Insulated lean-to extension ? what to expect?

[Pete 75C]

I live in a large old house which had to have a brick-built lean to shed demolished recently. I’ve rebuild the shed using the original bricks up to a height of around 1000mm (the dwarf wall is 2 bricks thick). Topping this is a timber frame insulated with foil backed foam core insulation complete with breather membrane. The roof uses 125mm by 50mm joists at 400mm centres topped with 22mm flooring grade chipboard. This is covered with an underlay felt and finally a covering of thick “super” shed felt. The lean-to is guttered and vented properly. The end wall of the house (the 4^th wall of the shed) is further insulated and boarded over.

I’ve ended up with a usable space of around 12ft by 6ft and I'm using 9mm ply on top of softwood for the baseboards. The one thing I’m not sure about is what to expect regarding temperature changes and moisture levels? The longest wall of the shed faces west and just under a little further north of us is the North Norfolk coast (North Sea).

If anyone’s gone down a similar road, I’d be interested to hear of your experiences.

I have the use of a good quality dehumidifier (if needed) that’s plenty big enough for this space and I will have some form of heating for the winter months.

 

It occurs to me that this might not be posted in the correct forum section - apologies if that is the case.

[chris p bacon]

The more insulation you can get in the better, (you don't say how much you have used ) there should be a 20 -25mm airspace between the insulation and the outside (roof and walls) then you can pack the rest with Celotex/kingspan. If you think in terms of a timber framed house rather than a shed then you'll have a cosy year round room to use. you may have to ventilate the room but you could look at venting from and back into the house rather than outside. need a bit more detail for that though.

 

You're on the right road though by the sound of it.

[jcm@gwr]

I agree, lots of insulation but don't compromise the air gap, (you must

have air-flow or you will have problems with the timber rotting)

My biggest concern is the use of chipboard (of any grade) as a roof

panel, so I would suggest a good few coats of wood preserver first.

As the structure is a lean-to and you are going have good airflow,

it should be okay to just add heating, not a de-humidifier. If you are

lucky, you might be able to extend the central heating system and add

a radiator ( this would be the cheapest option in the long run).

Good luck, Jeff

[chris p bacon]

My biggest concern is the use of chipboard (of any grade) as a roof

panel, so I would suggest a good few coats of wood preserver first.

Good point there I just presumed it was the modern caberfloor (Green in colour) which is moisture resistant, we use it as a working platform now instead of erecting an internal scaffold, you'd be surprised what weather it can take nowadays.

As the structure is a lean-to and you are going have good airflow,

it should be okay to just add heating, not a de-humidifier. If you are

lucky, you might be able to extend the central heating system and add

a radiator ( this would be the cheapest option in the long run).

Good luck, Jeff

Agree with you here Jeff, extending the house Central Heating if possible will be the cheapest in the long run and a lot more comfortable.

[Pete 75C]

Good point there I just presumed it was the modern caberfloor (Green in colour) which is moisture resistant, we use it as a working platform now instead of erecting an internal scaffold, you'd be surprised what weather it can take nowadays.

Agree with you here Jeff, extending the house Central Heating if possible will be the cheapest in the long run and a lot more comfortable.

Hi Jeff and Dave. Thanks for your replies.

There's 75mm of insulation in the wall panels with a 25mm air gap and 100mm of kingspan in the roof, again with a 25mm air gap.

The ultra thick chipboard roof panels are the green moisture resistant panels, but as a belt and braces approach, they've been treated and sealed prior to felting. I'm not anticipating any problems with the structure itself - I just didn't know what conditions to expect inside. I hadn't thought of tapping through to the central heating and installing a rad - that's a really good idea.

Thanks again.

[Black Sheep]

extend the central heating and fit a thermostatic valve to it, that way the temperature of the room is regulated by the temperature in the room (provided the heating is on)

 

all a thermostatic valve does is turn the water flow on if the room gets below a set temperature, closing and stopping again when it gets to temperature, it needs to have hot water supplied by the rest of the system to provide heat, it doesn't communicate with the boiler or main thermostat.

