Roof insulation options - Large Vented Eaves

[robinbanks]

Is there a good reason why anyone would've opted not to do that in the first place? Why would some one knowingly go for a cold roof setup like this given the choice?

warm roof is more complex to do, takes longer and requires more materials. It is best done as a planned exercise during roof construction. Cold roof is simpler; put trusses up, membrane, battens, slate. That gets the place watertight quickly so the finish trades can get on with boarding, plastering, tiling. Insulating is towards the end, just roll out wool on the floor, takes a couple of hours. Creating an insulation layer in warm roof is definitely more than a couple hours work and requires skill and thinking ability beyond rolling out wool (which I frequently see a hash made of too)

If I block them up won't I get issues with damp and moisture etc?

It depends on what other follow up work is done. It's not just about stopping the draught; this area is cold because also you've got that white membrane, then tiles and then world. There is nothing in that buildup that insulates. A warm roof is a composition of an insulated deck, a breathable outer and a vapour barriered inner. The vapour barrier resists human generated moisture from entering the roof build up in the first place. The breathable layer lets out any water vapour that is in there.

A cold roof can deal with huge amounts of moisture (cooking, breathing, drying washing on rads) by huge ventilation. A warm roof deals with small amounts of moisture by small ventilation or breathable fabric, but to make sure it is small moisture you need to barrier a lot of moisture away from entering the roof construction and then deal with this redirected majority another way; PIV, MVHR, ventilation of the warm moist space

 

[GeddyMortgage]

Yeah, hmm...I was afraid you was afraid I was going to say that.... Nothing but less than ideal news with this house, i've come to expect the news to always be bad now.....

I see what you mean, in the example of converting this to a warm roof, the floor insulation is lifted to put a moisture barrier, that's what you mean by 'deck'?..... reading your comment it looks like it also would involve removing all the white membrane and swapping that out? so tiles off, basically re-working the entire roof? If so i'm assuming 5 figures and likely out of budget.. Meaning the original plan of insulating the hell out of the barriers that separate the warm and cold spaces of this property, albeit some of them being a pain in the a*se to access.

Check out this latest video as does relate to the above given the warm roof is looking probably less likely.

Video 3 - insulation issues

Note: Near the end of the vid when I say "parcel shelves", I meant to say "shelf brackets"

 

[GeddyMortgage]

Also @robinbanks ... throwing the finances aside for a moment, the uninsulated loft floor spaces.... Can I use insulation boards here instead of roll? given all the cables etc. 150mm of celotex would be alot easier than 300mm of roll in this scenario despite the increase in cost.

Does anyone generally do this? Cant recall seeing it

 

[robinbanks]

that's what you mean by 'deck'?....

Sorry, I shouldn't have said deck there as it too often implies a floor. Warm roof means the thing that carries the slates also carries the insulation it can be on the world side of the roof timbers or the house side, and with or without insulation between the timbers. In all cases, as with the walls, we either make a structure that breathes moisture, or we barrier moisture as close to the point of generation and then make the rest of the composition able to breathe out any moisture that may get into it

 

[robinbanks]

Teading your comment it looks like it also would involve removing all the white membrane and swapping that out? so tiles off, basically re-working the entire roof?

That's probably the extreme case; I was more thinking it prudent to access and insulate the roof from the outside for the parts that are inaccessible from inside..

Meaning the original plan of insulating the hell out of the barriers that separate the warm and cold spaces of this property, albeit some of them being a pain in the a*se to access.

But the part that concerns me there is that you're deeming some things to be barriers that it doesn't make sense to deem a barrier. The back of the plasterboard wall between the bedroom and the boiler space for example. Bring the boiler and piping into the warm envelope, that way any heat lost is lost to the warm side. If you insulate the wall and leave the boiler cold you're using more insulation (distance along the floor and up the wall is greater than the slope) and you're putting a lot of plumbing in where it's cold

You asked about insulating both; to me it makes only partial sense. Let's say I have two sheets of 100mm kingspan, a bedroom wall, a boiler room wall, and a world. I could put 200mm on the bedroom wall, leave the boiler in the world. I could put 100mm on the bedroom, 100mm on the boiler, have the boiler room a lower temperate. I could put 200mm on the boiler room and have the boiler room the same temp as the bedroom. In all those scenarios the bedroom is the same temp - there is 200mm of insulation between it and the world.
Where the insulation is dictates the warmth of the boiler room. Choice given, I'd make the boiler room the same temp as the bedroom, trap in any waste heat from the boiler, pipes etc. The hot water tank rate of heat loss is related to the ambient temp of the room the tank sits in. If that tank is at 50 and in a room that is 0, it'll lose heat faster than it will in a room at 20 degrees.

