hitting the base and splashing back onto the lower parts of the walls or potentially running under the building, thereby meaning you will have to always pay extra special attention to treatment of these areas and damp could become a possibility.

 

Your paving comes in front of the cabin walls.

 

After you undertake any further wood preservative treatments and external stain coats, paying particular attention to the end-grain and overlapping log ends, I would suggest you then run a bead of mastic down the 3 vertical edges which are on each of the lapped corners, especially to the elevations facing the prevailing wind. Because of the way these cabins are made, the overlapping logs on each external corner will form 3 'Right-Angles'. Does this make sense?

 

I cast the slab then built a 1 brick high sleeper wall to support the timber bearers (for the floor and the external walls). This kept the timber bearers off the concrete slab so if water found its way onto the slab it wouldn't be in contact with the timber. The strip of DPC (Damp Proof Course) is then on top of the timber bearers. (I would have preferred if the builder had listened to me and this was on top of the brick sleeper wall / under the timber bearers).

 

In case you are wondering why the weird arrangement for the brick sleeper wall, this was to provide support for some insulation boards I wanted to place between the floor bearers. This kept the insulation above the slab, maintaining the ventilated air gap without the need for additional fixings. By alternating the bricks I had full support for the timber bearers and stop-start support for the insulation. Method in my madness. You'll understand better in the next picture.

 

 

 

Edited January 29, 2021 by Damo666

[Nick C]

40 minutes ago, Damo666 said:

After you undertake any further wood preservative treatments and external stain coats, paying particular attention to the end-grain and overlapping log ends, I would suggest you then run a bead of mastic down the 3 vertical edges which are on each of the lapped corners, especially to the elevations facing the prevailing wind. Because of the way these cabins are made, the overlapping logs on each external corner will form 3 'Right-Angles'. Does this make sense?

 

Yeah, I've seen advice to add a bead of mastic/silicone down the external corners, that's on my list as soon as I've done the preservative (which of course I can't do until it's warm enough for it to dry properly!)

 

Should I sand the existing coating on the end-grains right back (or as much as possible anyway) before applying the new coating? The instructions of the Cuprinol just say to sand back any loose or flaking paint.

[Damo666]

55 minutes ago, Nick C said:

Should I sand the existing coating on the end-grains right back (or as much as possible anyway) before applying the new coating? The instructions of the Cuprinol just say to sand back any loose or flaking paint.

I would be looking to get as much of the wood preservative into the end-grain as possible. Obviously remove friable / flaking paint as you know, but if sanding gets me access to more end-grain then I'd do that.

 

At least 2 coats of wood preservative (I must have put 4-5 localised coats on the joints before final stain coat went on). I did the end-grain, then the log sides and went back to the end-grain again. Repeated the next day.

[Damo666]

Is there a gutter on the roof? This makes a huge difference. Wind will just blow the rain that falls off the roof onto the logs.

[Nick C]

Thanks - I'll do that then, as soon as I get three days of +5c and no rain forecast! (at least working from home I can do it in my lunchbreak instead of only having weekends...)

 

There's no gutter at the moment, that's also on the list to fit.

[chris p bacon]

I'm not that familiar with Log cabins. @Damo666 has put forward some really good suggestions   which I can't really add much to. 

Good idea to ask @AY Mod about the sticky too, having info and links in one place has to be better.

[Damo666]

16 minutes ago, Nick C said:

There's no gutter at the moment, that's also on the list to fit.

I did a bit of research for a gutter & downpipe system last summer and ended up going to this company. https://www.guttering-expert.co.uk/

There was a company on eBay or Amazon, cannot remember, which supplied / manufactured the whole kit in the Netherlands (IIFC) but 'Guttering Expert' seemed to have the same product. A little more expensive but arrived sooner.

 

Now with us having left the EU, there might be additional import costs if you were to go direct.

 

The nice thing about the kit is that it was complete and sizes to suit the cabin, I needed the extended brackets for the gutter. Screwfix / Travis Perkins only seemed to have the domestic gutting, too large and OTT (and would have been more expensive too).

[AY Mod]

7 minutes ago, chris p bacon said:

 

Good idea to ask @AY Mod about the sticky too, having info and links in one place has to be better.

