Converting wooden workshop to office

[Adthrawn]

Looking for some advice…

 

Have a wooden workshop building, that’s been built with 2x4s, 8-10mm internal plywood skins, insulated voids (fibreglass? Not checked), and external horizontal lapped cladding. It’s about 12-15 years old, but in excellent condition (and seems to have been built pretty well). Has an industrial roof as well, made of 100mm insulated metal panels. 

 

I’m converting to be an all-year office/studio space, and looking for some advice/reassurance that my plans are sound. 

 

There’s a musty smell - the majority of the smell actually reduced when I removed old workbenches and shelving. The internal walls don’t feel damp - there’s no damp or mould etc. Theres also no airflow/trickle vents - one job will be to add some kind of venting. 

 

The plywood skin is screwed in, and there’s sealant etc so I’m loathed to pull the internal skin off; the cladding nails feel loose, so I think removing boards externally will be much easier, and I can repair/replace any external boards at the same time. I’ll be able to see the inside of the plywood and frame, so if any do need replacing, it can be addressed.

 

So my plan is to take the cladding off, remove the old solid plastic membrane and replace with a proper tyvek/building paper for moisture control and breathing. The old membrane can be seen in gaps, and appears to have degraded in a few places, albeit, it seems a good/thick membrane. 

 

In the process, I’ll check the internal wood frame for damage, moisture, woodworm etc and treat it as well. I’ll likely spray woodwork preventive treatment into the frame as well. 

 

I’ll strip the old insulation out, and replace with PIR insulation panels.

 

The plan will be to skin the existing plywood skin with plasterboard; screwed directly on use PB screws. I’m assuming I should use plain plasterboard and not one with a foil back etc?

 

I’ve taken shelves mounted to the plywood off, and the screws were all rusted were they were in the plywood. Is this typical, or indicative of damp/trapped moisture? We’ve only just moved in, and the weather has been pretty good, so I have no idea how damp the building is. It feels pretty dry up to now. 

 

The frame appears to be sitting directly on a concrete base, no plastic etc under the frame. The concrete pad is about 6” above ground level at its lowest point (and about 12” on the furthest side). There has been Ivy ingress on one corner (I’ve removed all traces externally), but doesn’t appear to have been any water ingress. 

 

So, I’m hoping that the <cladding> <vapour barrier> <insulation> <plywood> <plasterboard> order will work to provide good insulation and minimise damp/moisture? Plus ventilation!

 

• what kind of membrane should I fit? I’ve seen recommendations ranging from breathable to solid plastic sheets, to no membrane and relying on the foil on the PIR plus foil tape over the wood frame. 
• Where should the membrane be? On the outside of the insulation, or on the inside?
• Being 4” framing, I’m planning to use 90mm PIR (I’ve heard that 100mm doesn’t fits into 4”  frames, so 90mm works best. 

 

I’ve been reading datasheets for insulation, but any mention of wood frames is related to brick cavity walls with internal wooden walls, so hard to get a feel for the correct assembly sequence. 

 

Thanks!

[tonyshouse]

DO NOT PUT POLY ON THE COLD SIDE OF THE INSULATION!  Vapour control needs to be on the warm side. if poly it needs to go I prefer no membrane but you could use breather layer. 
 

inside I would add insulated plasterboard or insulation and plasterboard, Best to get this from the US as sounds like 24” centres and we use 600c/c 

 

thicker the insulation the better and so could you add more externally? 
 

 

[Adthrawn]

51 minutes ago, tonyshouse said:

DO NOT PUT POLY ON THE COLD SIDE OF THE INSULATION!  Vapour control needs to be on the warm side. if poly it needs to go I prefer no membrane but you could use breather layer. 
 

inside I would add insulated plasterboard or insulation and plasterboard, Best to get this from the US as sounds like 24” centres and we use 600c/c 

 

thicker the insulation the better and so could you add more externally? 
 

 

Thanks!

 

I don’t know the stud centres yet, I should probably measure the screws in the plywood. I’m U.K. based, so would hope the guy who built it did 600mm. 
 

Regards to vapour/breather membranes, which is better to fit? An external breather or internal vapour, or both? If I use PIR, then effectively I have a vapour barrier with the foil backing?

