Another Insulation Post

[spearson92]

Hi guys,

Just me again. I've pretty much finished my workshop now, and starting to look to the inside construction, but first I have a couple of questions:

1) The wall construction is (outer to inner): featheredge cladding, battens, breathable membrane, OSB, stud framework (no insulation or internal cladding/boarding yet. Question is: have I correctly added OSB to the outer side of the studwork?

From videos and pictures I have seen of shed construction, all other projects seem to do the same however, looking at posts on this site, it suggests that by doing this, and then adding insulation in between studs then boarding, that moisture could get trapped on the inside part of the outer layer of OSB, and run down creating damp and rot. I guess that the answer to that question partly depends on my next question which is...

2) what is the best way to insulate the shed to give best value for money? I don't want to go overboard on the cost with insulation, but don't want to go too cheap so as it doesn't sufficiently insulate the place. With the wall construction as above, is there any other way I should be insulating between studs so as to prevent rot?

Sorry for bringing up the same old questions!

Cheers,

Steve

 

[Woody2Shoes]

Hi - you need a vapour-impermeable (a 'vapour-check') barrier on the 'warm' (that is, inside) side of any insulation. OSB is considered vapour-impermeable, but since you've got it on the "wrong" (of course there's nothing wrong with it!) side of any insulation you put between the studs - you'll need to add another vapour-barrier. The whole idea is to stop warmer (moisture-bearing) air from inside migrating into the structure and cooling on its way towards the outside and dropping excess moisture on the way - https://en.wikipedia.org/wiki/Interstitial_condensation

As a vapour barrier you could use, for example: polythene sheet sealed with tape, foil-backed plasterboard, or more OSB (say 9mm or 12mm).

As far as insulation is concerned, the best insulation value for a given thickness (U or R value) is probably foil coated polyisocyanurate board (Kingspan/Celotex) e.g. https://www.insulationsuperstore.co.uk/ ... lotex.html

It's not the cheapest, but you get what you pay for, but - for me - money spent on the best insulation is not wasted.

It's worth paying attention to both airtightness and ventilation if you're going to do the best job you can.

Cheers, W2S

PS given that you're probably in a relatively exposed location (in Inverness) your first outer layer of OSB is probably a really good idea - making the whole structure very robust.

 

[spearson92]

Hi W2S,

Thanks for the quick reply!

Ok that's good to know, I suddenly had a panic thinking that the outer layer of OSB would cause me problems in the future by trapping moisture, but panic over

My plan as you say would be to use OSB internally, with insulation of some description in between the studs. I know with wool type insulation you have to ensure an air gap, is that the same with foam insulation as well? Or can you stuff the whole cavity with foam board (Kingspan etc) and then OSB?

Cheers,

Steve

 

[Woody2Shoes]

Hi - I think you could fill the gap between the studs completely if you wanted to, but two thoughts cross my mind:

1) the studs will probably be CLS sizes or similar so, for example, a nominal 100mm will be say 95mm - the celotex on the other hand would be exactly 100mm with an annoying discrepancy to get rid of with firring pieces/battens or somesuch.
2) You might find it handy to have a small gap between the insulation and the OSB for wiring etc.

So, maybe an option would be to go with say 80mm Celotex between the studs - with gaps sealed with PU gun foam (helps with airtightness which makes a huge difference overall) - and then say 12mm OSB3 on top directly onto the studs (you could add a polythene vapour check membrane under the OSB as belt and braces but it would probably be overkill e.g https://www.insulationsuperstore.co.uk/ ... -roll.html ).

That's what I'd do anyway! Cheers, W2S

 

[spearson92]

Hey W2S,

That seems like solid advice to me, so will take that on board and do that.

Thanks for the help, much appreciated!


Cheers,
Steve

 

[MikeG.]

.......Question is: have I correctly added OSB to the outer side of the studwork?.......

No, it is very much the wrong way if you are proposing to heat the workshop and work in it during the winter. You have effectively put the vapour barrier outside the insulation (ie on the cold side of the insulation), when it should be on the warm side (the inside). People you watch on Youtube might well live in different climates, but more likely, are just ignorant of the issue.

