Membrane to go behind cladding

[Pete Bannister]

Kevin,

In that particular case there was a body of water stewing away within thick masonry walls; so there was an unintended moisture source. However, the MVP membrane greatly restricted venting of the roof void that would otherwise have occurred naturally via the roof finishes.

Its a common problem in refurbs. and conversion work were it may be difficult to provide compensatory ventilation at the eaves and ridge for example

 

Codlasher,

It not my roof thankfully! I investigate building defects professionally. The example in my photo is actually a ‘cold roof’ ie a traditional form with thermal insulation at ceiling level well below the roof. You might be surprised to know that most of the modern MVP membranes you see in the UK are specifically marketed for use in un-vented roofs, ‘cold roofs’ or ‘warm roofs’ . Many of them now have BBA certs for use in unvented construction. Crazy in my view but it stems from the concern to conserve heat energy in our buildings. Truth is that when a well-designed building remains in good condition an unvented roof may be fine..however, when a fault is introduced the lack of venting tends to result in a small local problem going exponential .

The ‘warm roof’ has popular because its cheap to build. There’s no other good reason for using it in my view.

The link you posted shows cold roofs’,’ warm roofs’ and ‘inverted roofs’. I think it might confuse the OP!

 

Jamesd,

The great advantage of vented air gaps is that it provides a ‘safety valve’ in the event of imperfect design, imperfect construction, or imperfect maintenance. Consider that the perfect building has yet to be built!

Codlasher and I may share the view that ventilation is generally good for the health of the fabric but you have to decide if you can afford it!

On the matter of thermal insulation; in broad terms, you get what you pay for. Do you need good heat retention in a shed? if its small it won’t cost much to heat for occasional use…depends want you intend to build it for. Anyways, the key point to consider is that when you introduce thermal insulation it generally increases the risk of interstitial condensation: so that needs to be addressed in the design.

[Mull]

Kevin,

 

In that particular case there was a body of water stewing away within thick masonry walls; so there was an unintended moisture source. However, the MVP membrane greatly restricted venting of the roof void that would otherwise have occurred naturally via the roof finishes.

 

Its a common problem in refurbs. and conversion work were it may be difficult to provide compensatory ventilation at the eaves and ridge for example

 

 

 

Codlasher,

 

It not my roof thankfully! I investigate building defects professionally. The example in my photo is actually a ‘cold roof’ ie a traditional form with thermal insulation at ceiling level well below the roof. You might be surprised to know that most of the modern MVP membranes you see in the UK are specifically marketed for use in un-vented roofs, ‘cold roofs’ or ‘warm roofs’ . Many of them now have BBA certs for use in unvented construction. Crazy in my view but it stems from the concern to conserve heat energy in our buildings. Truth is that when a well-designed building remains in good condition an unvented roof may be fine..however, when a fault is introduced the lack of venting tends to result in a small local problem going exponential .

 

The ‘warm roof’ has popular because its cheap to build. There’s no other good reason for using it in my view.

 

The link you posted shows cold roofs’,’ warm roofs’ and ‘inverted roofs’. I think it might confuse the OP!

 

 

 

Jamesd,

 

The great advantage of vented air gaps is that it provides a ‘safety valve’ in the event of imperfect design, imperfect construction, or imperfect maintenance. Consider that the perfect building has yet to be built!

 

Codlasher and I may share the view that ventilation is generally good for the health of the fabric but you have to decide if you can afford it!

 

On the matter of thermal insulation; in broad terms, you get what you pay for. Do you need good heat retention in a shed? if its small it won’t cost much to heat for occasional use…depends want you intend to build it for. Anyways, the key point to consider is that when you introduce thermal insulation it generally increases the risk of interstitial condensation: so that needs to be addressed in the design.

 

 

Good points indeed!

 

All this discussion for a 12x7 shed???

 

Only on here, haha, brilliant!!

[codlasher]

Kevin,

In that particular case there was a body of water stewing away within thick masonry walls; so there was an unintended moisture source. However, the MVP membrane greatly restricted venting of the roof void that would otherwise have occurred naturally via the roof finishes.

Its a common problem in refurbs. and conversion work were it may be difficult to provide compensatory ventilation at the eaves and ridge for example

 

Codlasher,

It not my roof thankfully! I investigate building defects professionally. The example in my photo is actually a ‘cold roof’ ie a traditional form with thermal insulation at ceiling level well below the roof. You might be surprised to know that most of the modern MVP membranes you see in the UK are specifically marketed for use in un-vented roofs, ‘cold roofs’ or ‘warm roofs’ . Many of them now have BBA certs for use in unvented construction. Crazy in my view but it stems from the concern to conserve heat energy in our buildings. Truth is that when a well-designed building remains in good condition an unvented roof may be fine..however, when a fault is introduced the lack of venting tends to result in a small local problem going exponential .