Will check video 3 anon

 

[GeddyMortgage]

I agree, whatever happens with the loft space on the other side of that boiler room 'wall' Even if that does stay as a cold space, the boiler room will be made into a warm space. As shown on the video, roof-wise in that section it looks like someone started to do it and give up as it does have 100mm celotex in the rafters. Im going to insulate that properly and double layer plasterboard for good measure.

It's the separation of the boiler room and the cold loft on the other side of the boiler room wall that I think is going to be harder to insulate entirely. Partly as the 'wall' is just a big piece of plywood with a big section missing which needs to stay for loft access.

And even though all the boiler pipes start in the boiler room they do run through that plywood 'wall' to the cold side before wrapping back around. Without having those entirely re-routed they're always going to largely exist in the cold space, even if we insulate the boiler room incredibly well. That said we have lagged all the 22mm pipes with the fatter variant lagging (19mm thickness)

Only alternative I guess is back to Warm loft everywhere, but im a little unclear on exactly what has to happen here logistically to convert this. That white membrane has to change right?

 

[robinbanks]

Doesn't need to be everywhere but it would likely be easier to warm roof the bit the other side of the boiler room. I don't really see the plywood wall as a wall; more like "a large board to screw things to in a cold loft area". By making it a part of the warm envelope you take away a whole load of fiddly work moving or insulating pipes. Remember too that pipe insulation is 19mm (not very large in the grand scheme of things;my air source heat pump has 75mm insulation on its feed pipes); your loft wool (roughly same thermal performance) is supposed to be 300mm; that'll outperform the 19mm pipe lagging by a huge margin. Lag by all means but cover the pipes in wool too. Avoid insulating power cables unless you've calculated that they can still carry the power they currently do after insulating (look up cable derating)

Insulating the ply wall will involve insulating the sloping roof above the boiler. if using board for it ensure it's a good fit between rafters and board/tape over them too to create a vapour barrier. You're essentially performing a loft conversion of the boiler room and installing the insulation the builders should have done

If the white membrane is non breathable, leave a gap between it and whatever you insulate between the rafters with. Eg if your rafters are 200mm, put 150 insulation between, flush with the ceiling.

 

[GeddyMortgage]

Ahh, right, im with you now @robinbanks . I thought you meant making the whole of the cold loft space into a warm space. You mean just the portion on the other side of the plywood where the pipes are.

I've put some photoshopped images below now of the current setup, a version where just the boiler room would be warm and a version that I think is what you're proposing... The images also include the the location of the hot water / central heating pipes up there. Unfortunately they do span pretty round most of the loft space. See the 3 images below.



If i am understand you correctly, you're proposing the last of the 3 images, where I was originally planning to do closer to image 2.

So if I went with your proposal on image 3 and made everything on the right side of the boiler room into a warm space. If Im putting 100mm kingspan in the rafters and assume im blocking the vented soffits etc... What keeps the timber dry and where would the ventilation come from? ...Is this why the white membrane between the rafters would need to change?

Also based on how far the heating / hot water pipes span it'd only fix a portion of it anyway I guess. Lots of the pipes would still be in cold areas.


Lastly did you happen to see the stuff in video 3? For the vertical wall sections Im wanting to put 100mm kingspan over the 50mm kingspan that's there currently... I wanted to ask...as the timbers there are a little deeper than the kingspan (60mm) should the 50mm kingspan boards have an airgap behind them between them and the plasterboard of the bedroom walls? or should they be flush against the plasterboard with no air gap?

If the former I assume air can just escape down the back of them bypassing them entirely, if the latter then they are shallower than the timbers meaning when I put the 100mm kingspan over the top there will be a 10mm gap between the 50mm king span and 100mm king span.... is that the lesser of 2 evils? Confused which I should be doing here.

thanks again!

 

[robinbanks]

What keeps the timber dry and where would the ventilation come from?