 

We've got them in each room to balance the house out so if one room is cold, they're all cold and the heating should be coming on in response and to cut off and stop any rooms becoming roasting before the others are up to temperature.

[chris p bacon]

Hi Jeff and Dave. Thanks for your replies.

There's 75mm of insulation in the wall panels with a 25mm air gap and 100mm of kingspan in the roof, again with a 25mm air gap.

The ultra thick chipboard roof panels are the green moisture resistant panels, but as a belt and braces approach, they've been treated and sealed prior to felting. I'm not anticipating any problems with the structure itself - I just didn't know what conditions to expect inside. I hadn't thought of tapping through to the central heating and installing a rad - that's a really good idea.

Thanks again.

Good, thats a decent amount of Insulation, I'm guessing you have 4 x 2 studs (75 plus the 25 airspace). In housebuilding what we are required to do now is cover over the 75 and the studs with another layer (Minimum 25 usually 40mm) as the timber studs are supposedly a "cold spot" (walls and roof) but for your purposes unless you really want to go to town you probably have sufficient.

An easy job to do unless you have plasterboarded/covered it in, is to tape the joints, may not seem like much but you'd be amazed at the heat loss. The more you do now the more comfort you will have.

If you can't extend the Central heating then you could look at an electric panel rad with thermostat and timer, if they are set up correctly (position mainly) they are not expensive to run.

 

What have you/do you intend to board the inside with ? For fire rating plasterboard is the best to use. Fitted neatly there is no need to skim plaster it, just fill the screw holes and a coat of emulsion will do the trick.

[PaternosterRow]

Sounds like you know your stuff.  Wouldn't worry too much about extreme temperature changes as the area is attached to the house and you've used good insulation and construction methods.  Agree about ventilation - it's the key to any good build.  Many people set up in their lofts without additional boarding or insulation and their layouts and stock seem to cope well with the massive extremes generated - you'll get nothing like that in your lean to.

[Pete 75C]

Good, thats a decent amount of Insulation, I'm guessing you have 4 x 2 studs (75 plus the 25 airspace). In housebuilding what we are required to do now is cover over the 75 and the studs with another layer (Minimum 25 usually 40mm) as the timber studs are supposedly a "cold spot" (walls and roof) but for your purposes unless you really want to go to town you probably have sufficient.

An easy job to do unless you have plasterboarded/covered it in, is to tape the joints, may not seem like much but you'd be amazed at the heat loss. The more you do now the more comfort you will have.

If you can't extend the Central heating then you could look at an electric panel rad with thermostat and timer, if they are set up correctly (position mainly) they are not expensive to run.

 

What have you/do you intend to board the inside with ? For fire rating plasterboard is the best to use. Fitted neatly there is no need to skim plaster it, just fill the screw holes and a coat of emulsion will do the trick.

Just time between clearing up all the kids' wrapping paper and peeling the spuds to reply to the posts above...

 

I won't be adding 25-40mm over the existing wall insulation - I'm happy that I've done enough on that score. The walls will just be overboarded with plasterboard and I'll skim over the joins with a jointing knife then paint. I'll still got around 40 litres of Wickes "Paint for New Plaster" which went on really well whilst doing the main house. Electrics are good to go. I've spent the past 6 months stripping this old house (1894) back to bare brick. Everything's rendered, plastered, new roof, new electrics, plumbing etc. I'm now doing "the shed" as a means to get back some "me" time and resume modelling.

I still think connecting a rad to the central heating is a good idea, but I will probably go for the electric option with thermostat. I may want some heat in the shed without turning the house heating on. As Dave says, they don't have to be expensive to run.

You've all put my mind at rest that I won't have to defrost my Bachmann blue 25 before an operating session or that my 2EPB won't melt in the heat!

Thanks again for your input guys.

Now it's time to see if my track plan actually works and fits the space available. Urban London decay in the 70s - 80s here we come.

Happy Christmas everyone - now where's the potato peeler?

Pete.

