I want to add some insulation to an outbuilding. The external walls have a stud framework of 3 x 2 timber and the outside is clad in feather edge timber. I plan to add friction fit insulation panels and clad the inside in thin ply. Do I need to add a water resistant membrane before I fit the insulation? If so any suggested products?
The outbuilding has a pitched roof of slate with a rubberised felt under. I plan to fit friction fit insulation and then 12.5mm plasterboard with the silver film on. Again do I need a moisture barrier?
The soffits are currently open and will have to be finished off in some way. Is it best to leave them ventilated?
If you use celotex and tape the joins over the studs, that has its own WPM, but if you just use rockwool I'd use foil backed plasterboard, or, if you prefer, a sheet of DPM tacked up before you line the interior.
I have a similar situation with a barn. I planned to put a polythene membrane on the outside of the studs then Kingspan or similar finished with battens and feather edge.
OK so far but what if I infill the studs with more insulation and plaster leaving the studs part exposed?
Somewhere between the studs and the feather edge will be a dew point. Presumably if it is outside the polythene then OK, inside and NBG.
Is polythene membrane the same thing as polythene sheeting? If so then if you keep the building as a barn then it might be OK but if you want to turn it into a closed building then the polythene sweats and doesnt result in a good living space. CAT did just that on one of their early houses with the result that the windows are always covered in condensation and the residents end up leaving exterior doors open in all but the worst weather
The point with the modern approach to insulation, is to hermetically seal, and then punch such holes are are needed through the sealing, in order to guarantee the correct humidity level.
It is also important to put the vapour barrier in conjunction with INSULATION. otherwise it gets cold and actually CAUSES condensation.
If you like, the modern approach is to live inside a plastic or other waterproof box covered in insulation, with a carefully adjusted flow of air in to and out of it.
Get any bit of it wrong and its easy or people to knock the whole concept. Done properly it results in a warm dry house.
Yes. It would prevent moist air migrating through the walls.
Something similar was done to the farmhouse, here. You may remember I said that the builders had to clean the *Snowcem)* off the pargetting before adding exterior insulation.
I haven't fully grasped the problem with inter wall condensation. I understand that air above zero centigrade will carry moisture up to a level of saturation. As the temperature falls some of this moisture will condense out either as dew or on the nearest cold surface. In the case of a permeable wall, warm moist air is able to penetrate. Somewhere, within the wall, the temperature will not support that level of moisture and condensation will occur.
Advice in here seems to advocate preventing moisture from entering the wall and keeping the inside (warm moist side) above any possible dew point.
I guess condensation on windows points to inadequate glazing insulation /insufficient ventilation. It is not a problem here although all sorts of things might be happening behind the plaster:-(
The worst of all worlds is to have e.g. a timber frame with a vapour barrier outside it, and insulation inside it. And a floor with no DPM in it at all.
Moisture boils off the floor when the thing is heated, goes into the walls, through the insulation, and hits a cold impermeable surface and condenses.
Rots the frame in a few years. Adds lovely mould spores to the air as well.
Windows - especially single glazed windows - are a natural place for condensation as they represent just such an impermeable and cold surface.
Hence trickle ventilation slots above them to let cold dry air in from outside is conventional.
Also the lack of open fires in modern properties - which naturally sucked all the stuffy moist air up the chimney, replacing it with ankle level icy draughts from ill fitting windows and doors - has much to answer for.
Like I said, once you decide to do thing to a level of comfort you have to do it ALL.
That means forced extraction at points of maximum humidity - kitchens bathrooms and utility rooms. That means full insulation everywhere with the DPM inside of it ..and ventilation provide at the right level by judicious use of ventilation inlets..especially where fossil fuel burning appliances are near, and proper flue design to carry the water products of combustion outside and through the impermeable and insulative layers.
Barn conversions are fraught with possible ways to c*ck this up. They were never designed to do more than keep the rain and the wind mostly off whatever was inside them. At the lowest possible cost. And people then want to preserve the 'character' of living in a cowshed, by punching huge windows in them and plating them inside with T&G or the like.
I can't see the point frankly...if you want a timber structure with large double glazed windows, start from scratch and design one...
The trouble is that the people who built those barns knew enough to build a barn, and the people who convert them know even less, usually. Slap in some insulation and CH, block all the draughts, and wonder why they end up in a Turkish bath atmosphere.
Its so NOT *artistic* to have to bother with DPMS, U values, and the like, when what *really* counts its an atmosphere of romantic fantasy from a 17th century idealized and totally wrong perspective.
Now I am very happy to build a dream house - I have done so, and it does indeed look like an idealized fantasy of a 17th century timber framed house. BUT it is done on solid principles of engineering, and heat and moisture control straight out of the building regulations. And steel beams carefully inserted wherever the dream and reality would have collided head on into a crashing pile of timberwork..and it has things they never would have considered in the 17th century, like underfloor heating and insulation, and underfloor vents to feed the open fires, and fans to remove the steam from the very un 17th century showers and bathrooms..