Depends how you're doing it but if you're infilling between the rafters with a depth of insulation that leaves a vent gap under the white membrane, and you're boarding over and taping the joints or you're fitting a vapour membrane then the space is still ventilated and you've massively reduced the amount of warm moist air (from the house) that can reach the timbers. If you're doing it from the outside you're relaxing the non breather membrane with a breather type so you can get away with less of a gap between the membrane and the insulation (so, more insulation) because moisture can diffuse through the membrane

Roof timbers don't automatically get wet (unless you have a leak that you should fix) but cold roof constructions are typically massively over ventilated so they can cope with very still, very cold days when the people in the house are generating loads of moisture (cooking, washing, drying) and the ceiling between the house and the loft doesn't have any particular barrier to prevent it getting into the loft and condensing when it cools

Lots of the pipes would still be in cold areas.

im not advocating leaving that untreated. Those areas need filling up with insulation that covers the pipes and the dwarf walls. If you can't get access by crawling consider laying wool out by rolling it and pushing it with a stick (drain rods where you can make the stick progressively longer if space is limited) or by having a sledge made of plastic membrane onto which rolled out insulation is placed, then the head end of the sheet is pushed (by stick again) so that the sheet slides along the floor taking the insulation with it. It's going to be a compromise compared to taking walls or roof out to gain access, but it gains a reduced amount of work in making good

Lastly did you happen to see the stuff in video 3?

Not yet; haven't been anywhere quiet/free time enough to think about it

there are a little deeper than the kingspan (60mm) should the 50mm kingspan boards have an airgap behind them between them and the plasterboard of the bedroom walls? or should they be flush against the plasterboard with no air gap?

it doesnt matter so long as the top and bottom are effectively sealed. Double glazing units have a huge gap between the panes sometimes but they still insulate well because they are sealed. Air is a very good insulator if you can stop it moving. If it moves, it convects heat. When close to a surface, friction keeps it in place. Wool insulation works by being full of trapped air, kingspan a network of closed foam bubbles etc. regardless where your 10mm gap is, if you've expanding foamed the top and bottom of the gap then the air won't move much and will contribute to the insulation. Personally for ease I'd push the 50mm against the plasterboard, foam a line top and bottom to take up the 10mm, give it a bit of time/few mins to acquire some structure (expanding foam works best if you apply it to a damp surface and leave some of the solvent in it to evaporate before squashing it) then fit the 100mm over and fix with a few 120mm screws with big washers

If the 50mm boards are loose, foam them in

 

[GeddyMortgage]

and the ceiling between the house and the loft doesn't have any particular barrier to prevent it getting into the loft and condensing when it cools

Ok gotcha, yes im assuming ours doesn't have a barrier between house and loft due to it being a cold loft they're just using the the ventilation from the soffits to clear any rising moisture away.

On the topic about insulating the roof. The rafters in our roof are 200mm, if I went with 150mm of kingspan flush with the bottom of the rafters leaving the 50mm between the white membrane and kingspan, I assume i'd run the kingspan as far down to the eaves as possible, atleast so they're snug with the wool on the loft floor so all the air from the vented soffits there could only get up the air gap behind them, between kingspan and membrane.... OR would it be best to put wool in the soffits too essentially blocking the vents?


That said, even if I did do one of the above for 1/4 of the house / insulating an entire run of rafters all facing the same direction, im assuming it'd do very little to help the thermal performance of the house? As the vented soffits perpendicular to the then insulated/boarded rafters would still be letting lots of cold air in still bypassing the insulated rafters entirely? tried to display that on the image below (i've used the other side of the loft for this image as it's less busy, incase you're wondering where the pipe work has all gone)

So I guess i'd have to completely block off the portion of loft that had has the insulated rafters, to stop the vented soffits blowing through to this section entirely, Like an insulated stud wall either side, something like the image below?

Then if I understand you correctly, with all the air from the soffit vents not able to reach these insulated/boarded rafters, they'd still be OK moisture wise due to the airgap between the kingspan and membrane? the air from the world side of the roof tiles would be enough to keep them dry?

Personally for ease I'd push the 50mm against the plasterboard, foam a line top and bottom to take up the 10mm, give it a bit of time/few mins to acquire some structure (expanding foam works best if you apply it to a damp surface and leave some of the solvent in it to evaporate before squashing it) then fit the 100mm over and fix with a few 120mm screws with big washers

If the 50mm boards are loose, foam them in

Thats great, thats exactly what we planned to do except use wool in place of the foam and tape over the wool using that insulating tape. But yeah thanks for confirming.

