Garden Room Build - Cladding and Ventilation

[efunc]

Anyone here built one of these, or something similar. I'm halfway through a small Garden Room which is a lean-to structure against the corner of my garden. This is how it stands now and I'll be fitting vertical T&G larchwood cladding:

 

 

 

 

 

 

 

I was starting to think about ventilation and just realised I've probably got the wrong end of the stick so need to get back to basics and re-learn what I need to do here. There are 3 main structural cavities in the building:

 

1 - The one between the outer timber cladding and the exterior OSB face. (I'm just using 19mm battens, not counter-battens, before fitting vertical T&G larchwood cladding)
2 - The one between the interior OSB face and the PIR insulation. About 15 - 20mm in my case
3 - The room itself

 

The only ventilation I was considering was just the interior of the room (3) with trickle vents on the doors and one 229x154mm 'passive' hit and miss vent. I'm now wondering if the additional vent is really necessary at all? Cavity 3 (the actual room) doesn't really need additional ventilation I believe for the room - it's more for the benefit of the inhabitants rather than the building itself, so trickle vents are sufficient and in the summer the doors can just be opened up. Is that right? I mean with an internal volume of about 15.5m3 how much airflow is required? If in the summer the doors could be opened out and overnight there will be nobody sleeping in it. Is it dangerous to have no other ventilation?

 

Cavity 2 functions in tandem with the insulation and should be air sealed, so no need to ventilate this.

 

It's Cavity 1 that I've probably got wrong. This should have been counter battened to enable unimpeded airflow from bottom to top. I'm just fitting horizontal battens however, onto which my vertical cladding will be secured. Ergo, no airflow. To try to improve this I propose cutting out several 2 inch gaps into the horizontal battens, just less than the width of a cladding strip, and also secure stainless steel rodent mesh at the bottom batten gap and at top one too. At the rear of the structure I won't fit cladding right up the rear back wall of the lean-to. Instead I'll leave a 1 inch gaps here and block it off with another strip of mesh. I'll also install a louvre vent in the cladding in each of the two side walls to vent cavity 1. None of this is quite as effective as counter-battens, but is it likely to be effective enough? Any better tricks I can employ instead? I don't want to do counter battens at this stage because of the additional wall thickness and the door/window sills are already going to be too short.

 

Thanks for any advice.

[the_r_sole]

.

Edited September 26, 2019 by the_r_sole

[andyscotland]

Does it have to be T&G cladding? I believe you can do board-on-board cladding without counterbattening although you need to chamfer the tops of the horizontal battens to direct water outwards.

[efunc]

1 hour ago, andyscotland said:

Does it have to be T&G cladding? I believe you can do board-on-board cladding without counterbattening although you need to chamfer the tops of the horizontal battens to direct water outwards.

 

Thank you both. Yes, it should have been counter-battened, however this hasn't been done. Partly due to the fact that the wall thickness would increase even more and my 150mm sills barely cover the cladding I'm doing now, etc. So what I was proposing was creating air channels in the horizontal battens to allow air to pass through. The only problem is that it might not be enough and I'm not sure how many or wide that would need to be. I would also leave the back open (where it leans against the brick wall) to allow air to pass through. Finally a couple or louvre vents in the cladding. I'm not sure what else I can do, or that will be enough.

 

Board on board cladding is a good call, but I've already got the T&G stuff here so I need to stick with it to manage the budget. 

 

I'm even a bit worried about installing a couple of louvre vents in the cladding too, since there's not much protection against rain coming in through them. But maybe these are better than no airflow at all.

[Ferdinand]

I would slot by trimming alternate ends to give a more thorough continuous air path, and make sure there was access bottom and top to give better coverage and some natural draught.

 

Particularly in the side panels.

 

F

Edited August 29, 2019 by Ferdinand

[efunc]

Thanks, I will try this.

 

Do you think these vents are OK to place towards the top and bottom of the cladding? I'm worried they may let in too much rain. Are there more suitable vents around?

 

[Ferdinand]

In my similar I used soffit vents.

 

[efunc]

You mean the 70mm little round ones, or the long vent strips?