 

I could look at insulating the frame, and also fitting plasterboard with integral insulation, but then with the plywood sandwiched in there, where does the vapour barrier go? Should the vapour barrier go under the plywood layer, as there’s also foil backed plasterboard that could be vapour barrier, but again that’s over the plywood. 
 

Making sure the place stays dry is the objective, so whether that’s no membranes or putting new membranes etc somewhere - I just don’t want to end up causing/continuing issues by putting something in the wrong layer. 

🥴

 

[Adthrawn]

I’ll also add, I may need to work from the inside if I’m not going to get a straight run to do outside (kids etc, so don’t want to leave exposed). I’m hoping I can cut the barrier from the inside, and pull as much from between the frame edges and the cladding. 

[tonyshouse]

Cutting and removing poly is ok 

plywood is 4’ wide 🙂

vapour barrier warm side, if pir then damp will go through studs and the warmer and longer it is warm inside the more this will happen 

you could poly on ply, I would do re insulate between, ply on again, more sheet insulation, poly vb, plasterboard 

 

I would do a floating floor with vb on top and joined to one in wall

[Adthrawn]

On 03/05/2022 at 08:58, tonyshouse said:

Cutting and removing poly is ok 

plywood is 4’ wide 🙂

vapour barrier warm side, if pir then damp will go through studs and the warmer and longer it is warm inside the more this will happen 

you could poly on ply, I would do re insulate between, ply on again, more sheet insulation, poly vb, plasterboard 

 

I would do a floating floor with vb on top and joined to one in wall

Hi,

 

Is it worth putting a breathable membrane under the cladding? Or just cut the poly out, re-insulate the studs, and put a poly over the ply?

 

So I’m guessing the objective is to leave the insulation and the studs open to the external air, and stop moisture from passing into the plasterboard/top layer (we might skin in plywood as the decor finish)?

 

Your comment regarding the PIR - is this a bad thing that the damp will go through the studs? Or do you mean that it will need to be exposed in order to dry out?

 

Thanks

[tonyshouse]

I think no breather membrane

 

need a vapour barrier to prevent moisture getting into wood studs and plates no problem with pir except the cost and difficulty of working with it 

 

 

[SimonD]

On 03/05/2022 at 07:29, Adthrawn said:

I’m U.K. based, so would hope the guy who built it did 600mm. 

 

If so, he/she would have had to order 2400 x 1200 plywood sheets. Most suppliers still stock 8' x 4' so you end up with 16 or 24" centres. It's so annoying... you can however just batten horizontally to the inside of the plywood at 600 centres, a space you could use for more insulation and services.

 

If any help, I have an all seasons garden office I insulated with sheepswool insulation, no vapour barrier, no breather membrane, just osb to the inside of the frame. It's held up fine for years.

Edited May 4, 2022 by SimonD

[AnnieG]

On 04/05/2022 at 19:21, SimonD said:

 

If so, he/she would have had to order 2400 x 1200 plywood sheets. Most suppliers still stock 8' x 4' so you end up with 16 or 24" centres. It's so annoying... you can however just batten horizontally to the inside of the plywood at 600 centres, a space you could use for more insulation and services.

 

If any help, I have an all seasons garden office I insulated with sheepswool insulation, no vapour barrier, no breather membrane, just osb to the inside of the frame. It's held up fine for years.

I am looking at converting a large shed into an office. It is cedar lap on 2x4 struts and seems very solid. I’d like to use sheep wool insulation, but was assuming I would have to line the inside first. I’d be very happy if that was not the case!

So if I understand you correctly, you have the outer wooden shell, then insulation and osb forming the walls on the inside? Nothing more?

 

 

[Iceverge]

 

 

40 minutes ago, AnnieG said:

I am looking at converting a large shed into an office. It is cedar lap on 2x4 struts and seems very solid. I’d like to use sheep wool insulation, but was assuming I would have to line the inside first. I’d be very happy if that was not the case!

So if I understand you correctly, you have the outer wooden shell, then insulation and osb forming the walls on the inside? Nothing more?

 

 

Pics always help. 

 

What level of use do you expect to get from it? Will it be used 40hrs per week or 30 mins every couple of days? 

 

Sheep wool is a nice product but if you just shove it between the studs as is (assuming no membrane externally) , it'll get wet and infested with creepy crawlies. Also your timber frame and cladding will rot and decay without proper ventilation. Post some pics and a solution will be forthcoming!! 