[shortliner]

I have to say that I will never again have Felt roofing on a shed - having replaced th felt on SWMBOs shed 3 times - the last after it was ripped off by BIG gales a year ago - that was fun trying to cover it with a tarpaulin as a temporary measure in a gale and driving rain, by myself! - I came to the conclusion that I'm getting too old for that silly game, and I have had it replaced with metal sheathing - which I suspect will outlast the wooden shed, and probably us both as ell!

[Pannier Tank]

A most interesting topic with some very useful advice. I'm currently in the process of constructing an outbuilding for a railway room, so the insulating information is very relevant to me. I have trench fill footings with a brickbase up to DPC Level then a timber frame with plywood cladding and the roof is Marley Fibre Slates. My main concern now is the amount of water I have in the base of the building (about 3"  deep!!); I intend to have wooden joists and Chipboard suspended flooring. I have Air Vents at each end of the building.

[Pete 75C]

A most interesting topic with some very useful advice. I'm currently in the process of constructing an outbuilding for a railway room, so the insulating information is very relevant to me. I have trench fill footings with a brickbase up to DPC Level then a timber frame with plywood cladding and the roof is Marley Fibre Slates. My main concern now is the amount of water I have in the base of the building (about 3"  deep!!); I intend to have wooden joists and Chipboard suspended flooring. I have Air Vents at each end of the building.

When you say "plywood cladding", do you mean small individual pieces (similar to cedar shingles) or large sheets? Hopefully it's at least marine ply.

Good luck with the project! Everyone's been really helpful here.

Pete.

[Pannier Tank]

When you say "plywood cladding", do you mean small individual pieces (similar to cedar shingles) or large sheets? Hopefully it's at least marine ply.

Good luck with the project! Everyone's been really helpful here.

Pete.

Hello Pete,

I bought the 8' x 4' sheets from the local timber merchants, I can't remember the exact name (it was a while ago now), but we did discuss the usage and that it needed to be smooth on both sides.

Looks like you will be in your room a lot sooner than I will.

My thoughts were to either add 3" of ballast with 2" of sand on top or go the whole hog and concrete over a DPC. The later more time consuming but hopefully solve the problem with water under the flooring.

[Pete 75C]

Hi David. Someone will soon come back to you with advice about the floor and drainage, not something I know much about - I was lucky that there was an existing structure originally in place. My floor is identical to the original ground floor of the rest of the house. One less thing to worry about! I'm still not sure about using sheets of ply as an external cladding - maybe it's ok, but I'd be tempted to overclad with shiplap, shingles or even featheredge.

It is really nice to have a dedicated modelling space though! We're going to convert the middle floor of our place into bedrooms with ensuites and maybe consider offering B&B for 6 months of the year (seaside town with steam railway - VERY busy in the summer). So a spare bedroom is not an option for me. Just need a combination lock to keep the wife and kids out! I've already said that if anyone thinks they're storing a lawnmower or a mountain bike in my room, there will be trouble!

Pete.

[Pannier Tank]

! I'm still not sure about using sheets of ply as an external cladding - maybe it's ok, but I'd be tempted to overclad with shiplap, shingles or even featheredge.

 

That's probably what I will do long term, I used the Plywood to give a little more security than just Shiplap. Also with a quick lick of Acrylic Barn Paint it made the building waterproof so protecting my investment whilst I moved on to other things (internals). I've certainly lost some sleep over the building project and will be glad when it's finished.

 

Your B&B sounds a good idea to fund further projects!

[jcm@gwr]

I used a mixture of materials and techniques when I built my shed.

I extended the original concrete raft (8' x 6' to 13' x 9') so that I had

a stable base for my 12' x 8' unit.

2 courses of brick topped with a DPC, then 4" x 2" (treated) as a

wall plate, 4" x 4" corners and 4" x 2" stud walls. 6" x 2" roof joists

(plus firring's), due to the weather at the time, the inside filled with

water, so I just drilled a hole in the concrete to let it drain away!