Soffits are sheltered, so I'm not worried about those so much, these vents are for the side walls to vent the cavity behind the vertical T&G cladding. Normally that's vented behind the cladding along the top edge and bottom edge, not on the actual wall face which is pretty exposed to the prevailing wind, so just trying to think creatively here.

Edited August 29, 2019 by efunc

[Ferdinand]

2 hours ago, efunc said:

You mean the 70mm little round ones, or the long vent strips?

Soffits are sheltered, so I'm not worried about those so much, these vents are for the side walls to vent the cavity behind the vertical T&G cladding. Normally that's vented behind the cladding along the top edge and bottom edge, not on the actual wall face which is pretty exposed to the prevailing wind, so just trying to think creatively here.

 

I used multiple soffit vents in the bottom half of a cabin type sun lounge, half glazed half panelled.

 

F

[efunc]

Do you have a picture of this by any chance Ferdinand? Sounds interesting!

[efunc]

This academic study (Ventilation of the Cavities in Timber Framed Walls) from Glasgow University is very interesting, and seems to play down the need for ventilation it seems:

https://www2.gov.scot/resource/doc/217736/0090835.pdf

• “If no VCL is present, ventilating the cavity will not affect the risk of severe interstitial condensation.
• If an incomplete VCL is present ventilating the cavity behind timber cladding will make little difference.
• If there is a complete VCL, there will be no condensation whether the cavity is ventilated or not.”

 

So is this suggesting that the main source of moisture in the cavity is that which leaks from the room? What about atmospheric humidity and rain from outside?

 

"ventilation does not increase the drying rate of wet sheathing behind timber cladding." " If there are no rain impacts on external walls, the moisture content of external

cladding is determined by the ambient relative humidity and is little affected by ventilating the cavity. Cladding that has been wetted by rain impacts, will dry slightly faster if the cavity is ventilated, however the difference is not dramatic."

 

The recommendation is the following: "Ventilation at the base alone will be sufficient in areas which are not exposed to high levels of driving rain.  Ventilators top and bottom should be installed in highly exposed areas."

 

I must say I don't quite understand the findings of the report. How can ventilation at the base perform any function if the air doesn't circulate and leave somewhere?

Edited August 30, 2019 by efunc

[Ferdinand]

I don’t believe that; I think that an elasticated waist, a belt and suspenders is a good thing.

 

Or perhaps they did the study the Scottish season when having it ventilated to outside will make it even wetter rather than drying it ?.

Edited August 30, 2019 by Ferdinand

[Ferdinand]

On 29/08/2019 at 21:15, efunc said:

Do you have a picture of this by any chance Ferdinand? Sounds interesting!

 

Afraid not. I mean the little round 70mm ones.

 

I repurposed a ‘plastic panel bottom half’ conservatory as a sun lounge to avoid having to build any dwarf walls or foundations, and insulated all the surfaces, incl. the floor. Did the lower half of the walls by glueing on treated cls battens on the inside, then 50mm of taped joint Celotex, then finishing with plastic shiplap, with a windowsill above. Done such that each side across several panels was a single cavity.

 

Then I ventilated the void behind the Celotex to the outside with I think a soffit vent on each panel. This was 5+ years ago. If the cavity have been say 2m high I would have done two of them at say 225 and 1825 to get some natural flow; that is what I sometimes on similar cavities when insulating a half brick wall.

 

The theory was that should anything leak a little in my renovated 2nd hand conservatory, there would be a route out for the moisture from there and a degree of resilience built in. Did vaguely similar things for other aspects.

 

The only pooch in the pumpkin is that one of T’s dogs pulls them out and chews them, so I supplied a bag of ten spares which cost £2.99 and it has been quiet since. I did not glue them so that I would have access with a hoover just in case.

 

F

Edited August 30, 2019 by Ferdinand

[efunc]

Good theory there, glad to hear it's working out OK too.

 

My thoughts are turning back to whether or not I really need to ventilate the inside space now.

So... is it sufficient to rely on only trickle vents for the sole ventilation, or is that potentially dangerous? The room is 2.1m x 3.1m and 2.1m high, so about 15.5m3 in volume. It is well insulated all the way round with 100mm PIR and fully vapour sealed on the inside. None of the windows can open, only the doors, consequently the only ventilation possible, apart from opening the doors, is the two trickle vents. So the room is very small, pretty sealed, and will be used in mild weather with no boiling kettles, showers, moisture generating activities taking place, just a desk and chair. However I'm concerned about having oxygen to breath should anyone fall asleep or something for a few hours and only the trickle vents are open.