[SimonD]

23 hours ago, AnnieG said:

So if I understand you correctly, you have the outer wooden shell, then insulation and osb forming the walls on the inside? Nothing more?

 

Yep, that's correct. It's obviously not the recommended buildup and due to various building delays has been in use for a lot longer than imagined and planned. My upcycled garden office has just a slightly different filling of insulation in that the studs weren't deep enough to accommodate 100mm of insulation so I installed two layers of 50mm, one between the studs and the other within counter battens to the inside, then the osb. It gets used, fully heated for upwards of 60 hours per week, has never had a problem with condensation and my wife says it's comfortable all seasons.

 

I've used the garden office as a bit of a test to see if it does attract bugs and stuff, including leaving a gap created by damaged cladding where I can see the insulation. It's still there, intact and no sign of any bug activity anywhere and no damp. One spring there were some little birds that pulled some out from this gap, which I later found in a nest in the bushes, must have been very comfy for them.

 

I used Thermafleece which isn't a pure wool as it uses a percentage of recycled polyester. Very happy with how it has performed.

 

In the house I have almost the same buildup but it adds a breather membrane and ventilation space behind the cladding which is really how it should be done.

[Adthrawn]

I've been back and forth on what to do, but thinking of something like this:

 

[external cladding] -- [2x3" frame]+[50mm PIR inside frame] -- [18mm plywood] -- [vapour barrier] -- [62.5mm PIR insulated plasterboard]

 

I want to avoid removing the existing 18mm plywood internal walls, as the frame was built with it in panels, and has been overlapped, so in corners I'll have to cut the plywood in order to remove it. Instead, I'll just be removing the external cladding, pulling the old membrane out and removing the old insulation. I'll check the inside face of plywood and the frame as well. The old membrane is definitely in the wrong place, so that really has to go.

 

I then plan to fit 50mm PIR into the frame, pushed against the internal plywood skin - and PU expanding foam in gaps. That'll give me a 20mm air gap between the cladding and the PIR. I'm hoping the exposed frame is enough to allow moisture in the frame and plywood to dry. At the top of each frame void, I'm planning on cutting a ventilation vent, with something like a 50-60mm vent. I don't think I need a bottom vent, as the overlap of the cladding isn't amazing anyway so should air out. If there are noggins, need to figure out how to allow ventilation around them.

 

Over the internal plywood skin, I'll fit a membrane - and will seal that with the membrane I'll lay on the concrete pad (I've made assumption it's not insulated). I'll then put 50mm PIR onto the floor, and will fit 50mm insulated plasterboard (it's 62.5mm finished thickness) over the plywood.

 

Now, my next concern is if the plasterboard needs to be kept away from the vb/vb away from the plywood layer? The drawings I've seen show the plasterboard on the vb, which is fine - but rather than having OSB etc on the outside of the frame, I've got plywood on the inside of the frame. I don't know if I'm trapping the plywood to the degree it'll never be dry... Or if the exposed timber frame will be sufficient to allow the plywood to dry out.

 

Frustratingly, the manufacturers provide very contradictory information - Kingspan's site says one thing in the PDF, their calculator shows completely different installation, and then advice is differing again.

 

 

[Adthrawn]

I've drawn up the plan of attack...

[Nickfromwales]

2 hours ago, Adthrawn said:

I've drawn up the plan of attack...

Better than my shed 😭. I wish I’d insulated. Tbh, I’ve only feather-edged one 1/4 so I may pull that off and just stuff the 4x2 frame with rock wool. Either that or pour vermiculite into the voids and just remove the top couple of planks. 
 

Anyone know if vermiculite has any reasons to not use it? Do vermin like the stuff etc?

[Adthrawn]

14 hours ago, Nickfromwales said:

Better than my shed 😭. I wish I’d insulated. Tbh, I’ve only feather-edged one 1/4 so I may pull that off and just stuff the 4x2 frame with rock wool. Either that or pour vermiculite into the voids and just remove the top couple of planks. 
 

Anyone know if vermiculite has any reasons to not use it? Do vermin like the stuff etc?

From what I’ve read up, ventilation in the insulated void is key. I think a rock wool is your best bet. You could always internally insulate as well, and put a vapour barrier over it (or just seal the edges of PIR with silver tape). 

[Adthrawn]

I’ll add, that the conversion for us is to turn it into a proper office/studio building - we’ll have print equipmen in there, and some needs to be kept

around 20C. I’m trying to do what I can to retain as much heat as possible. 
 