The outside was 'stirling' board, then covered in (overlapping)

roofing felt, then finally clad in featherboard (salvaged from old

fence panels!). Roof was treated the same, but I used plastic

corrugated sheeting to finish it.

Inside it was insulated with Celotex/Kingspan and plasterboard.

Power was supplied by 40mm armoured cable and a garage

consumer unit, multipe sockets and down-lights (sounds posh,

but like most of the materials, were left-overs or no longer needed

items) which give a good light and, being recessed, are have a

lesser chance of being smashed when moving stuff about!

A proper door (surplus old back door) and a double glazed window

completed the build, it is heated with a couple of skirting board

heaters (thermostatically controlled, of course)

Hope you can get some ideas/inspiration from this.

Jeff

[N15class]

For the roof I would not use felt unless it is the torch on stuff. If it is a pent roof I would go with slates or something that will keep the weather out with a good roofing felt underneath it.

[Pete 75C]

For the roof I would not use felt unless it is the torch on stuff. If it is a pent roof I would go with slates or something that will keep the weather out with a good roofing felt underneath it.

Using felt on a large (over 100 sq ft) roof with a relatively shallow pitch caused me some concern, but like jcm@gwr, the entire structure was built with materials that just happened to be "lying around". Until very recently my garden has resembled a builders merchant with everything from 110mm drainage pipe to 4.8m 6x2s lying around!

I've made a conscious decision that if the roof shows any sign of failure it will be replaced with something maybe more suitable. I guess time will tell.

I have to go now... I have to have a beer and then play with some Lego (my son's... honest...) Merry Christmas.

[jcm@gwr]

On the subject of materials 'lying around', the whole job

was remarkably cheap. I seem to remember that the

total cost was under £100 (more than half that was the

consumer unit and 30m of armoured cable).

I also forgot to say that the inside finish is plastered

(and painted), mainly for practice before I did certain

jobs around the house.

Jeff

[chris p bacon]

A most interesting topic with some very useful advice. I'm currently in the process of constructing an outbuilding for a railway room, so the insulating information is very relevant to me. I have trench fill footings with a brickbase up to DPC Level then a timber frame with plywood cladding and the roof is Marley Fibre Slates. My main concern now is the amount of water I have in the base of the building (about 3"  deep!!); I intend to have wooden joists and Chipboard suspended flooring. I have Air Vents at each end of the building.

You need to get rid of the water, rather than try and keep it out by mass filling the oversite why not dig a french drain to stop it filling ? is your room in a dip or low part of the garden or do you have a high water table ?

Dig a trench through the oversite and backfill with Shingle or gravel when you are outside of the footing you can change the gravel to a 110mm drainage pipe, this must have a  fall to a soakaway sited as far from the room as possible.

If you have a high water table then it would be easier to Line the oversite with a DP membrane and backfill with a lean mix of concrete 100-150mm thick, this will create a dry area (sort of swimming pool in reverse) you can still set a suspended floor over it as long as it has ventilation underneath.

 

As for exposed ply as a finish, I'm afraid this is not long term, you must cover the ply sooner rather than later with a breathable membrane such as Tyvek and then use shingles, featherdge or shiplap as a covering. Ply or Sterling (OSB) board will delaminate quite quickly when exposed to sun and rain, water gets in the end grain at joints and capillary action draws it along the internal structure until you see it wrinkling, by then it's too late. I doubt it is "Marine grade ply" as if you could afford that you'd be on Gauge 1 live steam in the garden anyway

 

Chris Dave

[Pannier Tank]

You need to get rid of the water, rather than try and keep it out by mass filling the oversite why not dig a french drain to stop it filling ? is your room in a dip or low part of the garden or do you have a high water table ?

Dig a trench through the oversite and backfill with Shingle or gravel when you are outside of the footing you can change the gravel to a 110mm drainage pipe, this must have a  fall to a soakaway sited as far from the room as possible.

If you have a high water table then it would be easier to Line the oversite with a DP membrane and backfill with a lean mix of concrete 100-150mm thick, this will create a dry area (sort of swimming pool in reverse) you can still set a suspended floor over it as long as it has ventilation underneath.