W2S has indicated the correct redemptive procedure......a less permeable vapour barrier on the inside of the studs. However, this is very much a second-best solution, particularly in a workshop, because there will be all sorts of penetration s of this for electrics etc, and from everything that gets screwed to the wall.

 

[Rorschach]

If he didn't put OSB on the studwork though, what would his insulation have gone up against? The back of the featheredge boards?

 

[MikeG.]

No, the breather membrane (or building paper). This is critical for forming a ventilated void behind the cladding, and for keeping the frame dry in all circumstances.

 

[Benchwayze]

My garage shop has only one outside wall. West facing, cavity wall, single-brick and thermalite inner courses. It's about four feet from the next door property and is clearly sheltered, except when it's very windy, when it becomes a wind-tunnel. Would insulation help at all. At the moment I don't get any condensation in my shop, due mainly I suspect to the metal, south-facing up and over door .
which gets blister hot in the summer and still collects heat in the winter.

John (hammer)

 

[MikeG.]

It'd help a bit, but frankly the ventilation heat losses from around an up-and-over door are probably a bigger issue in the winter.

 

[Benchwayze]

Thanks Mike.

I am leaving well alone. I can work in there at any time of year in relative comfort. If it's extremely cold weather I have a heater, and in summer I have to have the door open or I'd fry. Yet still I get no condensation despite the heat exchanges. My bench is well toward the rear of the shop of course and not too close to the open door. Planing soon gets a sweat on!

John (hammer)

 

[Rorschach]

No, the breather membrane (or building paper). This is critical for forming a ventilated void behind the cladding, and for keeping the frame dry in all circumstances.

So what about houses that use sheathing over the framework? Every house in the US is built that way.

 

[MikeG.]

Yep, and it's a massive store of potential problems they're creating for themselves. They're building standards are so crap that no-one should be trying to copy them.

 

[spearson92]

Thanks Mike,

It's my first project and safe to say I have made a few mistakes but definitely learnt some lessons.

I shall have to go down the preventative route as Woody suggested. I'm not sure about heating it during the winter at this point in time (although I know will still be cold in the winter). If I don't heat the workshop at all I guess there will be a lot less risk of damp on the inside of the exterior layer of OSB, owing to no heat actually escaping the workshop?

Cheers,
Steve

 

[Rorschach]

Yep, and it's a massive store of potential problems they're creating for themselves. They're building standards are so rubbish that no-one should be trying to copy them.

I'm confused, you wanted to stop moisture ingress, which he has done with the outer layer of OSB, and the membrane on the outside of that will help stop the OSB getting wet and he has battens providing a vented void behind the cladding. As long as he makes sure he has a vapour barrier on the inside too he is fine. The insulation is protected from moisture in both directions then.

According to this page his wall construction is correct.

https://www.designingbuildings.co.uk/wi ... _buildings

 

[MikeG.]

Thanks Mike,

It's my first project and safe to say I have made a few mistakes but definitely learnt some lessons.

I shall have to go down the preventative route as Woody suggested. I'm not sure about heating it during the winter at this point in time (although I know will still be cold in the winter). If I don't heat the workshop at all I guess there will be a lot less risk of damp on the inside of the exterior layer of OSB, owing to no heat actually escaping the workshop?

Cheers,
Steve

The thing to understand, and which most people don't get, is that a heated building is wetter on the inside than the outside.

......I'm confused, you wanted to stop moisture ingress, which he has done with the outer layer of OSB, and the membrane on the outside of that..........