I did look at "Approved Document F Ventilation" on The Planning Portal yesterday and it seemed to only be concerned about kitchens, bathrooms, utility rooms, etc. Places where a lot of humidity might lurk. The only figures I could find was for the smallest space (<50m3) recommending a vent size of 35000mm2, so there doesn't seem to be that much requirement for ventilation. My trickle vents are 2 x 30cm long so that alone may cover things. The trickle vents amount to about 6000mm2 of vent space, which doesn't amount to much, but may be just enough for a room volume of 15.5m3. I could really do with some tables to confirm though.

Installing an additional little louvre (sliding, hit and miss vent), about 127x75mm is a possibility, but I don't want to have to cut into the wall unless it's dangerous just to rely on the 2 trickle vents. Anyone any ideas?

[Ferdinand]

You need to think about heat as well as ventilation.

 

My Ts sun lounge also has 100mm rockwool in the roof, and an aspect that does not get sun until mid afternoon (NW side of the house - deliberately). What she finds is that it is a 4 season room due to the insulation (also has a big rad) and ventilation (2 x 0.8m square opening windows at opposite ends), but that when it is just *hot* for several days in summer (eg as we had recently) the heat soak gets her and makes the doggies uncomfortable. Ventilation does not work because it is also too hot outside.

 

She has a portable air conditioner for this situation, which may get occasional use in the summer. This is a mini implementation of what @JSHarris just did with his aircon, and what I will be doing at home.

 

I also have a similar issue with my conservatory, which is well insulated and on the N side (the only place I would put a conservatory), and gets some sun in the afternoon. That runs up to a high temperature quickly for the short time it gets sun. I manage that because it is really a link room between lounge and kitchen, not a living room. That has a similar thing in that it is usable 3.5 seasons, and only not in winter as we choose not to use the heating - could be switched on eg for a party.

 

So shading helps make it useable for the vast majority of time, but you will have the same issue.

 

Most of the time it will be fine, but having gone for a "warm" garden room means that heating will be cheap in autumn/winter, but that you risk not being able to cool it when it is warmed up and being uncomfortable when the outside is say above 25-26C. I do not think however much ventilation will address that.

 

This is the problem that several BHers have found last year and this year. As your garden room is shed-size, it will have less time-buffer. My house can take several days before it becomes uncomfortable; a garden room should be less.

 

I would suggest considering your possible usage patterns (and others if it may be used differently in future) and consider:

 

1 - Arranging vents such that you can purge-vent securely overnight (consider height, wind, orientation) at opposite ends, to give you a nice morning temp during hot periods to let you partially use it.

2 - A couple of (second hand?) solar panels on the roof and a small split aircon, so that you can heat or cool when needed, and when you need to cool it is cost-free. Depends slightly if you have house solar. Run the numbers. Or a portable. Perhaps make provision, but wait until the first time you get lobstered to see if there is actually a problem in your circs.

 

Ferdinand

 

Edited September 2, 2019 by Ferdinand

[efunc]

Thanks, excellent suggestions. I wish I'd thought of some of this in the design phase. Now I'm just nearing completion and have sort of missed the boat. Still, plenty there to chew on. I like the addition of solar panels and A/C. I might be able to retrofit that next year after watching its use through the next four seasons.

[Ferdinand]

I actually left a 50mm gap right across the back under the flashing for ventilation, then used a wider than planned Ubiflex.

 

How are you damp proofing and insulating your rear wall? That is an obvious thing to ask.

 

If it is a garden wall you will need to lap your slab dpc up behing the skirting and screw not glue to make sure you can check it in say 10 years. And make that slice of floor removable so you can also check that. I ran electrics and gas in a void along the back for that reason, and left the rear (house) wall as brick so that it was easier to look after.

 

T asked what I was going to use to finish it - "wire brush then pva".

 

My design phase lasted until it was built, as it was a "learning about conservatories" project. I try and build one of everything myself.

 

F

 

Edited September 4, 2019 by Ferdinand