And it does NOT suffer from condensation, cold draughts, rot, and cold spots.
Perhaps this detracts from its 'character' - I don't know - but its what I wanted.
Read The Whole House Book and Out of the Woods, both by CAT. Out of the Woods is a cheaper book and covers all this sort of stuff. The Whole House Book is the bible..and well worth having.
The poly vapour barrier has to be behind the plaster board on the "inside", NOT on the outside. It acts a barrier for water vapour getting into the wall and condensing. The layers of material from the inside out must be the highest vapour resistance to the lowest from inside to out, with the exception of the inner plasterboard.
If the outer cladding is leaking, then replace it, don't install poly barriers on the outside. In fact if new outer cladding is being used it is worth cladding the outside part of the barn, covering the studs, which are a cold bridge, to create a complete heat envelope.
The CAT house was not ventilated properly. "Build tight, ventilate right". They know much more now at CAT, 30 years on. Just follow the Canadian R2000 approach, best in the world, and you can't go wrong.
Huh! Additional fenestration will be resisted..... local planners.
Umm... the point usually is that barn conversions are one of the few ways to build domestic accommodation in rural areas. This is metropolitan green belt so twice damned.
Romantic fantasy? I must work that into the conversation next time I am trying to convince a planner that my about to tumble down shed is worthy of retention and that milking 12 cows in fixed standings was uneconomic anytime after 1950.
I'm not actually hoping for domestic use but would like to get something more commercial than B&B.
Well done.
I'm only at the character building stage of sorting the logs before working them down into usable sizes. I have a head start as another contributor *slabbed* an Oak limb for me when he called to collect something else.
Back to the problem... The bulk of the insulation will be external. There will be a membrane under the floor. There will be a membrane between the framing and the external insulation. The only unresolved bit is whether to fit an additional 25mm of insulation between the framing timbers i.e. behind the plaster board and inside the membrane or just leave an air gap?
Yes. However there is a practicality issue. Assuming one wishes to go down the romantic road of having at least part of the timber exposed, the barrier can only be fitted on the outside of the frame. A layer of external insulation should keep the frame warm enough to avoid condensation.
External insulation is great as it prevents cold bridging. To expose the studs and beams insert foil backed plasteboard between the beams/studs and insulation behind. And install poly at pionts that are exposed. The foil is the vapour barrier.
If you use foil backed plasterboard, use the insulation.
I'd be a little bit wary of using it without...but 25mm unless its celotex won't be a huge difference..and you might want to put cables in the void as well, so think it through.
Google for 'housewrap'/breather membrane. Should be fairly straightforward to get an airtight structure which lets water vapour out and doesn't suffer condensation.
Also try a Google on that and some insulation trade names, eg
OK. There seems to be a slight conflict in views expressed:-) I don't anticipate large amounts of water going into a building intended for workshop/office use so I think foil backed plaster board, 25mm insulation, vapour barrier, Celotex type insulation, permeable membrane (back to the Canadian building paper) and feather edge.
Excellent point. For workshop use, I will just slap up some steel trunking at waist height. Studwork framing does not lend itself to horizontal cable runs.
If the building ever gets a change of use I will need to insulate and screed the floor and I had vaguely considered skirting trunking for power and data cabling. I suppose it might be practical to disguise waist height ducting as a shelf. Either way it conflicts with wall standing furniture. It might be better to follow your hint and pre-install a duct at eaves level with conduit behind the plaster board. The emphasis given to sealing round sockets outlets in the sites given by Pete is a bit worrying.
Anyway, I think I have enough confidence now to get on with the job. Ta.
Also probably well worth contacting the technical advice dept of the relevant company and getting their opinion on what you plan to do.
If leaving exposed beams the tricky bit might be to get a good airtight seal between Celotex and beams that will allow a bit of seasonal movement in the beams. Some strips of closed cell foam and/or aluminium tape, sealant might do the trick.
Alternatively ignore air migration into what should be a warm zone and concentrate on a sound moisture barrier in front of the insulation.
If this building were going straight into conventional domestic use, these precautions would pay back time and effort. But, for a farm workshop with possible change of use 5 years on, a bit overkill:-) If gaps have opened up they can be filled with some sort of flexible mastic later.
I have moved on to start worrying about eaves ventilation:-)
Horizontal cabling in studwork is not hard, and surely the best option. Yes you gotto drill some holes, no big. 5-10 minutes work drilling and youve got a nice neat result. Use lip & spur bits or better use standard twist drills reground with steep angles and a lot of the rear supporting metal behind the cutting edges ground away. I use these lots and they cut several times as fast as standard all purpose twist drills. Great use for all those knackered bits.
Surface mounted thingies remove any complications with vapour barriers, insulation and condensation, are quicker to fit, and look fine.
Yes. However, where most of the stud is exposed, only a small space remains for cabling. Certainly impractical to bury at 50mm. Vertical conduit dropped behind the plaster board would be safe if a bit inflexible.
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