Warm air holds more moisture than cold air. A lot more. Because 20 degree air can hold a lot more moisture than 0 degrees air, condensation occurs at the point where air is cooled down such that it can no longer hold all the moisture it was holding. So if you think about your wall...........on one side it will be say 20 degrees C, and on the outside it will be, say, 0 degrees C. Somewhere in the depth of the wall there is a point where condensation would occur if there was any moisture in the air within the wall. So, there are a couple of strategies for keeping the wall dry: firstly, don't allow any moisture in from the wet side (the inside), by putting in a vapour barrier (nowadays a Vapour Control Layer -VCL-, because it is impossible in ordinary construction to provide a complete barrier). Secondly, allow any vapour that gets in to the wall (and we've just seen the impossibility of keeping it out) to get out of the wall again into fresh air on the outside. Those constructions which have OSB on the outside disallow the latter, leaving you with only one strategy for fighting interstitial condensation.

Because of service penetrations and stuff fixed to walls, identical vapour-resistant layers on the inside and the outside of the wall will have differing performances. The one on the inside is almost universally less effective, so you have the situation where moisture is allowed into the structure from the wet side, and let out from the outside at a slower rate. Again, this is a recipe for interstitial condensation, which almost always manifests itself as moisture condensing on the inside of the external sheathing. This can then run down the inside face of the board and gather on plates or noggins, allowing firstly the growth of mould and then secondly the development of rot. There is a rule of thumb, therefore, that the vapour resistance of the interior of a wall should be 5 times that of the exterior of a wall (there's a whole essay on where the exterior of a wall is, BTW).

Your wall, when finished, will have roughly the same vapour barrier inside and out, albeit the interior one will be compromised as I've described. Moisture will get in, and will be trapped in. In an irregularly heated space with a poor wall design used intermittently in winter the best way of ensuring the long term health of the structure is to have good ventilation so as to remove the warm damp air. This can amount to little more than letting all the heat out when you've finished in there by, say, leaving a top light open in a window, or running a slow extraction fan, or running a dehumidifier, or opening hit and miss grills, or somesuch other. The fact is that these regimes will always rely on human behaviour, and so will never be fully followed. When you sell the house, the next guy isn't going to have read this post and is not going to have a clue how he should use the building. This is why it is better to design the problem away in the first place, and why I advocate using sheathing on the inside of the wall and a membrane on the outside.

 

[Woody2Shoes]

Hi Mike - point taken - I guess you're not a fan of SIPs then (accident waiting to happen?) which usually seem to be a PU sheet sandwiched between two OSBs. If the celotex is properly installed in an airtight way (and I realise that's a reasonable sized if 99% of the time) surely that helps to keep moisture migrating outwards? My motto is 'where there's a temperature difference, there's a vapour pressure'. Cheers, W2S

 

[spearson92]

Thanks for the posts guys!

Based on what you've said Mike, would it help at all to drill a small ventilation hole in each cavity, just above the noggins (being careful not to penetrate the membrane), so that if/when any moisture does build up behind the outer layer of OSB, it can evaporate quicker?

Cheers,
Steve

 

[Woody2Shoes]

Thanks for the posts guys!

Based on what you've said Mike, would it help at all to drill a small ventilation hole in each cavity, just above the noggins (being careful not to penetrate the membrane), so that if/when any moisture does build up behind the outer layer of OSB, it can evaporate quicker?

Cheers,
Steve

I'm not Mike - I know! - but my 2d's worth is that: 1) it would be almost impossible to do without damaging the membrane; 2) halfway up (I assume that's where your noggins are) would be the least best place - I'd say top and bottom to allow a convection circulation of air; and 3) how would you arrange your celotex to allow circulation of air on the cold side of it - quite tricky. What you're currently planning to do is no worse than a lot of buildings that get built all over the place - especially in Scotland, I suspect - (Mike is pessimimistic about their long-term prospects, and time will tell) - as we've both mentioned - good ventilation and airtightness are helpful mitigations. Cheers, W2S

 

[spearson92]

Hi Woody,

I see your point, I'll go with the original plan then.

You're absolutely right though, all the new builds across the road have OSB exteriors and then cladding or some sort of rendering on them.

If it can go a good few years I can accept that. The next workshop will just have to be bigger and better!

Thanks for the advice Woody and Mike, much appreciated!

Cheers,

Steve