 

 

Chris Dave

Hello Dave,

 

Thank you for the valued advice.

 

My outbuilding is approx 7.5M x 2.5M; the ground is sloping and drops by 150mm over the 7.5M length. When digging the footings I was aware of the high water table and it is now confirmed with the building filling with water. I've just been out to the building and after a couple of days of dry weather there are no puddles in the building, the 50mm or so of loose ballast is just a bit damp as are the walls up to DPC Level. So the water does drain away, when the lawns are saturated so is the inside of the building. I have two vents the size of one brick in each end of the building.

 

It is my intention to clad the building in Shiplap or similar, the painting of the plywood was just to keep the building water tight whilst I get the internals finished. 

[chris p bacon]

Hello Dave,

 

Thank you for the valued advice.

 

My outbuilding is approx 7.5M x 2.5M; the ground is sloping and drops by 150mm over the 7.5M length. When digging the footings I was aware of the high water table and it is now confirmed with the building filling with water. I've just been out to the building and after a couple of days of dry weather there are no puddles in the building, the 50mm or so of loose ballast is just a bit damp as are the walls up to DPC Level. So the water does drain away, when the lawns are saturated so is the inside of the building. I have two vents the size of one brick in each end of the building.

 

It is my intention to clad the building in Shiplap or similar, the painting of the plywood was just to keep the building water tight whilst I get the internals finished. 

As it's a High water table then it's a bit pointless digging a drain,  I did forget to put that as long as the underside of the floor is at least 150 -200mm above the highest point of the ground then it shouldn't be a problem.

One thing you could do it place some battens/offcuts on the underside of the joists and infill with panels of polystyrene floor insulation ( one name is "Jablight") this is the cheapo board not to be confused with Celotex and is a 1/4 of the cost. you'll only need about 5 sheets and this will give you an insulated floor.

 

If you need one I'll see if I can do a sketch or something.

[Pannier Tank]

As it's a High water table then it's a bit pointless digging a drain,  I did forget to put that as long as the underside of the floor is at least 150 -200mm above the highest point of the ground then it shouldn't be a problem.

One thing you could do it place some battens/offcuts on the underside of the joists and infill with panels of polystyrene floor insulation ( one name is "Jablight") this is the cheapo board not to be confused with Celotex and is a 1/4 of the cost. you'll only need about 5 sheets and this will give you an insulated floor.

 

If you need one I'll see if I can do a sketch or something.

First of all apologises to Pete for hijacking this thread.

 

The ground level is at least 300mm from the DPC Level, so what do you reckon I should do. Apply 100mm of ballast on top of the ground to try and soak up some of the water or go with concrete on polythene membrane?

 

It is my intention to fill between the joists with some form of insulation in the manner you describe.

 

I appreciate your help and advice has this project has cost me more than a few hours in lost sleep!

[Pete 75C]

First of all apologises to Pete for hijacking this thread.

Not a problem!

I'm pretty much sorted - I'm finding your dilemma far more interesting than mine ever was!!! Good luck.

[chris p bacon]

First of all apologises to Pete for hijacking this thread.

 

The ground level is at least 300mm from the DPC Level, so what do you reckon I should do. Apply 100mm of ballast on top of the ground to try and soak up some of the water or go with concrete on polythene membrane?

 

It is my intention to fill between the joists with some form of insulation in the manner you describe.

 

I appreciate your help and advice has this project has cost me more than a few hours in lost sleep!

I wouldn't worry too much as you have plenty of room between the underside of the joists and the groundlevel, sometimes we put a DP membrane over the ground held down with a bit of shingle, this cuts down mould growth on the wet ground, or you can spread a bit of lean mix over it. You won't stop the ground water but with a 300mm air gap it shouldn't present any problems for you, you could for a bit of extra vent put 2 more brick sized vents along the 7.5M, the more the merrier.

 

Along with Pete you have the makings of a top notch room coming along there, look forward to seeing what happens on the inside.