Workshop insulation

I have a substantial wooden building I want to turn into a workshop. Its about 16' x10'. Frame is 4" X 4" clad with featherboard and lined under the featherboard with tar paper. I want to insulate it and clad the inside. Where should the vapour barrier go? I think the correct thing would be to cut out the tar paper between the frames, install insulation, then vapour barrier ( polythene sheet?) and then the inner cladding. This would stop vapour passing through to the insulation, which can breathe through the many gaps under the bottom edge of the featherboard. Is this correct?

Tar paper is the vapour barrier at the moment, but general opinion and building regs says DPM on the warm side of the insulation, that is between the insulation and the lining. I wouldnt bother removing the tar paper, you only get condensation when warm moist air hits a cold surface, the cold surface will be the outside boarding, and the cavity behind the boarding will be at the same temperature as ambient outside. leave a 1" gap next to the boarding to allow air to circulate, if the joists are 4", use 3" jablite, or cellotex/kingspan if you can afford it! you could use 75mm rockwool batts, but dont use loose glasswool, it just gets damp and slumps. I used visqueen dpm on my brick walls, but put the insulation in contact with the brickwork, and put the dpm over the top, immediately under the lining. With a timber building,which already has a tarpaper dpm, it is debatable whether you really need another DPM. No point in using breathable under tile type, as it has nowhere to breathe too!

Phil

We have just had a new shed from Crane buildings. It came with a vapour barrier between the outside skin shiplap planks and the frame frame. The walls were then Rockwool insulated and the inner planing tongs and grooved. The roof was the same but had celotex insulation. No other insulation except the floor had solid insulation fastened to the underside. Celotex. I asked about any further barrier but was told it would just trap condensation and cause rust etc.

David

Thanks for the reply Phil. As you say, current thoughts are dpm behind lining but I can see your thought train. So, to summarise from inside out, lining, insulation (possibly expanded polystyrene) , gap, tarpaper, featherboarding?

Sheds that are just sheds are expected to be for storage only. Therefore no moisture being created inside so you are trying to keep the outside damp outside so the barrier is next to the outside cladding. A house is definitely for people who each produce several pints of vapour from their bodies plus cooking and washing and have plants and damp coats and part dried washed clothes. The problem comes when you have a shed as a workshop - where is the most moisture going to be most of the time.

Personally I always recommend vapour barrier on the inside and a dehumidifier in the UK. In a hot but damp climate with aircon in the building I would put the vapour barrier on the outside.

I agree with Bazyle and Phil.

The tar paper is not a true DPM but if any water gets in through the FE boarding the paper will stop the water reaching the structural frame.

You can use fibreglass or rockwool between the timbers and line internally with foil backed plaster board or a decent vapour barrier overlaid by a lining of your choice. The foil helps reduce heat loss through radiant heat too.

What tends to get overlooked by Building Regulations (and the BREAMM energy performance model) is that buildings do not often sit in a steady state - they get wet on the outside, get exposed to dry winds which cause wet surfaces to evaporate water quickly (chilling them), and get erratically heated and steamed up internally.

It has been shown that thermal mass is also important in control of moisture. A 'breathable' masonry inner surface and unsealed concrete floor will take up moisture in the short term and gradually release it when internal air is dry.

This effect can be useful as our machines have very high thermal mass and will suffer from condensation when their temperature remains below that of the dew point at that time.

Keeping the workshop reasonably heated will overcome this by keeping the temperature above dew point, but having the fabric able to absorb moisture will help keep dew point at a low temperature (by helping keep the air dry)

It is a complex subject but if going the low thermal mass / low heat input route I would advise to ensure robust vapour barrier and well sealed doors and windows combined with a dehumidifier. Be careful with fumes in such a building!

Remember that when choosing insulation, Foam type insulating boards have a thermal value nearly 4 times better than rockwool. It is not expensive from the builders merchants. If you can find one open. I recently bought some to make female moulds/ molds for making fibre glass patterns to repair a motorhome. Good stuff. Cuts like butter. Does not make you itch.

Steve.

No profile, so just my purchase details. Well over 20 square metres of 100mm insulation for around fifty quid from a regular collective/dispersal sale in the UK. Clearly no sales at the moment, but not many builder’s merchants operating either. Another 5 full sheets of 100mm kingspan cost me about forty quid from another regular sale venue.

More or less what I built my garage with. Industrial freezer room panels.

Steve.

Steve, that build looks very interesting in its minimalism. More details please. Are they metal clad? From your other photo it looks like the roof is flat and no other support than the walls.

Hi Bazyle. Have a look at this. I built from scratch. The panels are Cold room panels & come in various thickness & height . I found mine on ebay. The garage is 2nd hand panels. The roof was new from Forgales in Liversedge. The walls are 100mm galv steel sandwich. The roof is 50mm insulated.

Anyway have a look. Size is 9M x 4M. The upvc window & door were given to me. The roller door was 2nd hand off ebay. I made my own hanging brackets. Build your own insulated garage

Steve.

A quick comment on board type insulation products - these work best when accurately cut to the space they fill, so accuracy of framing is important, otherwise heat escapes round the edges. This is easily seen when scanning with thermal imaging cameras. Rockwool is much easier to cram into irregular shaped spaces.

The value of insulation is in moderating the cost of heating the workshop, and making it easier to keep the workshop temperature above dew point.

One other comment is that, when designing for energy conservation, for many of us it is also a good idea to design for noise insulation. Some of what we do creates noises that might irritate neighbours.

A dense masonry wall (better still cavity wall), tiled roof and well fitting windows and doors do much to limit noise transmission. Another case where mass is good (especially for controlling low frequency sound and transmission of 'impact noises'. Even small gaps allow noise to bypass an otherwise noise resisting structure.

My best performing workshop had dense concrete inner leaf, cavity insulation and brick outer leaf, 12mm plasterboard ceiling with insulation over and concrete tiled roof and salvaged double glazed UPVC windows. Mind you that was a really nice space (albeit quite small) and even had salvaged woodblock flooring which was a delight when you dropped a chisel!

It required very little heating and never suffered condensation problems.

My workshop was, until recently, a 5.2m x 3m Yard Master garage of plastic coated galvanised steel construction.
I devised a means to fix wood into the U shaped internal supporting members of the walls, filled the spaces with insulating sheets cut to fit and boarded out the interior walls with 18mm OSB. The roof was insulated with 25mm Celotex.
The workshop stayed above 10C in all weathers in Yorkshire with just a desktop PC running and I never had any issues with condensation.

Thanks forr the video link Steve. Very interesting construction.

No problem. At the time cost was an issue. (divorce settlement etc). Although now I think I would still take the same route. Pretty soundproof. Easy to construct . Fireproof. Heat insulating. Easy to clean. No brainer. But everyone has there own idea of a good workshop. I can see why many choose wood. Looks nice when finished. How about a Swedish log cabin. With an open fire.

Steve.