 

[robinbanks]

Took a look at video 3- re the first half of the video: I would pull those boards out from between the studs, I'd cut a chamfer on the top of them that mates as closely as possible with the angle the ceiling boards are at (if the ceiling boards are at 30 degrees I would cut a 60'degree angle on the stud board), wipe the end of the ceiling board with a damp cloth, apply some good lines of expanding foam on the ceiling board, wipe the recently cut end of the stud board, and refit the stud board. Push it carefully into place so it doesn't flatten the expanding foam any more than it has to; if you push it too far and have to pull it back the foam might not expand to fill the gap any more. The stud boards will this be mounted higher and will have a gap at the bottom; you can fill that with wool

For travesties like that socket powering the router I'd pull it off the wall and move it so it wasn't in the way of the board then fill in the hole it left, use a longer bit of cable if necessary and have the cable come out at the side of a stud so the insulation can pass by its side. Bashing a massive hole for it really just goes to show how rough arse those builders were

Don't forget to insulate the back of the access door

 

[robinbanks]

I assume i'd run the kingspan as far down to the eaves as possible, atleast so they're snug with the wool on the loft floor so all the air from the vented soffits there could only get up the air gap behind them, between kingspan and membrane.... OR would it be best to put wool in the soffits too essentially blocking the vents?

Yes, have your kingspan that way, and foam on the ends to join them together then slide into place, or if you have space for it board over the studs instead; might as well have a larger vent gap and continuous insulation. In the small spaces any boarding over the studs you can do will help too

That said, even if I did do one of the above for 1/4 of the house / insulating an entire run of rafters all facing the same direction, im assuming it'd do very little to help the thermal performance of the house? As the vented soffits perpendicular to the then insulated/boarded rafters would still be letting lots of cold air in still bypassing the insulated rafters entirely?

If only doing part of the roof you then have to completely create a room, so install walls that join up with the insulated part of the roof and enclose the boiler room, joining eg an insulated dwarf wall to the insulated roof slope with a triangle of insulated wall

Then if I understand you correctly, with all the air from the soffit vents not able to reach these insulated/boarded rafters, they'd still be OK moisture wise due to the airgap between the kingspan and membrane? the air from the world side of the roof tiles would be enough to keep them dry?

They're dry already so it won't take much to keep them that way; they're barriered from the house generated moisture by the overboarding of the rafters (and if you have no space for insulation overboarding you can install a membrane)

A few pictures show a boardwalk on top of the wool. If you're woolling the floor, don't forget to wool over it as you exit the job; right now it's compressing the floor wool and reducing its performance but it's convenient for access. Woolling over it leaves it there for future access but improves the thermal layer (top wool can be moved for any future access needed

 

[GeddyMortgage]

Took a look at video 3- re the first half of the video: I would pull those boards out from between the studs, I'd cut a chamfer on the top of them that mates as closely as possible with the angle the ceiling boards are at (if the ceiling boards are at 30 degrees I would cut a 60'degree angle on the stud board),

Ah Yes i've already started this in earnest, though I didn't chamfer the edges at the top of the 50mm wall boards already in the studs as I've bought a load of 100mm board that will be going over the top (appreciate this may be overkill, better than underkill i guess!)....the plan was to chamfer the 100mm boards to the angle of roof allowing me to get them as high as possible too / flush with the roof rafters, in turn covering all of the 50mm wall boards. I've drawn a crude cross section below to show what i mean.

The 60degree angle that remains where the roof and wall boards meet i've put some wool in there for now, but my plan was to use off-cuts from the 100mm overboard pieces to slot and foam into that gap like below.

Though even as i draw it, I can see really for it to be consistently thick, I should have something insulative in this space here between the roof rafters... I guess i could just put some wool there.

Don't forget to insulate the back of the access door

Yeah, this is a point i realised yesterday too. There are 5 access hatches, all uninsulated. They're all hinged opening doors too so will either stick a chamfered piece of kingspan to the back of them as to still allow them to open without hitting the frame..... But as this won't insulate the gaps around the hatch doors, I had a quirky idea of hanging some thick material behind them, like a thick throw blanket, which you could just roll up and hang or even Velcro to a location directly above the hatch, sort of like the door of a tent, when you need to go through.

 

[GeddyMortgage]

A few pictures show a boardwalk on top of the wool. If you're woolling the floor, don't forget to wool over it as you exit the job; right now it's compressing the floor wool and reducing its performance but it's convenient for access. Woolling over it leaves it there for future access but improves the thermal layer (top wool can be moved for any future access needed

Everything in this reply made total sense and already in hand, after i'm done with insulating the walls properly, my next focus is the loft floor, i'll be raising as much of it as possible to get 270-300mm of wool (currently 100mm) beneath the boarded sections (plus some atop the boarded sections as you recommend) and likely 300-400mm of wool for the un-boarded sections (currently 150mm)