Garden building insulation advice

phantombantam · 2 Aug 2021

johnny said:
the problem is that you have constructed the shed without seeking advice until you've nearly finished and in so have now severely limited your options . There is little to gain in now being petulant with those that are trying to help you .

Your problem now is not trying to exclude water vapour from outside the shed with a vapour check . What you need to do is to extract the water vapour which will form every night when the temperature drops . Warm air during the day will store a lot of water in the form of vapour but when the temperature drops the air molecules have to shed that vapour because they are unable to retain the moisture .

You can either install an extractor in the roof of the shed ,(preferably at the highest point ,) or you can install a dehumidifier which will extract and store the moisture and will require emptying periodically.

These are simple,cheap and very effective options for your situation that will cause minimal installation disturbance .

The other thing you might consider is how hot the inside of the shed will become when the outside temperature rises and the sun is beating down on the timber flat roof. Some form of extraction system will be essential if you intend to work in the shed in comfort during Summer months . Opening the door and windows is unlikely to provide adequate through ventilation

I'm pretty confident of the water tightness of the wall and render so I wonder if this will be helpful or not. What do you think please?

phantombantam · 2 Aug 2021

johnny said:
the problem is that you have constructed the shed without seeking advice until you've nearly finished and in so have now severely limited your options . There is little to gain in now being petulant with those that are trying to help you .

Your problem now is not trying to exclude water vapour from outside the shed with a vapour check . What you need to do is to extract the water vapour which will form every night when the temperature drops . Warm air during the day will store a lot of water in the form of vapour but when the temperature drops the air molecules have to shed that vapour because they are unable to retain the moisture .

You can either install an extractor in the roof of the shed ,(preferably at the highest point ,) or you can install a dehumidifier which will extract and store the moisture and will require emptying periodically.

These are simple,cheap and very effective options for your situation that will cause minimal installation disturbance .

The other thing you might consider is how hot the inside of the shed will become when the outside temperature rises and the sun is beating down on the timber flat roof. Some form of extraction system will be essential if you intend to work in the shed in comfort during Summer months . Opening the door and windows is unlikely to provide adequate through ventilation

Hi Johnny,

it is going to be used as a garden room as opposed to a workshop so will have furniture, TV etc in. I was under the impression rightly or wrongly that it would be cooler in summer and warmer in winter once insulated. Is that not likely to be true?

RobinBHM · 2 Aug 2021

phantombantam said:
Hi, no petulance here from me. I'm just trying to weigh up the different opinions and advice which seem to be complete opposites.
Hindsight is a marvellous thing and I obviously did not know what I have since been told about a number of things

TBH most builders don’t know, thousands of flat roofs are built every year…wrongly. In fact there are plenty of new build houses that have suffered such severe interstitial condensation, the walls have had to be gutted and redone.

My gut feeling is, if you can’t get much ventilation in the roof void, is to not have a void at all and use a high performance vapour control layer like resitrix 600

johnny · 3 Aug 2021

phantombantam said:
Hi Johnny,

it is going to be used as a garden room as opposed to a workshop so will have furniture, TV etc in. I was under the impression rightly or wrongly that it would be cooler in summer and warmer in winter once insulated. Is that not likely to be true?

Insulation is fine for cold weather when you want to retain heat and minimise heating needs and costs however in the hot weather the reverse is true. Insulation is going to keep warm air in your shed so you will need some form of induced air flow to make it bearable.
In a traditional stone cottage with 18" walls and a tiled roof the inside will stay cool all day long without any thermal insulation whatsoever during hot weather. The construction is also quite effective it retaining heat due to its mass. Your shed roof has no dense mass to absorb heat.

With a shed that has a timber flat roof you have no effective means of reflecting heat from the sun so it will rapidly heat up. Internal thermal insulation is not going to help too much . Ideally what you need is a through ventilated secondary layer fixed over your roof covering which will keep your roof from getting hot . Think Landrover where they have an additional layer over the roof and cab ,open at the edges to keep the interior cool in hot climates. Alternatively you are going to need forced ventilation and extraction in hot weather or you'll bake in there .

johnny · 3 Aug 2021

RobinBHM said:
TBH most builders don’t know, thousands of flat roofs are built every year…wrongly. In fact there are plenty of new build houses that have suffered such severe interstitial condensation, the walls have had to be gutted and redone.

My gut feeling is, if you can’t get much ventilation in the roof void, is to not have a void at all and use a high performance vapour control layer like resitrix 600

I had to Survey several new houses on a housing estate in Yeovil not so long ago where occupants complained of running water in the lofts...it transpired the builder had not installed any ventilation at all and the insulation was pushed up tight to the eaves so all the condensation in the house got trapped in the lofts. It was like an Amazon rainforest in the roof .

phantombantam · 3 Aug 2021

johnny said:
I had to Survey several new houses on a housing estate in Yeovil not so long ago where occupants complained of running water in the lofts...it transpired the builder had not installed any ventilation at all and the insulation was pushed up tight to the eaves so all the condensation in the house got trapped in the lofts. It was like an Amazon rainforest in the roof .

A friend of mine bought a relatively newly built house and I was helping him move some stuff into the loft. It was wet inside and we discovered his bathroom extractor fan was sucking straight into the loft space.