Even now, during very hot days, it’s warm, but it’s not hot. That gives me hope that during winter the heat loss won’t be too bad. With the refurb we’re doing, it should perform better than our concrete block/steel frame office in a business park 😂

[Iceverge]

Following the example of the States I don't think there's anything wrong with an air barrier external to the insulation provided the wall can dry to the inside and you do a top job on airtightness. In fact I think @IanR has an external airtightness layer, albeit with a breathable board outside. 

 

If the existing structure is ok I would avoid dismantling it, especially with kids and limited time. Building has a habit of taking 3 times longer than you expect. 

 

I would install the final windows and doors in the structure as is. Make your own blower door fan and make the external plywood totally sealed with tape at all the joints and to the floor. This isn't hard and is ideally suited to the DIY'er. Given it's your own project you're lightly to get a top result too.  

 

Then build a stand alone stud wall (63mm CLS minimum) stood off the ply internally by say 100mm. (can be increased/decreased) This will create a handy void for all your services. Fill this with mineral wool/ rockwool insulation and the gap between the studs. 

 

Finally plasterboard internally. You may need vapour retarder plasterboard/ foil backed to keep the vapour in check. 

 

To really add polish use blown cellulose  in the void. 

 

Pretty cheap and very easy to do in bits as it's all internal. Equally importantly, given your busy life at any point in the progress you can close the door and walk away as all the work is internal. 

 

As for the floor, Insulation board (as much as you can fit) and then a floating floor or 2 layers of staggered OSB. If you have the head height you could use a pumped screed with UFH. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Adthrawn]

On 27/05/2022 at 18:46, Iceverge said:

Following the example of the States I don't think there's anything wrong with an air barrier external to the insulation provided the wall can dry to the inside and you do a top job on airtightness. In fact I think @IanR has an external airtightness layer, albeit with a breathable board outside. 

 

If the existing structure is ok I would avoid dismantling it, especially with kids and limited time. Building has a habit of taking 3 times longer than you expect. 

 

I would install the final windows and doors in the structure as is. Make your own blower door fan and make the external plywood totally sealed with tape at all the joints and to the floor. This isn't hard and is ideally suited to the DIY'er. Given it's your own project you're lightly to get a top result too.  

 

Then build a stand alone stud wall (63mm CLS minimum) stood off the ply internally by say 100mm. (can be increased/decreased) This will create a handy void for all your services. Fill this with mineral wool/ rockwool insulation and the gap between the studs. 

 

Finally plasterboard internally. You may need vapour retarder plasterboard/ foil backed to keep the vapour in check. 

 

To really add polish use blown cellulose  in the void. 

 

Pretty cheap and very easy to do in bits as it's all internal. Equally importantly, given your busy life at any point in the progress you can close the door and walk away as all the work is internal. 

 

As for the floor, Insulation board (as much as you can fit) and then a floating floor or 2 layers of staggered OSB. If you have the head height you could use a pumped screed with UFH. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Thanks for that!

 

The building is definitely trapping moisture inside - I don’t think the polythene sheet is helping matters. Ventilation will help of course. 
 

I’ve ordered both the PIR and thermal plasterboard (plus vapour barrier). Will be using the 50mm on floor too. 
 

Taking the cladding off, affords me the opportunity to check the structure. Ivy has gotten under the frame in a few places, plus there may be some damp patches too. As I’ll need to take the metal roof trim off as well, it’s also going to give me a chance to refit that and improve water tightness all round. I’m also fitting a large bifold door, so will have rework the front wall anyway!

 

I know we’re putting quite a bit of money into it, but it’s going to be our workspace for the next 5-7 years easily and we’ll have some specialist printers in there that need to be kept warm…

[Adthrawn]

On 27/05/2022 at 18:46, Iceverge said:

Following the example of the States I don't think there's anything wrong with an air barrier external to the insulation provided the wall can dry to the inside and you do a top job on airtightness. In fact I think @IanR has an external airtightness layer, albeit with a breathable board outside. 

 

If the existing structure is ok I would avoid dismantling it, especially with kids and limited time. Building has a habit of taking 3 times longer than you expect. 

 

I would install the final windows and doors in the structure as is. Make your own blower door fan and make the external plywood totally sealed with tape at all the joints and to the floor. This isn't hard and is ideally suited to the DIY'er. Given it's your own project you're lightly to get a top result too.  