The ‘warm roof’ has popular because its cheap to build. There’s no other good reason for using it in my view.

The link you posted shows cold roofs’,’ warm roofs’ and ‘inverted roofs’. I think it might confuse the OP!

 

Jamesd,

The great advantage of vented air gaps is that it provides a ‘safety valve’ in the event of imperfect design, imperfect construction, or imperfect maintenance. Consider that the perfect building has yet to be built!

Codlasher and I may share the view that ventilation is generally good for the health of the fabric but you have to decide if you can afford it!

On the matter of thermal insulation; in broad terms, you get what you pay for. Do you need good heat retention in a shed? if its small it won’t cost much to heat for occasional use…depends want you intend to build it for. Anyways, the key point to consider is that when you introduce thermal insulation it generally increases the risk of interstitial condensation: so that needs to be addressed in the design.

 

Thank you for your informative post. A little worrying that these modern membranes are marketed to do something that they aren't really up to. The old adage of 'if you insulate you gotta ventilate' still stands then with folk relying on the membranes doing this and its not happening! Hence your picture and really quite a nasty problem.

codlasher

[codlasher]

Good points indeed!

 

All this discussion for a 12x7 shed???

 

Only on here, haha, brilliant!!

Thanks.

I like sheds and I hope the discussion that we've been having helps jamesd build a beautiful tiny shed which he will be proud of.

I once built a little 8' x 18' miniature replica of a 75' x 24' barn just so I could do all those wonderful cut joints. The best part was that the timber all came from a load of oak firewood that I bought and had milled!

I'm planning to try building another this summer using some douglas I have left from another project.

I have an idea how it will look and it will enhance our holiday camping area.......

codlasher

[Mull]

Thanks.

 

I like sheds and I hope the discussion that we've been having helps jamesd build a beautiful tiny shed which he will be proud of.

 

I once built a little 8' x 18' miniature replica of a 75' x 24' barn just so I could do all those wonderful cut joints. The best part was that the timber all came from a load of oak firewood that I bought and had milled!

 

I'm planning to try building another this summer using some douglas I have left from another project.

 

I have an idea how it will look and it will enhance our holiday camping area.......

 

codlasher

 

 

Nice work, good on you!

Is it felt shingles on roof?

[jamesd]

After reading through the above posts, (thank you all for your advice! it has been VERY helpful!) and thinking it through for a while would it be easier to simply put a membrane on the stud work followed by the feather board and not insulate the inside? and maybe just put a radiator or something inside? That ,ight stop the condensation problem i was worried about?

As i said before i am a complete novice to any sort of building and am just looking for any advice before i go any further with my build!

[Pete Bannister]

Good luck with it Jamesd,

Looks like Codlasher has set the bar high with his delux shed.

You realise you're now under a moral obligation to send us the photos of your completed job.

No pressure mate!

[SteveA]

Thank you for your informative post. A little worrying that these modern membranes are marketed to do something that they aren't really up to. The old adage of 'if you insulate you gotta ventilate' still stands then with folk relying on the membranes doing this and its not happening! Hence your picture and really quite a nasty problem.

codlasher

 

Saying that I chatted with Knauf tech dept about using their Polyfoam for roof insulation and they said it's fine for that BUT very important that it's butted right up next to breather membrane otherwise there would be an interstitial condensation problem!

 

I'd go with whatever the manufacturers recommend.

[jamesd]

On a side note what sort of boarding would you use for the roof? its just going to be felted

I was thinking sterling board or plywood which would be best suited?

[agg221]

I would insulate personally - it is expensive trying to heat the whole district otherwise.

 

There are two basic approaches - one is to stop water getting in, the other is to let it out if it does.

 

I am not a big fan of relying on a perfect seal. When it is breached the water will get in and then not get out again.

 

I would infill the walls with something breathable - eg reed board, sheep's wool or fibreglass (compare prices and lambda values). I would cover the outside in thin ply (wbp) if I hadn't used reed board, with a breathable membrane if I didn't trust my cladding to be fully waterproof. I would then overlay 20mm battens, lined up with the frame below, and clad over this, so there was an air gap behind the cladding. Inside I would board out the walls with half inch wbp ply to minimise absorption of water into the structure. For a Rolls-Royce job I would paint the back of the ply first. The ply doesn't need to be this thick but you can then screw shelves etc directly to it.

 

Roof, I would use Kingspan or Celotex expanded foam boards - thin seconds are pretty cheap. I would then put wbp ply over the top, probably 9mm so I could climb on it and nail it straight through with helical nails. Felt or corrugated bituminous sheet to finish. Infill between rafters or ply under, to cover the foil surface and prevent condensation.

 

Perhaps overkill for a shed, but it would be reasonably quick and cheap on a construction that size.

 

Alec