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Insulation Question

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Insulation Question

Postby the bear » 26 Jul 2017, 21:51

Its looking like building a custom workshop is going to be on the back burner for a fair bit while I substantially remodel my house. Since I moved I have been workshop less and unable to use any of my machines but I could do with them to help the renovation.

So I'm going to get set up in the garage. It is a double, separate from the house. My old workshop was about single garage size and built by my own fair hands, insulated concrete floor, aircrete walls and insulated slated roof. Very toasty in winter.

This garage is unusual in that for a detached it has cavity walls. I want to tackle the roof first. The tiles are fine if not to my choice. The under felt however is disintegrating and I constantly have bits falling off. Luckily those tiles don't leak so its not a weather problem at the moment.

So my plan is to leave the tiles alone. Which means I'll be leaving the under felt alone until such a time I re-tile the roof.

In order to make the garage a bit more toasty I still want to insulate it in the meantime. I have reasonable access to the underneath. The rafters are 100mm. My plan is to celotex or similar between the rafters then line with ply underneath.

My question is what depth of insulation. I presume I need an air gap so how much insulation and how much air gap in that 100mm? This needs to work with the current under felt and with the new stuff when I get round to re-roofing in a few years.

Opinions on the thickness?

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Re: Insulation Question

Postby the bear » 26 Jul 2017, 21:53

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Re: Insulation Question

Postby the bear » 26 Jul 2017, 21:55

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Just trying tapatalk for the first time for the pics

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Re: Insulation Question

Postby TrimTheKing » 27 Jul 2017, 10:50

Hi mate

I can't answer in terms of what's 'right' but my 'shop is being built to Regs and I have put in 100mm inside 150mm rafters so 50mm gap, then a further 50mm beneath that.

I would think for yours either 500mm between rafters and another 50mm underneath, or 75mm between rafters would be sufficient if you wanted insulation but not necessarily to Regs level. HTH.

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Re: Insulation Question

Postby RogerS » 27 Jul 2017, 12:18

I shall be watching this thread with interest (and so will Malcolm, I think) as I have a similar situation.
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Re: Insulation Question

Postby Malc2098 » 27 Jul 2017, 15:11

I've articulated the issue in my 'shop build. I think I have an answer for my particular issue.

I will be creating a minimum of 50mm gap from eaves, over the ridge and down the the opposite eaves. I shall have gaps on top of the rafters and I shall have ventilation holes on the gable tops and the eaves.

BTW, I have 12 sheets of 1200x1200x120 paper faced pir insulation boards. I shall put them on the For sale thread.
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Re: Insulation Question

Postby the bear » 27 Jul 2017, 17:35

Interesting both Trim and Malc have gone for a 50mm gap. I'm not trying to build this to habitable room regs, just trying to avoid the moisture/condensation issue and will then probably go with whatever insulation that leaves me with between the rafters. I'm wondering if 60/65 mm insulation and 40/35 mm gap would be OK.

My old workshop had 100mm rockwool a bit like Mikes so without looking up U values I'm guessing this would be a similar level of insulation, which I would be happy with, but I suspect the air gap is more important.

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Re: Insulation Question

Postby RogerS » 27 Jul 2017, 18:08

I think the critical thing is to make sure that you find a way of bypassing the ridge so that the air can flow up one side and out the other. If you don't allow for that then the insulation will butt up against the ridge thus blocking through flow of air.
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Re: Insulation Question

Postby TrimTheKing » 27 Jul 2017, 20:10

Without any education in this at all, I would imagine anything over an inch of gap would be fine but I'm sure Mike will confirm either way.

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Re: Insulation Question

Postby RogerS » 27 Jul 2017, 21:47

TrimTheKing wrote:Without any education in this at all, I would imagine anything over an inch of gap would be fine but I'm sure Mike will confirm either way.

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But it does have to go 'up-and-over', I reckon. If you take the insulation hard up to the ridge then that blocks any air circulation. I think.
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Re: Insulation Question

Postby Malc2098 » 27 Jul 2017, 22:09

RogerS wrote:
TrimTheKing wrote:Without any education in this at all, I would imagine anything over an inch of gap would be fine but I'm sure Mike will confirm either way.

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But it does have to go 'up-and-over', I reckon. If you take the insulation hard up to the ridge then that blocks any air circulation. I think.



That's my understanding, too. Just to make sure, I'm going introduce some lateral air movement as well as up an over.
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Re: Insulation Question

Postby TrimTheKing » 27 Jul 2017, 22:16

Yep I would agree with you both on that.

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Re: Insulation Question

Postby Malc2098 » 01 Aug 2017, 17:28

I have invested in a Mterotile Aquapan system for my roof covering on my 'shop build.

I have been looking for ventilation solutions as in the discussion above. I have just found this on Metrotile's website.

https://www.metrotile.co.uk/wp-content/uploads/Ventilation-Guide.pdf

It sort of answers the theory side of things to me. I've now just got to execute the practise!
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Re: Insulation Question

Postby RogerS » 02 Aug 2017, 09:44

That's really useful thanks, Malcolm. Interesting to not see any reference to a 50mm gap anywhere, at least at a first pass, so where did that figure come from ?
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Re: Insulation Question

Postby Malc2098 » 02 Aug 2017, 10:36

RogerS wrote:That's really useful thanks, Malcolm. Interesting to not see any reference to a 50mm gap anywhere, at least at a first pass, so where did that figure come from ?


I'm still not sure, Roger. I've seen so many drawings and pages over the last few days, I remember seeing 'minimum 50mm gap' so many times, but never found the source that these documents used.

So, we all seem to go for this 50mm gap, which seems reasonable and doable in most situations.

If anyone is thinking of a DIY outbuilding or extension, I can recommend Metrotile. Their customer support for a DIYer like me has been brilliant. They are currently helping me with a vent solution through their product.
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Re: Insulation Question

Postby Deejay » 02 Aug 2017, 14:27

Hello Roger

Page 4, third dawing. Associated text is above the drawing.

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Re: Insulation Question

Postby Malc2098 » 02 Aug 2017, 15:08

Deejay wrote:Hello Roger

Page 4, third dawing. Associated text is above the drawing.

Cheers

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Thanks, Dave. I knew I'd seen it somewhere!!
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