 

Then build a stand alone stud wall (63mm CLS minimum) stood off the ply internally by say 100mm. (can be increased/decreased) This will create a handy void for all your services. Fill this with mineral wool/ rockwool insulation and the gap between the studs. 

 

Finally plasterboard internally. You may need vapour retarder plasterboard/ foil backed to keep the vapour in check. 

 

To really add polish use blown cellulose  in the void. 

 

Pretty cheap and very easy to do in bits as it's all internal. Equally importantly, given your busy life at any point in the progress you can close the door and walk away as all the work is internal. 

 

As for the floor, Insulation board (as much as you can fit) and then a floating floor or 2 layers of staggered OSB. If you have the head height you could use a pumped screed with UFH. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

What software are you using for calcs as well? I’m trying to figure out the layers, and where the vapour barrier should go, and if it’s okay to sandwich the existing plywood, a barrier and then thermal plasterboard together - or if there needs to be an air gap for the plywood. 
 

My principle aim is to reduce/mitigate damp AND massively boost the thermal properties of the building. 
 

I could put a 63mm gap between the plywood and vapour barrier with studs.  
 

Also addressing your comments about the existing external barrier - I wanted to remove it (mostly as it seems to be a potential cause for any damp there is now), but also my thought was to fit ventilation for the exterior void. I might fit a breather membrane in to stop water ingress. The cladding is not perfect, so water ingress is possible. 
 

I’m very confused now, and back to square one having thought I’d figured this out! Materials on the way too 🥴

Edited May 29, 2022 by Adthrawn

[Iceverge]

Can you cancel the materials?!

 

PIR is expensive and not suitable for anything other than floor insulation in my opinion. 

 

[SteamyTea]

Have you considered heating/cooling and humidity control for when it is finished?

This may change your wall buildups.

Air2Air heat pump combined with a basic MVHR unit may be all you need.

Basically make the inside airtight, then control that.

[Adthrawn]

5 hours ago, Iceverge said:

Can you cancel the materials?!

 

PIR is expensive and not suitable for anything other than floor insulation in my opinion. 

 

Yeah, I can cancel them tomorrow. I do plan to use some for the floor, so will keep that. 
 

The tricky bit is the current wall build up.

 

Starting with cladding/poly/void/plywood, I’m not sure now what to do. Not sure if I can loose 160mm internally by building out, but could be the best option now.

 

Either way, a big concern is outfitting internally to find the external shell has a damp issue etc. or all of the internal materials get damaged. 
 

 

[Adthrawn]

5 hours ago, SteamyTea said:

Have you considered heating/cooling and humidity control for when it is finished?

This may change your wall buildups.

Air2Air heat pump combined with a basic MVHR unit may be all you need.

Basically make the inside airtight, then control that.

It’s definitely something I’ll be fitting, but not quite sure what form it’ll be yet.

 

To what degree would that change the wall buildups?

[Iceverge]

9 minutes ago, Adthrawn said:

Not sure if I can loose 160mm internally by building out, but could be the best option now.

 

 

How much can you spare? 160mm was just a suggestion. More is better but any is better than none. 

 

You could remove one or two external cladding boards and have a look to see the inside. If its ok I'd leave well enough alone. If there's significantly moisture ingress you'll need to solve that long before think I about insulation. 

 

Maybe you could post a few pics of the wall as it is, internally and externally. Plenty of sets of knowing eyes here. 

 

Make sure to capture the floor wall junction and the roof/wall junction both internally and externally. 

 

 

 

 

 

 

[Adthrawn]

1 minute ago, Iceverge said:

 

How much can you spare? 160mm was just a suggestion. More is better but any is better than none. 

 

You could remove one or two external cladding boards and have a look to see the inside. If its ok I'd leave well enough alone. If there's significantly moisture ingress you'll need to solve that long before think I about insulation. 

 

Maybe you could post a few pics of the wall as it is, internally and externally. Plenty of sets of knowing eyes here. 

 

Make sure to capture the floor wall junction and the roof/wall junction both internally and externally. 

 

 

 

 

 

 

I’ll post some pics in a minute…

 

The plywood is painted internally - if that’s taped at joints, will that not create a trapped void between the existing poly sheeting under the cladding and the ply?