 

I need to have a think where it should be placed, a sticky in Wheeltappers wouldn't be much use.

[Ron Ron Ron]

4 minutes ago, AY Mod said:

 

I need to have a think where it should be placed, a sticky in Wheeltappers wouldon't be much use.

 

Skill & Knowledge Centre perhaps ?

 

.

[Ron Ron Ron]

7 minutes ago, Damo666 said:

I did a bit of research for a gutter & downpipe system last summer....

......and sizes to suit the cabin, I needed the extended brackets for the gutter.

Screwfix / Travis Perkins only seemed to have the domestic gutting, too large and OTT (and would have been more expensive too).

 

B&Q and Wickes have sold mini guttering sizes, with matching fitments and downpipes, for sheds and outbuildings for years.

Very cheap too.

Basic simple pattern though.

 

B&Q Miniflow

 

Wickes MiniLine

 

 

.

 

[Damo666]

13 minutes ago, AY Mod said:

 

I need to have a think where it should be placed, a sticky in Wheeltappers wouldn't be much use.

That would be great, thank you.

 

I envisage it as a go-to depositary for factual information on the subject, not something that would be diluted with a lot of cross-talk and off-topic discussions. Somewhere where a forum member just goes to for the information and the construction principles are available there. I know that @chris p bacon has contributed good factual advice in the past on other threads. Sometimes it just seems to be the same questions been asked again and again.

 

Maybe Chris could nominate one of his articles as the basis of the sticky, I'd be happy to contribute too. Then the article could then be locked unless amendments are needed.

[Nick C]

13 minutes ago, Ron Ron Ron said:

 

Skill & Knowledge Centre perhaps ?

 

I agree - perhaps a section in there for all things 'outside' the layout, i.e. the railway room itself (sheds, garages, lofts, etc), baseboard construction, lighting rigs, and so on?

 

I wasn't sure where to put this thread in the first place - there's some related stuff in "modelling musings and miscellany" and others in "modelling quetions, hints and tips" as well as here in wheeltappers...

[Nick C]

So, some progress to report! Bought the materials last week, and with a decent forecast, stripped off the old felt this morning. My initial suspicions were indeed correct, in that much of the ingress was coming in around the nails holding the felt. More positively, none of the wood seems rotten, testing with a screwdriver showed it all to be firm.

 

It was quite windy, which proved to be somewhat advantageous as it dried off nicely. I brushed on the wood preserver - and just as I finished it started raining, which wasn't supposed to happen! It's now under a tarp to protect it overnight before, hopefully and weather permitting, the new layers go on tomorrow.

[37114]

I would definitely go for an EPDM roof, they are easy to apply as well, I recommend rubber4roofs.co.uk. who I used to supply the roof materials for my garden office.

 

I also went for a variant of option 1 when I did my roof namely  Epdm, Osb,Joists, Insulation, Moisture barrier, plasterboard. 

[tomparryharry]

On 29/01/2021 at 15:18, Ron Ron Ron said:

 

Skill & Knowledge Centre perhaps ?

 

.

 

I would certainly agree with the topic being located within skills & knowledge.  A considerable number amongst us  would dearly love owning a shed.  I still haven't fully finished mine, but being able to work undercover is an absolute  bonus.  More the merrier, I'd say. 

 

The Workshop Practice Series numbers 22 & 23 cover some of the practicalities involved. 

 

It's now 21:27 here, and I'm off to do an hour out there. 

 

Best wishes,

Ian. 

[Nick C]

Yesterday saw the vapour barrier, framing, insulation, taping and OSB go up - ran out of daylight (and energy!) before we could do the EPDM, so it's back under a tarp again.

[Quarryscapes]

For anyone thinking of building a layout shed, then I can whole heartedly recommend this series on Youtube. Very thoroughly covers the  building a decent Garden Room from planning Permission through construction. 

 

 

[37114]

2 hours ago, Quarryscapes said:

For anyone thinking of building a layout shed, then I can whole heartedly recommend this series on Youtube. Very thoroughly covers the  building a decent Garden Room from planning Permission through construction. 

 

 

I watched that series and they are excellent, I built my own Garden room last year and followed most of what Ali did: